The art of glass-making, unlike that of pottery, would appear not to have been discovered and practised by different nations independently, but to have gradually spread from a single centre. No trace of it was observed among the inhabitants of America at the time when that continent was discovered, although considerable progress in the arts had been made by some among them, e.g., the Mexicans and Peruvians ; but the steps by which it reached China may be indicated with much probability. The credit of the invention was given by the ancients to the Phoenicians, as is shown by the well-known story of its fortuitous discovery by Phoenician merchants, who rested their cooking pots on blocks of natron (sub-carbonate of soda), and found glass produced by the union under heat of the alkali and the sand of the shore (Pliny, Nat. Hist., xxxvi. 26, 65). A glassy mass may, however, be produced in the smelting of many metallic ores, silica being present, while the fuel supplies the alkali; or by the combustion of great masses of reeds or straw, in which the elements of glass are present,lumps of coarse imperfect glass being often found on the spot where a stack of wheat has been burned. Now the Egyptians practised metallurgic opera-tions from a very early period, and vast heaps of straw are, and no doubt have been from the earliest times, accumu-lated in that country, and probably not unfrequently set on fire. The adoption of glass as a substance capable of being made subservient to the use of mankind may therefore be due to the intelligence of some one who noticed its fortui-tous production there. Be this as it may, by far the earliest examples of glass existing of which the dates are attested Egyptian by inscriptions are of Egyptian origin. The earliest of glass. these, a small lion's head of opaque blue glass of very fine colour, but changed externally to an olive green, was found at Thebes by Signor Drovetti, and is now in the British Museum; on the underside are hieroglyphics containing the name of Nuantef IV., whose date according to Lepsius's chronology was 2423-2380 B.C. A bead of dusky green glass bears the prasnomen of Hatasu, a queen who is con-jectured to have lived about 1450 B.C. (Wilkinson, Manners and Customs of the Ancient Egyptians, vol. iii. p. 88). That such may be the real dates of these objects is con-firmed by the fact that glass bottles containing red wine are represented on monuments of the fourth dynasty, more than 4000 years old; and in the tombs at Beni Hasan, dating from the reign of Usurtesen I., at least 2000 years B.C., the process of glass-blowing is represented in an unmistakable manner (Wilkinson, vol. iii. p. 89). Very many examples of glass found in Egypt may be seen in museums, but, as they rarely bear inscriptions, it would be difficult to trace the progress of the art through them ; no competent person has hitherto undertaken the task. The manufacture probably continued to flourish as well during the period of the native monarchy as in that of the Greek dynasty; and its importance after the subjugation of the country to Pome was probably even increased by the new market then opened to its products. Martial (Ep., xxi. 74) alludes to the importation of Egyptian glass into Rome ; and it is mentioned in an ordinance of Aurelian. Hadrian in a letter addressed to the consul Servianus men-tions glass-blowing as one of the chief industrial occupations of the inhabitants of Alexandria. The manufacture was not confined to that city, but was also carried on in the lower Diospolis on Lake Mensaleh, as appears by a passage in the Periplus Maris Erythrcei (c. 6).
Much of the Egyptian glass was uncoloured and of a somewhat dusky hue; of the coloured and ornamental varieties perhaps the most characteristic examples are the small vases usually in the form of either alabastra or am-phora?, but occasionally in that of an Egyptian column. Tn these the prevailing colour is a deep transparent blue ; but not unfrequently the colour of the body of the vase is Boir.e shade of pale buff, fawn, or white (an imitation probably of arragonite, Egyptian alabaster), sometimes deep green, and in rare cases red. In almost every example the surface is ornamented by bands of colour, white, yellow, or tor-quoise blue, forming zigzag lines ; in some examples- there are only two or three such lines, in others the whole surface is covered by them. These lines are incorporated with the surface of the vessel, but do not penetrate through its entire thickness. By the Greeks and Etruscans such vessels were evidently much valued; the amphora? have been occasion-ally found in tombs, furnished with a stand of gold. In Rhodes and elsewhere they have been found associated with objects which probably do not date from an earlier period than the 3d or 4th century before Christ, and it does not appear that they are met with in tombs later than the Christian era; when coloured or ornamental glass vessels are discovered in these last, they are of a different character. Another species of glass manufacture in which the Egyptians would appear to have been peculiarly skilled is the so-called mosaic glass, formed by the union of rods of various colours in such a manner as to form a pattern ; the rod so formed was then reheated and drawn out until reduced to a very small size, a square inch or less, and divided into tablets by being cut transversely, each of these tablets presenting the pattern traversing its substance and visible on each face. This process was no doubt first, prac-tised in Egypt, and is never seen in such perfection as in objects of a decidedly Egyptian character in design or in colour. Very beautiful pieces of ornament of an architec-tural character are met with, which probably once served as decorations of caskets or other small pieces of furniture, or of trinkets ; also tragic masks, human faces, and birds. Some of the last-named are represented with such truth of colouring and delicacy of detail that even the separate feathers of the wings and tail are well distinguished, although, as in an example in the British Museum, a human-headed hawk, the piece which contains the figure may not exceed three-fourths of an inch in its largest dimension. Works of this description probably belong to the period when Egypt passed under Roman domination, as similar objects, though of inferior delicacy, appear to have been made in Rome.
The Phoenicians probably derived their knowledge of the art from Egypt; whether this be so or not, they undoubt-edly practised it from a very early period and to a very large extent. Probably much the same processes were em-ployed in Phoenicia and Egypt during some centuries before the Christian era, as they certainly were in Phoenicia, Egypt, and Rome for some centuries after. It seems probable that the earliest products of the industry of Phoenicia in the art of glass-making are the coloured beads which have been found in almost all parts of Europe, in India and other parts of Asia, and in Africa. The " aggry " beads, so much valued by the Ashantees and other natives of that part of Africa which lies near the Gold Coast, have pro-bably the same origin. These coloured beads are usually of opaque glass; they exhibit great variety of colour and pattern, and very different degrees of skill in manipulation. Their wide dispersion may be referred with much proba-bility to their having been objects of barter between the Phoenician merchants and the barbarous inhabitants of the various countries with which they traded. It is probable, however, that many of the specimens which exist in our museums date from times several centuries later than those in which Tyre and Sidon flourished; for, as we may learn from the Periplus and Strabo, glass in various forms was an article imported in the 1st and 2d centuries, as well into the emporia of the Red Sea as into the ports of Britain. Even at the present day beads are very extensively made at Venice for export to Africa, which bear a resemblance, doubtless not accidental, to those which we have reason to believe to be of very early date. Assyrian. Next in date to the earlier Egyptian examples mentioned above would appear to be the vase of transparent greenish glass found in the north-west palace of Nineveh, and now in the British Museum. On one side of this a lion is engraved, and also a line of cuneiform characters, in which is the name of Sargon, king of Assyria, 722 B.C. Frag-ments of coloured glasses were also found there, but our materials are too scanty to enable us to form any decided opinion as to the degree of perfection to which the art was carried in Assyria. Many of the specimens discovered by Layard at Nineveh have all the appearance of being Roman, and were no doubt derived from the Roman colony, Niniva Claudiopolis, which occupied the same site. Greek. The Greeks, excellent in the ceramic art, do not appear to have cultivated the art of glass-making at a very early period; but it was probably made in many places on the shores of the Mediterranean for some centuries before the Christian era. At Mycenae many disks of opaque vitreous pastes were found by Schliemann, and very similar objects at Ialyssus in Rhodes ; but it is not certain that these may not have been brought from Egypt, where very similar objects have been found, or whether they ought not to be attributed to Greek or to Phoenician artisans. At Camirus in Rhodes, however, many vessels of glass of very elegant forms have been discovered, which were probably made in the island.
In Etruscan tombs in Italy are also found glass vessels of peculiar character; these are small bowls resembling in form the half of an egg; they are usually of the variety of glass which is mentioned further on as "madrepore," the ground green and transparent, the stars yellow, while patches of colour of gold and of filigree glass are sometimes inter-spersed. They differ from and appear to be earlier than the madrepore glass, fragments of which are so often found in Rome. They are also said to be found in Magna Graecia. Another variety found in tombs in the same district is of blue and opaque glass, with much gold in leaf, all twisted together; the most frequent form in which this kind of glass has been found is that of a bottle several inches long and about one inch in diameter, without a neck, having probably had a mounting of gold. It remains to be deter-mined whether these should be attributed to a Greek or to a Phoenician origin. Glass, however, was occasionally used for purposes of architectural decoration during the best period of Grecian art, for Stuart and Revett, when describing the temple of Minerva Polias at Athens, give the following note :" A remarkable singularity observed in the capitals of this portico is in the plaited torus between the volutes having been inlaid at the interstices with coloured stones or glass." Mr H. March Phillips states that he well remembers having remarked these decora-tions, and that he believes them to be of blue glass.1
1 An example of the employment of glass in a like manner is indicated by the odd story which Pliny tells (Nat. Hist., xxxvii. 5, 17) that on the tomb of Hermias, a prince of the island of Cyprus,
In the first centuries of our era the art of glass-making Roman, was developed at Rome and other cities under Roman rule in a most remarkable manner, and it reached a point of excellence which in some respects has never been excelled or even perhaps equalled. It may appear a somewhat ex-aggerated assertion that glass was used for more purposes, and in one sense more extensively, by the Romans of the imperial period than by ourselves in the present day ; but it is one which can be borne out by evidence, It is true that the use of glass for windows was only gradually ex-tending itself at the time when Roman civilization sank under the torrent of German and Hunnish barbarism, and that its employment for optical instruments was only known in a rudimentary stage ; but for domestic purposes, for architectural decoration, and for personal ornaments glass was unquestionably much more used than at the present day. It must be remembered that the Romans possessed no fine porcelain decorated with lively colours and a beautiful glaze; Samian ware was the most decorative kind of pottery which was then made. Coloured and ornamental glass held among them much the same place for table services, vessels for toilet use, and the like, as that held among us by porcelain. o Pliny (Nat. Hist, xxxvi. 26, 67) tells us that for drinking vessels it was even preferred to gold and silver. Trebellius Pollio, however, relates of the emperor Gallienus that he drank from golden cups, despising glass, than which, he said, nothing was more vulgar. Glass was largely used in pave-ments, and in thin plates as a coating for walls. It was used in windows, though by no means exclusively, mica, alabaster, and shells having been also employed. Glass, in flat pieces, such as might be employed for windows, has been found in the ruins of Roman houses, both in England and in Italy, and in the house of the faun at Pompeii a small pane in a bronze frame remains. Glass of this description seems to have been cast on a stone, and is usually very uneven and full of defects; although capable of transmitting light, it must have given at best an indifferent view of external objects. When the window openings were large, as was the case in basilicas and other public buildings and even in houses, the pieces of glass were, doubtless, fixed in pierced slabs of marble or in frames of wood or bronze.
was a marble figure of a lion with eyes of emerald which shone so brightly into the sea that they frightened away the tunnies from the adjacent fisheries, so that it became necessary to change the eyes. In the great marble lion discovered by Mr Newton near Cnidus, and now in the British Museum, in the place of the eyes are deep sockets which probably, like those of the Cypriote lion, were filled with coloured glass.
The invention and ingenuity employed by the Roman artisans in producing variety in glass vessels are most re-markable; almost every means of decoration appears to have been tried, and many methods of manipulating glass, which have been considered inventions, have in reality been anticipated by the glass-workers of the period under con-sideration. The fertility of invention which devised so many modes of ornamentation and so many shades of colour, and the skill with which the manual execution is carried out, alike deserve great admiration. This prodigious variety seems to show that glass-making was at that time carried on, not as now in large establishments, which pro-duce great quantities of articles identical in form and pattern, but by many artificers, each working on a small scale. This circumstance enables us to understand why very pure and crystalline glass was, as Pliny tells us, more valued than any other kind. To produce glass very pure and free from striae and bubbles, long-continued fusion is required ; this the system of working of the ancients did not allow, and their glass is in consequence remarkable for the great abundance of bubbles and defects which it contains.
The Romans had at their command, of transparent ' colours, blue, green, purple or amethystine, amber, brown, | and rose; of opaque colours, white, black, red, blue, yellow, green, and orange. There are many shades of the former as well as of the latter, particularly of transparent blue, and of opaque blue, yellow, and green. Of opaque \ colours many varieties appear to be due to the mixture of one colour with another. In any large collection of frag-ments it would be easy to find eight or ten varieties of \ opaque blue, ranging from lapis lazuli to turquoise or to lavender, and six or seven of opaque green. Of red the varieties are fewer; the finest is a crimson red of very beautiful tint, and there are various gradations from this to a dull brick red. One variety forms the ground of a very good imitation of porphyry; and there is a dull semi-transparent red which, when light is passed through it, appears to be of a dull green hue. With these colours the Roman vitrarius worked, either using them singly or blending them in almost every conceivable combination, sometimes, it must be owned, with a rather gaudy and inharmonious effect.
These combinations of colour were effected in two ways : first, by glasses of two or more colours being combined so as to traverse the entire substance of the object; and, secondly, by the superposition of the one colour on the other.
To the former class belong all those termed mosaic and mille fiori, where the process of manufacture was the pre-liminary union, by heat, of threads of glass into a rod, which when cut transversely exhibited the same pattern in every section. Such rods were placed together side by side, and united by heat into a mass which was then formed into cups or other vessels. A vast quantity of small cups and pateras were made by this means in patterns which bear considerable resemblance to the sur-faces of madrepores, and are of the same kind as those which by the Venetians are termed " mille fiori." In these every colour and every shade of colour seem to have been tried in great variety of combination with effects more or less pleasing, but transparent violet or purple appears to have been the most common ground colour. Although most of the vessels of this mille fiori glass were small, some were made of large size; a fragment in the possession of the present writer must have formed part of a dish not less than 20 inches in diameter. Another variety of glass, evidently much U3ed, is that in which transparent brown glass is so mixed with opaque white and blue as to resemble onyx. This was sometimes done with great success, and very perfect imitations of the natural stone were produced. Sometimes purple glass is used in place of brown, probably with the design of imitating the precious mur-rhine. Imitations of porphyry, of serpentine, and of granite are also met with, but these were used chiefly in pavements, and for the decoration of walls, for which purposes the onyx-glass was likewise employed. Under this head must also be included the interlacing of bands and threads both of white and of coloured glass. Vessels are found composed of bands either so placed in sections as to present a plaited pattern, or simply arranged side by side; others, again, resemble the Venetian vitro di trina, threads of opaque white or yellow glass being twisted with clear transparent glass, and the vessel then formed by the welding together of the rods so made. Blue threads are occasionally intermixed, and several varieties of pattern may be found; but this branch of the art does not appear to have bsen carried by the Romans to anything like the perfection to which it was afterwards brought by the Venetians.
So few examples of glass vessels of this period which have been painted in enamel have come down to us that it has been questioned whether that art was then practised ; but several specimens have been recently described which can leave no doubt on the point; decisive examples are afforded by two cups found at Vaspelev, in Denmark, engravings of which are published in the Annaler for Nordisk Oldkyndeghed for 1861, p. 305. These are small cups, 3 inches and 2J inches high, 3| inches and 3 inches wide, with feet and straight sides ; on the larger are a lion and a bull, on the lesser two birds with grapes, and on each some smaller ornaments. On the latter are the letters DVB . R. The colours are vitrified and slightly in relief; green, blue, and brown may be distinguished. They were found with Roman bronze vessels and other articles. Vessels also are not uncommon on the surface of which enamel colours appear in the form of spots; it is probable that these were applied in the form of melted glass, not, as in true enamel painting, in that of a finely divided powder tempered with an essential oil and applied cold.
The first place among those processes in which one colour was superimposed on another may be given to that by which the cameo glass was produced. In this a bubble of opaque white glass was formed at the end of the tube used by the glass-blower; this was coated with transparent blue, and that again with opaque white, and the vessel required was formed from this threefold globe. The outer coat was then removed from that portion which was to constitute the ground, leaving the white for the figures, foliage, or other ornamentation; these were then sculptured by means of the gem-engraver's tools. Pliny no donbt means to refer to this when he says (Nat. Hist., xxxvi. 26, 66), " aliud argenti modo cselatur," contrasting it with the process of cutting glass by the help of a wheel, to w-hich he refers in the words immediately preceding, "aliud torno teritur."
The famous Portland or Barberini vase is the finest example of this kind of work which has come down to us, and was entire until it was broken into some hundred pieces by a drunken medical student some years ago. The pieces, however, were joined together by Mr Doubleday with extraordinary skill, and the beauty of design and execution may still be appreciated almost as well as when it was intact. A letter written by Wedgwood in 1786 to Sir William Hamilton has been published in the life of the former by Miss Meteyard (vol. ii. p. 577), which contains some interesting remarks upon this beautiful work of art. He concludes with the assertion, "I do not think ¿£5000 for the execution of such a vase, supposing our best artists capable of such a work, would be at all equal to their gains from the works they are now employed in." It is true that the gem-engravers of that day received very high pay for their work.
The two other most remarkable examples of this cameo glass are an amphora at Naples and the Auldjo vase. The amphora measures 1 foot § inch in height, 1 foot 7\ inches in circumference; it is shaped like the earthen amphoras with a foot far too small to support it, and must no doubt have had a stand, probably of gold; the greater part is covered with a most exquisite design of garlands and vines, and two groups of boys gathering and treading grapes and playing on various instruments of music; below these is a line of sheep and goats in varied attitudes. The ground is blue and the figures white. It was found in a house in the Street of Tombs at Pompeii in the year 1839, and is now in the Royal Museum at Naples. It is well engraved in Richardson's Studies of Ornamental Design. The Auldjo vase, a part of which is or was in possession of Mr Auldjo, and another in the British Museum, is an cenochoe about 9 inches high; the ornament consists mainly of a most beautiful band of foliage, chiefly of the vine, with bunches of grapes ; the ground is blue and the ornaments white; it was found at Pompeii in the house of the faun. It also | has been engraved by Richardson.
Costly as these beautiful objects must have been, a very great number of them existed, for even now fragments of ten to fifteen may probably be met with in the hands of the curiosity dealers in Rome in the course of three or four months. The same process was used in producing large tablets, employed, no doubt, for various decorative purposes. In the South Kensington Museum collection is a fragment of such a tablet or slab; the figure, a portion of which remains, could not have been less than about 14 inches high.
The ground of these cameo glasses is most commonly transparent blue (often lined with opaque white to throw up the colour), but sometimes opaque blue, purple, or dark brown. The superimposed layer, which is sculptured, is generally opaque white. A very few specimens have been met with in which several colours are employed.
At a long interval after these beautiful objects come those vessels which were ornamented either by means of coarse threads trailed over their surfaces and forming rude patterns, or by coloured enamels merely placed on them in lumps; and these, doubtless, were cheap and common wares. But a modification of the first-named process was in use in the 4th and succeeding centuries, showing great ingenuity and manual dexterity,that, namely, in which the added portions of glass are united to the body of the cup, not throughout, but only at points, and then shaped either by the wheel or by the hand. The attached portions form in some instances inscriptions, as on a cup found at Strasburg, which bears the name of the emperor Maximian (286-310 A.D.), on another in the Vereinigte Sammlungen at Munich, and on a third in the Trivulzi collection at Milan, where the cup is white, the inscription green, and the net-work blue. Probably, however, the finest example is a situla, 10J inches high by 8 inches wide at the top and 4 inches at the bottom, preserved in the treasury of St Mark at Venice. This is of glass of a greenish hue; on the upper part is represented, in relief, the chase of a lion by two men on horseback accompanied by dogs; the costume appears to be Byzantine rather than Roman, and the style is very bad. The figures are very much undercut. The lower part has four rows of circles united to the vessel at those points alone where the circles touch each other. All the other examples have the lower portion covered in like manner by a network of circles standing nearly a quarter of an inch from the body of the cup.
The art of glass-making no doubt, like all other art, deteriorated during the decline of the Roman empire, but it is probable that it continued to be practised, though with constantly decreasing skill, not only in Rome but in the provinces. Some few existing vessels, as two chalices of coarse blue glass in the British Museum, may perhaps be referred to this period, but the most remarkable production was mosaic for the decoration of churches. Examples of such decorations may be still seen in Rome dating from every century through the dark ages; and, though glass for mosaic was certainly made at Constantinople, and perhaps also at Ravenna, it is probable that it was also made in Rome. Glass was largely used in the immense windows of the churches built between the 3d and the 10th centuries. The first mention of coloured glass in a church window occurs in the time of Pope Leo III. (795-816); but pro-bably it was used at a much earlier period.
Some of the Roman artificers in glass no doubt emigrated Byzan-to Constantinople, and it is certain that the art was prac-tme' tised there to a very great extent during the Middle Ages. One of the gates near the port took its name from the adjacent glass-houses. St Sophia when erected by Justinian had vaults covered with mosaics and immense windows filled with plates of glass fitted into pierced marble frames; some of the plates, 7 to 8 inches wide and 9 to 10 high, not blown but cast, which are in the windows, may possibly date from the building of the church. Glass for mosaics was also largely made and exported. In the 8th century when peace was made between the caliph Walid and the emperor Justinian II., the former stipulated for a quantity of mosaic for the decoration of the new mosque at Damascus, and in the 10th century the materials for the decoration of the niche of the kibla at Cordova were furnished by Romanus II. In the 11th century Desiderius, abbot of Monte Casino, sent to Constantinople for workers in mosaic. The grounds of the Byzantine mosaics were usually either of gold or silver, a thin leaf of the metal having been enclosed between two layers of glass.
We have in the work of the monk Theophilus, Diversamm Artium Schedula, and in the probably earlier work of Eraclius, about the 11th century, instructions as to the art of glass-making in general, and then as to that of producing coloured, gilt, and enamelled vessels, which these writers speak of as being practised by the Greeks. But we look almost in vain for existing specimens of such works. Perhaps the only entire enamelled vessel which we can confidently attribute to Byzantine art is a small vase preserved in the treasury of St Mark at Venice, a very clever reproduction of which was exhibited by the Murano Glass Company at the Paris exhibition of 1878. This is decorated with circles of rosettes of blue, green, and red enamel, each surrounded by lines of gold; within the circles are little figures evidently suggested by antique originals, and precisely like similar figures found on carved ivory boxes of Byzantine origin dating from the 11th or 12th centuries. Two inscriptions in Cufic characters surround the vase, but they, it would seem, are merely ornamental and destitute of meaning. The presence of these inscriptions may perhaps lead to the inference that the vase was made in Sicily, but by Byzantine workmen.
Of uncoloured glass brought from Constantinople several examples exist in the treasury of St Mark at Venice, part of the plunder of the imperial city when taken by the crusaders in 1204. The glass in all is greenish, very thick, with many bubbles, and has been cut with the wheel; in some instances circles and cones, and in one the outlines of the figure of a leopard have been left standing up, the rest of the surface having been laboriously cut away. The in-tention would seem to have been to imitate vessels of rock crystal.
Probably at Alexandria, one of the great seats of glass- Alex-making, the art survived the conquest of Egypt by the andria-Saracens, for a glass disk serving as a weight has been met with in Egypt bearing the date 96 of the Hegira, corresponding with 715 A.D. (see memoir by Mr E. T. Bogers, Journal of the Royal Asiatic Society, vol. x. pt. 1). Numer-ous later examples leave no doubt that the manufacture of glass continued to exist in Egypt, though perhaps in a languishing condition. In the 11th century, however, we find in the life of St Odilo, abbot of Fulda (ob. 1049), mention of a "vas pretiosissimum vitreum Alexandrini generis," and in the same century Nasir Ibn Khusru (Safarnamah, published by Eoyal Asiatic Society), who visited Jerusalem in 1060, says that pictures of our Lord and others in one of the churches of that city were covered with plates of glass. No examples of ornamental vessels dating from this period have, however, come down to us. But we have many very remarkable examples of the skill of Egyptian and Syrian glass-workers in the 13th and 14th centuries,large bottles, basins, and lamps, very striking objects from the free use of enamel and gilding in their decoration. This is, as in most objects of Eastern art, chiefly composed of inscriptions written in large characters ornamentally treated; but figures of birds, lions, sphinxes, &c, may be found, especially on vessels made in Egypt. Although there may have been some indigenous practice of the art of glass-making in the East,for in the cup of Chosroes I. of Persia (531-579) preserved in the Bibliothèque Nationale at Paris are medallions of coloured and moulded glass,the arts of gilding and enamelling, as we see them exhibited in the Syria. Syrian and Egyptian works of the 13th and 14th centuries, were probably derived from the Byzantines. Damascus was also the seat of a like manufacture. In inventories of the 14th century, both in England and in France, mention may frequently be found of glass vessels described as of the manufacture of Damascus. That city was taken by Timur in 1402, and we are told by Clavijo, who visited his court in 1403-1406, that he carried off to Samarcand "men who made bows, glass, and earthenware, so that of these articles Samarcand produces the best in the world."
Glass no doubt continued to be made, as it still is, in Syria and Persia, but no very remarkable products of the manufacture are known in Europe, with the exception of Persia, some vessels brought from Persia, blue richly decorated with gold. These probably date from the 17th century, for Chardin tells us that the windows of the tomb of Shah Abbas II. (ob. 1666) at Kom, were "de cristal peint d'or et d'azur." At the present day bottles and drinking vessels are made in Persia which in texture and quality differ little from ordinary Venetian glass of the 16th or 17th centuries, while in form they exactly resemble those which may be seen in the engravings in Chardin's Travels.
India. Pliny states (Nat. Hist., xxxvi. 26, 66) that no glass was to be compared to the Indian, and gives as a reason that it was made from broken crystal ; and in another passage (xii. 19,42) he says that the Troglodytes brought to Ocelis (Ghella near Bab-el-Mandeb) objects of glass. We have, however, very little knowledge of Indian glass of any considerable antiquity. A few small vessels have been found in the " topes, " as in that at Manikyala in the Punjab, whicli probably dates from about the Christian era; but they exhibit no remarkable character, and fragments found at Brahminabad are hardly distinguishable from Roman glass of the imperial period. The chronicle of the Singhalese kings, the Mahawanso, however, asserts that mirrors of glittering glass were carried in procession in 306 B.C., and beads like gems, and windows with ornaments like jewels, are also mentioned at about the same date. If there really was an important manufacture of glass in Ceylon at this early time, that island perhaps furnished the Indian glass of Pliny ; or it is possible that it really came from China. Glass is made in several parts of Indiaas Behar and Mysoreby very simple and primitive methods, and the results are correspondingly defective. Black, green, red, blue, and-yellow glass is made. The greater part is worked into bangles, but some small bottles are blown (Buchanan, Journey through Mysore, vol. i. p. 147; vol. iii. p. 369).
The history of the manufacture of glass in China is obscure, but the common opinion that it was learnt from the Europeans in the 17th century seems to be erroneous. A writer in the Mémoires concernant les Chinois (vol ii. p. 46) states on the authority of the annals of the Han dynasty that the emperor Ou-ti (140 B.C.) had a manufactory of the kind of glass called "lieou-li," that in the beginning of the 3d century of our era the emperor Tai-tsou received from the West a considerable present of glasses of all colours, and that soon after a glass-maker came into the country who taught the art to the natives.
The Wei dynasty, to which Tai-tsou belonged, reigned in northern China, and at this day a considerable manufacture of glass is carried on at Po-shan-hien in Shantung, which it would seem has existed for a long period. The Rev. A. Williamson (Journeys in North China, i. 131) says that the glass is extremely pure, and is made from the rocks in the neighbourhood. The rocks are probably of quartz, i.e., rock crystal, a correspondence with Pliny's statement respecting Indian glass which seems deserving of attention.
Whether the making of glass in China was an original discovery of that ingenious people, or was derived via Ceylon from Egypt, cannot perhaps be now ascertained; the manu-facture has, however, never greatly extended itself in China. The case has been the converse of that of the Romans; the latter had no fine pottery, and therefore employed glass as the material for vessels of an ornamental kind, for table services and the like. The Chinese, on the contrary, having from an early period had excellent porcelain, have been careless about the manufacture of glass. A Chinese writer, however, mentions the manufacture of a huge vase in 627 A.D., and in 1154 Edrisi (first climate, tenth section) mentions Chinese glass. A glass vase about a foot high is preserved at Nara in Japan, and is alleged to have been placed there in the 8th century. It seems probable that this is of Chinese manufacture. A writer in the Mémoires concernant les Chinois (vol. ii. pp. 463 and 477), writing about 1770, says that there was then a glass-house at Peking, where every year a good number of vases were made, some requiring great labour because nothing was blown (rien n'est soufflé), meaning no doubt that the ornamentation was produced not by blowing and moulding, but by cutting. This factory was, however, merely an appendage to the imperial mag-nificence. The earliest articles of Chinese glass the date of which has been ascertained, which have been noticed, are some bearing the name of the emperor Kienlung (1736-1796), one of which is in the South Kensington Museum.
In the manufacture of ornamental glass the leading idea in China seems to be the imitation of natural stones. The coloured glass is usually not of one bright colour through-out, but semi-transparent and marbled; the colours in many instances are singularly fine and harmonious. As in 1770, cutting is the chief method by which ornament is produced, the vessels being blown very solid.
The chief source from whence a knowledge of the art of Diffusion glass-making spread through Europe was probably Rome ; of tne in the Roman imperial period glass was undoubtedly made, not only in Italy, but also in France, in Spain, and in all probability at or near Cologne, and perhaps in other places near the Rhine. Whether refugees from Padua, Aquileia, or other Italian cities carried the art to the lagoons of Venice in the 5th century, or whether it was learnt from the Greeks of Constantinople at a much later date, has been a disputed question. It would appear not improbable that the former was the case, for it must be remembered that articles formed of glass were in the later days of Roman civilization in constant daily use, and that the making of glass was tarried on, not as now in large establishments, but by artisans working on a small scale. It seems certain that some knowledge of the art was preserved in France and in Spain, possibly even in England, and it seems improbable that it should have been lost in that archipelago, where the traditions of ancient civilization must have been better preserved than in almost any other place. In 523 Cassiodorus writes of the " innumerosa navigia " belonging to Venice, and where trade is active there is always a pro-Vente» lability that manufactures will flourish. However this may and " be, the earliest positive evidence of the existence at Venice Mura;*» of a worker in glass would seem to be the mention of Petrus Flavianus, phiolarius, in the ducale of Vitale Falier in the year 1090. In 1224 twenty-nine persons are mentioned as friolari (i.e., phiolari), and in the same century "mariegole " or codes of trade regulations were drawn up (Monografía della Velraria Veneziana e Muranese, p. 219). The manu-facture had then no doubt attained considerable proportions; in 1268 the glass-workers exhibited decanters, scent-bottles, . and the like; in 1279 they made,among other things, weights and measures. In the latter part of this century the glass-houses were almost entirely transferred to Murano. From thenceforward the manufacture continued to grow in im-portance ; glass vessels were made in large quantities, as well as glass for windows. The earliest example which has as yet been describeda cup of blue glass, enamelled and giltis, however, not earlier than about 1440. A good many other examples have been preserved which may be assigned to the same century : the earlier of these bear a resemblance in form to the vessels of silver made in the west of Europe; in the later an imitation of classical forms becomes apparent. Enamel and gilding were freely used, in imitation no doubt of the much admired vessels brought from Damascus or Egypt. Many of the ornamental processes which we admire in Venetian glass were already in use or were invented in this century, as that of mille fiori, in which rods of various colours are joined by heat and so arranged as when cut transversely to form patterns resembling flowers or coral-lines. Such sections were then placed side by side and united by a lining of heated glass applied to them; the joint mass was then reheated and worked into the desired form. The beautiful kind of glass known as vitro di trina or lace glass, was made by a variation of the same process; the rods or canes, being composed of opaque white threads surrounded by transparent glass, were placed side by side in a mould, and a bubble of glass blown into the midst so as to adhere to the canes ; the whole was then reheated and fashioned in the same manner as any ordinary glass. Some-times two canes or cylinders were used, the lines of which ran in opposite directions, and a reticulated pattern was thus produced. An elaborate account of the process is given by M. Labarte (Histoire des Arts Industriéis, iv. 575 sq.). Many of the examples of this process exhibit surprising skill and taste, and are among the most beautiful objects produced at the Venetian furnaces. Glass was made by the Romans in like manner, but no ancient example which has come down to us equals in correct execution some of those made at Venice. That peculiar kind of glass usually called schmelz, an imperfect imitation of calcedony, was also made at Venice in the 15th century. Avanturine glass, that in which numerous small particles of oxide of copper are diffused through a transparent yellowish or brownish mass, was not invented until about 1600.
The peculiar merits of the Venetian manufacture are the elegance of form and the surprising lightness and thinness of the substance of the vessels produced. The glass on the other hand wants brilliancy, and is often tinged with yellow, or if, as is not uncommon, too much manganese (which neutralizes the yellow tinge imparted by the presence of iron) has been used, a faint purple. This slight coloration may not, however, appear a defect to eyes fatigued by the brilliancy of modern flint glass. The Venetian workmen perhaps somewhat abused their skill by giving extravagant forms to vessels, making drinking glasses in the forms of ships, lions, birds, whales, and the Tike.
Besides the making of vessels of all kinds the factories of Murano had for a long period almost an entire monopoly of two other branches of the art,the making of mirrors and of beads. Attempts to make mirrors of glass were made as early as 1317 A.D., but even in the 16th century mirrors of steel were still in use. To make a really good mirror of glass two things are required,a plate free from bubbles and striae, and a method of applying a film of metal with a uniform bright surface free from defects. The principle of applying metallic films to glass seems to have been known to the Romans and even to the Egyptians, and is mentioned by Alexander Neckam in the 12th century, but it would appear that it was not until the 16th century that the pro-cess of " silvering " mirrors by the use of an amalgam of tin and mercury had been perfected. During the 16th and 17th centuries Venice exported a prodigious quantity of mirrors, but France and England gradually acquired know-ledge and skill in the art, and in 1772 only one glass-house at Murano continued to make mirrors.
The making of beads was probably practised at Venice from a very early period, but the earliest documentary evi-dence bearing on the subject does not appear to be of earlier date than the 14th century, when prohibitions were directed against those wdio made of glass such objects as were usually made of crystal or other hard stones. In the 16th century it had become a trade of great importance, and about 1764 twenty-two furnaces were employed in the production of beads. Towards the end of the same century from 600 to 1000 workmen were, it is stated, employed on one branch of the art, that of ornamenting beads by the help of the blow-pipe. A very great variety of patterns was produced; a tariff of the year 1800 contains an enumeration of 562 species and a vast number of sub-species. Beads to the value of £200,000 are still annually made in Venice (Monografia della Velraria Veneziana e Muranese).
The efforts made in France, Germany, and England, in the 17th and 18th centuries, to improve the manufacture of glass in those countries had a very injurious effect on the industry of Murano. The invention of flint glass in England (about 1620?) brought in its train the practice of cutting glass, a method of ornamentation for which "V enetian glass from its thinness was ill-adapted. One remarkable man, Giuseppe Briati, exerted himself, with much success, both in working in the old Venetian method and also in imitating the new fashions invented in Bohemia. He was especially successful in making vases and circular dishes of " vitro di trina," one of the latter in the Correr collection at Venice, believed to have been made in his glass-house, measures 55 centimetres (nearly 23 inches) in diameter. The vases made by him are as elegant in form as the best of the Cinquecento period, but may perhaps be distin-guished by the superior purity and brilliancy of the glass. He also made with great taste and skill large lustres and mirrors with frames of glass ornamented either in intaglio or with foliage of various colours. He obtained a know-ledge of the methods of working practised in Bohemia by disguising himself as a porter, and thus worked for three years in a Bohemian glass-house. In 1736 he obtained a patent at Venice to manufacture glass in the Bohemian manner. He died in 1772.
The fall of the republic was accompanied by interruption of trade and decay of manufacture, and in the last years of the 18th and beginning of the 19th century the glass-making of Murano was at a very low ebb. In the year 1838 Signor Bussolin revived several of the ancient processes of glass-working, and this revival was carried on by C. Pietro Biguglia in 1845, and by others, and later by Salviati, to whose successful efforts the modern renaissance of the glories of Venetian art glass is principally due. Salviati revived the former processes and practised them, not in mere slavish imitation, but with freedom, invention, and fine artistic instinct. Every general industrial exhibition has con-tained brilliant examples of the products of the Venice and Murauo Glass Company, composed chiefly of English capitalists, and formerly managed by Salviati.
The fame of Venice in glass-making so completely eclipsed that of Italian cities that it is difficult to learn much respecting their progress in the art. It appears, however, that as early as 1295 furnaces had been established at Treviso, Vicenza, Padua, Mantua, Ferrara, Eavenna, and Bologna. In 1631 there were in Rome two and in Florence one glass-house ; but whether any of these produced ornamental vessels, or only articles of common use and window glass, would not appear to have as yet been ascertained.
The history of glass-making in France, Spain, Germany, and England offers many points of resemblance. In the Roman ^rst tnree» anc* probably in England also, glass was made at
ieriod. period of the Roman empire ; in France about Lyons, as is shown by a monument in the Musée Lapidaire to one Julius Alexander, described as an " opifex artis vitri," in Normandy and Poitou, and probably in many other parts. In Spain glass, according to Pliny, was manufactured (Nat. Hist., xxxvi. 26, 66) in many parts of the country, "per Hispanias," but the remains of Roman glass-making have been chiefly found in the valleys which run down to the coast of Catalonia, but also near the mouth of the Ebro, in Valencia and in Murcia (Senor Rico y Sinobus, Del Vidrio, p. 11).
Glass-making in Germany during the Roman period would seem to have been carried on extensively at Cologne, near which city many remarkable glass vessels of peculiar character have been discovered. The art was probably not lost during the period which followed the downfall of the Roman power. In the year 758 Cuthbert, abbot of Jarrow, wrote to Lullo, bishop of Mainz, to request him to send him. a maker of glass vessels. It is scarcely probable that the art had been forgotten and revived between the 5th and the 8th centuries.
It is not equally clear that glass was made in England, though it is probable that this was the case. Both vessels and window glass have been found in almost all parts of the country, and at Buckholt, near the Roman road from Winchester to Salisbury, the remains of a glass furnace, among which were numerous fragments of glass which may possibly have been of Roman origin, and a fragment of un-doubtedly Roman pottery. But associated with these were fragments of glass of the 14th and later dates, and of pottery of the 16th century. The dark Very little has been ascertained as to the practice of the aSes' art in either of the four countries during the dark ages, but it would seem to have been preserved in France and Germany, and perhaps also in England and Spain. The fact narrated by Bede, in his history of the abbots of Wearmouth, that Benedict Biscop about 675 procured workmen from France to make glass for his monastery, shows at once that it was preserved in France and lost or nearly so in England. But a great quantity of drinking vessels are found in the tombs of the Anglo-Saxons while still pagan ; and although the like are found both in France and in Germany, it is said that a greater number and more varieties occur in England, and it has thence been inferred that they were probably made in the country. Welsh poets of the 6th century (!), Aneurin and Llywarch Hen, both mention glass vessels by a name, "wydr," evidently derived from vitrum, and it is possible that the Welsh retained a knowledge of its manufacture. Some know-ledge of the art of manipulating glass existed in Ireland in and before the 12th century, as is shown by cameo heads and small pieces of mosaic glass of quite peculiar patterns which occur on objects of Irish workmanship (Transactions of the Royal Irish Academy, vol. xxiv., Antiq., part. iv.).
In France " vitrarii " are mentioned in several centuries from the 6 th to the 11th ; in Germany, as has been shown above, there is ground for believing that the art was prac-tised in the 8th century ; and in the 12th artisans are found at Cologne with the designation " ustor " attached to their names, which Merlo (Kunst und Kunstler in Koln, p. 563) suggests may probably mean maker of glass. Nothing seems to have been ascertained about the existence of the art in Spain between the Gothic conquest and the 13th century, when it was practised at Almeria by the Moors.
During the mediaeval period France produced large Mediaeval quantities of glass, as well in the form of vessels as in that period, of window glass. The first were made on a large scale in Dauphiné in the 14th century. In 1338 Humbert the dauphin granted a part of the forest of Chamborant to a glass-maker on condition that he should furnish him with more than 3000 vessels of glass annually (Hist. Dalph., ii. 363). In 1302 window glass was made at Bezu le Foret, in the department of the Eure, for the king of France ; a fragment of a roll of accounts for that year is preserved in the Bibliothèque Nationale. Glass was also made in Poitou, and a drageoir with the arms of Charles VIII. of France (1470-1498) has been engraved by M. Fillon (L'art de Terre, &c.) which is believed to have been made in that province.
Much glass was no doubt made for windows both in Germany and the Low Countries, during the Middle Ages, and in 1453 mention occurs of a fountain and four plateaux made for Philip the Good, duke of Burgundy, by a glass-maker of Lille; but if artistic objects were made, hardly any examples have been preserved. Glass-makers existed at Vienna in 1221 (Peligot, Le Verre, p. 342). In 1428 a Muranese artificer set up a furnace in the same city, and another was established by another Italian in 1486, which it is said was still at work in 1563. How far these efforts to rival the manufactures of Murano may have succeeded we have no information, but contemporaneously the native artificers continued to produce articles for common use, as we may see by the woodcut in the edition of George Agricola De Re Metallica, published at Basel in 1561, re-presenting the interior of a glass-house. In this the tall cylindrical drinking-glasses known as wiederkoms, bottles with big bellies and slender necks, and retorts may be seen. A glass-house was founded at Daubitz in Bohemia in 1442, others in 1504 (Peligot, Le Verre, p. 343).
In England vessels of glass seem to have been but little used during the Middle Ages ; they occur very rarely in inventories, and when they do, as in the Calendars of the Treasury of the Exchequer, they are usually described as mounted in gold or silver, or as painted, being probably enamelled glass vessels from Constantinople, Damascus, or Venice, objects rather of virtu than of daily use. It/has even been asserted that there is no evidence that any description of glass was made in England before the 15th century, but in the roll of the taxation made at Colchester in 1295 three of the principal inhabitants are designated "verrer"; and it would seem hardly probable that so many in such a town were glaziers only and not glassmakers. In the 14th century Andrew le glasswright is mentioned in the records of Great Yarmouth. In 1439 (or 1447) English glass is mentioned* in the contract for the windows of the Beauchamp chapel at Warwick, but disparagingly, as the contractor binds himself not to use it. In 1485, however, it is mentioned in such a manner as to lead to the conclusion that it was dearer and presumably better than either " Dutch, Venice, or Normandy glass "
(Domestic Architecture of the Middle Ages, by Hudson Turner, p. 78).
In Spain glass was made at Barcelona in considerable quantities in 1324; and Almeria, according to an Arab author of the 13th century, was famous for its manufacture of glass. In the 15th century the export of glass from Barcelona was considerable; Jeronimo Paulo in 1491 says that glass vessels of various sorts were sent thence to many places and even to Rome.
In the 16th century the fashion of using glass vessels of ornamental character spread from Italy into France, England, and the Low Countries. Henry VIII. had a large quantity, chiefly or wholly, it would seem, of Venetian manufacture (see inventory in 151-2, Archaeological Journal, vol. xviii.). This increasing use of glass led to the reflexion that large sums of money went annually to Venice for such wares, and to the question whether the manufacture might not be carried on at home. We therefore find that about the middle of this century attempts were made to introduce the Venetian methods of manufacture into the several countries; Henry II. of France established an Italian named Mutio at St Germain-en-Laye, and Henry IV. in 1598 per-mitted two "gentilshommes verriers " from Mantua to settle themselves at Rouen in order to make " verres de cristal et autres ouvrages qui se font a Venise. " A like attempt was made in England about 1550, for in that year eight glass-makers from Murano addressed from London a petition to the Council of Ten at Venice praying to be excused from the penalties decreed by that body in 1549 against Venetian subjects who taught the art to foreigners (Gal. State Papers, Venetian, No. 648). The council allowed the eight workmen to remain until the end of the term for which they had engaged themselves. Other attempts followed : Stow says (Chron., p. 1040) that Venice glasses were first made in London by one Jacob Vessaline about the beginning of the reign of Queen Elizabeth, and in 1565 one Cornelius de Lannoy (or Launoy) was working in the pay of the Government (Gal. Stale Papers, Dom.). Others, as Carre or Quarre, and Becku alias Dolin, from the Low Countries, were engaged in similar undertakings during the latter part of this century, but it does not seem that any great success was attained, as the importation of glass from Venice continued until long after.
These attempts to rival Murano seem to have been most successful in the Low Countries and in Spain. Ambrozio de Mongarda had a privilege in the former country to make " voirres de cristal a la faschon de Venise," which in 1599 was continued to Philippe de Gridolphi; his glass-house was at Antwerp. Glass-works also existed at Liege. Much glass in the 16th century was sent from Antwerp to England (Houdoy, Les Verreries a la facon de Venise). This manufacture went on during the following century, and many examples remain, more or less resembling the products of Murano.
In Germany Ferdinand I. established a glass-house at Weidlingen near Vienna, which was to work in the Italian manner ; but no great success it would seem attended these efforts, partly no doubt because about the same time the native glass-makers struck out a new and original style of ornamentation for the great cylindrical beakers, sometimes 20 inches in height, usually called wiederkom (come again), but which M. Peligot says ought to be called willkomm (welcome). This was a somewhat coarse but very effective system of painting in enamel such subjects as the emperor and electors of Germany, or the imperial eagle bearing on its wings the arms of the states which composed the empire, kc. The earliest example which has been met with bears the date 1553, but the system had great vogue, and con-tinued in use until about 1725.
Spanish writers have not as yet acquainted us with the precise means by which the Venetian methods of working were brought to their country; but Gaspar Baneiros in his Chronographia, published in 1562, says that the glass made at Barcelona was almost equal to that of Venice, and during this and the next century large quantities were exported. Venetian glass was imitated in several other places in Spain, and with considerable success, as several examples in the South Kensington Museum testify. The native forms and methods of working, however, went on contemporaneously and it would appear do so down to the present day.
The branch of glass-making in w"hich the greatest results 17th were obtained in France during the 17th century was that of century, the manufacture of mirrors. In 1665 the services of eighteen Venetians were obtained, and a factory established in the Faubourg St Antoine at Paris, and another factory was founded at Tour-la-Ville near Cherbourg. These were united and worked with great success ; the plates which ornament the " Galerie des Glaces" at Versailles were made at Tour-la-Ville. In 1688 the process of casting plates of glass was first adopted in modern times (for the window glass of Roman times was cast); and thus it became pos-sible to make mirrors of dimensions which could not be attained as long as the plates were produced by blowing. The manufacture was carried on at St Gobain, still the seat of an immense production of glass.
English glass-making of the 17th century is distinguished by one of the most important innovations in the practice of the art which has at any time been introduced, that of using a large proportion of oxide of lead in combination with potash. Glass so made is more brilliant than other kinds, and is known in England as " flint glass," in France as "cristal." The employment of lead as one of the ingredi-ents of glass was not a discovery, for it had been practised to a small extent and for certain purposes, as in the imi-tation of gems by the Romans, and through the Middle Ages. Neither the date when flint glass was first made nor the inventor of the process is known.
Sir William Slingsby before 1611 (Gal. State Papers, Dom.) had obtained a patent for making glass with sea coal; and in 1615 the use of wood for that purpose was forbidden by royal proclamation. How far this proclamation may have been obeyed does not appear, but Sir R. Mansel, who held a patent of monopoly for glass-making from 1616 until about 1634 (and perhaps even later), states in 1623 that furnaces erected in London, the isle of Purbeck, Milford Haven, and on the Trent, had all failed, but that- he had established them successfully at Newcastle-on-Tyne. Pro-bably coal was used at this last place, and it seems not unlikely that flint glass may have been first made there. Merret, however, writing about 1665 (in his edition of the Ars Vitraria of Neri), says that glass made with lead was not in use in the English glass-houses on account of its too great fragility; but in 1673 Evelyn notes in Irs diary a visit to the Italian glass-houses at Greenwich " where glass was blown of finer metal than that of Murano," and in 1677 a visit to the duke of Buckingham's glass-works (at Lambeth), "where they made huge vases of metal as clear, ponderous, and thick as crystal, also looking-glasses far larger and better than any that come from Venice." From this time much glass was made in England, and Dr Pococke, who travelled in Germany in 1736, gives the preference in point of quality to English glass over Bohemian.
During this century much art and labour were employed in Germany in the ornamentation of vessels for drinking, such as goblets and wiederkoms. Sometimes they were painted in grisaille, the subjects being battles, processions, and the like, sometimes engraved or etched; common sub-jects are escutcheons with arms, views of cities, ciphers, &c. Many excellent artists worked in these various styles (Beckmann, Hist, of Inventions, iii. 209 ; Doppelmayr, Nilmber_giscke Künstler, p. 231, 233, &c), and their works brought high prices. Dr Pococke mentions seeing some at Eispen, to which the glass-works formerly carried on at Potsdam had been removed, which cost from £100 to £150. Excellent material for these artistic efforts was furnished by the Bohemian furnaces; the art was patronized by several German princes; the celebrated Kunkel was in 1679 director of the glass-houses at Potsdam, which were carried on at the cost of the elector, and where the beautiful ruby glass was produced. Etching and engraving on glass was also much practised in Holland.
In Spain glass was made in 1680, at San Martin de Valdeiglesias, in imitation of Venetian; and Barcelona, Valdemaquada, and Villafranca are named in a royal schedule of the same date, fixing the prices at which glass was to be sold in Madrid, as places where wares imitative of Venetian were made. There was also an important manufactory at La Granja (see Introduction to Cat. of Spanish Glass in South Kensington Museum, by Sefior Riano). Some of the products of the Spanish furnaces closely resemble those of Murano, but rarely exhibit much beauty or much originality. Others again, attributed to the factory of San Ildefonso and to the 18th century, bear a very close resemblance to some of the Dutch glass of that period.
Although during the 18th and earlier part of the 19th century progress was made both in the purity and in the beauty of the material (especially in the case of glass for optical pur-poses), and in the organization and working of factories, it was a period marked in no country by much of novelty or of artistic effort in the manufacture of glass. M. Labarte even goes so far as to say (Hist, des Arts Industriels, iv. 597) that in France in 1759 the fabrication of "vases de verre" had so completely fallen into decadence that the Academy of Sciences offered a prize for an essay on the means by which the industry coula be revived. In the beginning of the present century cut glass was much in vogue, and was produced in England of great brilliancy, though the forms of the objcts often left much to be desired in point of elegance.
The manufacture of coloured glass for windows was a consequence of the revival of Pointed architecture, and England, France, Belgium, and Germany have in this century rivalled each other in its production.
The Exhibition of 1851 did not perhaps produce a more marked effect on any of the industrial arts than on that of glass. The progress made since that date in the fabrication of artistic glass wares (the verrerie de luxe of the French) has been surprising, and at the present moment enlightened and enterprising manufacturers of glass are in every country studying the products of the furnaces of all times and all countries, as their predecessors at Murano in that great period of the artthat of the Renaissancedid the relics of Roman glass-working (Biringuccio, Pirotechnia, lib. ii.) in order to glean from them lessons and suggestions of further advance in their art (A. NE.)
THE MANUFACTURE OF GLASS.
Glass, in its ordinary signification, is a brittle, transparent compound produced by the fusion, at a very high tempera-ture, of silica (silicic acid) with one or more basic substances, one of which, in all cases, must be an alkaline metal. But the silicatss of sodium and potassium, whether separate or combined, being soluble in water, and also readily acted on by other agents, are not in themselves suitable for most of the purposes to which glass is ordinarily applied. When, however, to these silicates, or to either of them, a silicate of an alkaline earth is added, the resulting body is not sensibly affected by water or ordinary solvents; and it is the fused amorphous mass thereby obtained that alone is glass in the restricted technical sense. Thus the definition given by Dumas that glass is a silicate of at least two metals belonging to different groups, one of which must be an alkaline metal, strictly embraces and limits all varieties of ordinary glass. Boracic acid, a substance closely allied in chemical properties to silica, has a similar influence on the alkalies and alkaline earths, producing by their mutual fusion a transparent amorphous compound ; and indeed, for certain special purposes, a glass in which borates to a certain extent supplant silicates is used for optical purposes. The substances, however, which form the essential basis of all varieties of common glass are (1) silica as the acid element; (2) soda or potash as the alkaline base; and (3) lime and oxide of lead as the alkaline earths. To the alkaline earths commercially employed there ought also to be added baryta and alumina, the former being used in the place of lead, and the latter being a common ingredient in certain kinds of glass.
The following tabular statement shows the bodies capable of yielding transparent glass :
== TABLE ==
Various authorities who have investigated the constitution of glass have endeavoured to establish a chemical formula for what they term normal glass. The results arrived at, however, by different investigators disagree among them-selves ; and the balance of opinion is in favour of the view that no such substance as normal glass exists, and that glass does not result from any definite chemical compounds, but is simply a mixture of silicates, with usually an excess of uncombined silica. The proportions in which the ingredients of glass are present, how-ever, have not only a very great influence on the fusibility of the mass ; but these conditions also very materially affect the qualities of the substance. In general the more nearly the proportion of silica approaches the amount necessary to form definite compounds with the basic ingredients, the better and the more stable is the quality of the glass. The conclusion of Otto Schott in his investigation of the constitution of glass, that the simplest formula for glass is represented by yCaO^ } 2^^2; may be accepted as a safe statement.
The phenomenon of devitrification, which is exhibited most readily by glass of inferior quality, has important bearings on the chemical constitution of glass, as wrell as on the working of the material. Devitrification is a change which may be induced in all varieties, but only with diffi-culty in the finer kinds of potash glasseither by slowly cooling the glass from the state of fusion, or by heating it in a mixture of sand and plaster of Paris till it softens, and then allowing it to cool by very slow degrees. Thereby it partly or entirely loses its transparent amorphous form, and by the formation of innumerable minute crystals it becomes opaque. When such a change penetrates the entire mass it assumes a milky and porcelain-like appear-ance, whence it is in this condition known as Reaumur's porcelainthe phenomenon having been first investigated by that observer. Devitrification renders the material much harder and less fusible than the same glass in a transparent condition; and it is less subject to fracture on the application of heat. According to Pelouze, Splittgerber, and others, devitrification results simply from a rearrangement of the molecules into the crystalline form while the mass is soft, no alteration in the proportions of the constituents taking place concurrently; but Dumas and his followers maintain, on the contrary, that in undergoing the change the glass loses part of its alkali, and that crystallization takes place through the formation of compounds infusible at the temperature existing at the moment of crystallization. These compounds may result either from the dissipation of alkaline matter or from the separation of the mass into two strata,that most highly alkaline retaining its amorphous condition more persistently than the other more siliceous portion.
The physical properties upon which the great value and utility of glass principally depend are. (1) its well-known prevailing transparency combined with a brilliant lustre and great hardness; (2) its fusibility at a high temperature; and (3) its softness and viscosity at a red heat, whereby it can be moulded and otherwise worked with facility into any shape desired. Of great value also is its resistance to the influence of common solvents. Properly-made glass is not sensibly acted on by any of the acids except hydro-fluoric acid, which attacks it powerfully, combining with and removing its silica. Water affects glass so feebly that for practical purposes its action may be disregarded; but when it is submitted for a prolonged period to water at a high temperature, it is slowly dissolved. Even prolonged exposure to moist air so acts on glass, particularly on highly alkaline varieties, that the surface becomes clouded and obscure, and the beautiful iridescent scaling off observable in ancient glass is due to the exposure of the substance for long ages to the influence of moist air or damp earth.
Glass is an extremely bad conductor of heat, and from that property springs, in great measure, its brittleness. Owing to this imperfect conductivity it necessarily results that a mass of glass, or a glass object cooling from a state of fusion, becomes cooled and set or solidified on its outer surface before the internal molecules have parted with their heat, contracted, and established themselves in a stable re-lation to each other. The solidification of the superficial stratum thus necessarily hinders the contraction of the internal portion of the mass, and as the internal molecules cool down a state of tension is created, the central portion tending to draw the surface stratum inwards with a force held in check by the strain in the contrary direction of the outer range of molecules. In this condition a very moderate impact is sufficient to determine the fracture of the glass, The high degree of brittleness which results from unequal cooling is exhibited in a very marked manner by the philo-sophical toy known as " Rupert's drops." Such pear-shaped masses of glass are prepared by allowing molten glass to fall, drop by drop, into cold water, when the drops assume a more or less spheroidal form, with a finely tapering point. Of course a very sudden and rapid cooling of the surface takes place, while the interior is still at a high temperature, and correspondingly much dilated, the consequence of which is that a state of great tension is established between surface and centre. The breaking off of a small portion of the tail is sufficient to destroy the equilibrium established between the hard superficial and the dilate internal molecules; and immediately the whole mass is shattered to dust with explosive violence. Excessive brittleness is overcome by the operation of annealing to which glass is submitted,a process which has been explained under ANNEALING, and which will be further referred to in deal-ing with a method of tempering or hardening glass which has been introduced by M. de la Bastie within the last few years.
It is difficult to give a consistent and systematic view of the manufacture of glass, because not only chemical constitution, but the mechanical operations by which glass is prepared, and the purposes to which the material is ap-plied, have also to be taken into consideration. A good classification, from a chemical point of view, is that given by Stein (in Bolley's Technologie), who distinguishes three classes:
(1.) Glass containing one or two bases belonging to the same group. This class embraces only the soluble silicates which do not fulfil the ordinary functions of glass.
(2.) Glass with several bases which belong to different groups, comprehending two classes:(1) calcium glass, i under which come sodium-calcium glass and potassium-calcium glass; and (2) lead glass, which constitutes ordinary flint glass or crystal and strass.
(3.) Coloured and opaque glass, of which there are also ! two classes,the first embracing the varieties of transparent coloured glass which may belong to any of the above classes, with the addition of colouring oxides, and the second being devoted to the various kinds of opaque glass.
The raw materials of the glass manufacture embrace the following principal ingredients. (1.) Silica is used in the forms of pure quartz (for very fine qualities of glass), crushed sandstone, pulverized flints, and especially sand of degrees j of purity varying in proportion to the quality of the glass to be made. The finest iron-free sand in the United King-dom is obtained from Alum Bay in the Isle of Wight, from Lynn, Norfolk, and from Leighton Buzzard, Bedfordshire; but much pure sand is imported into the United Kingdom from Fontainebleau in France, from Belgium, and other localities. (2.) Lime is employed in the form of chalk or marble, either burned or unslaked, and it also must for colourless glass be free from iron impurities. Of (3.) potash and (4.) soda any of the ordinary salts except chlorides, but especially the sulphates and carbonates, are indifferently utilized, the point of real importance being here also the free-dom of the compound from contamination when fine glass is being made. At no very remote date kelp was the principal source of soda alkali in glass, but this is now entirely dis-used, and the principal source of potash is the salt mines of Stassfurt and Leopoldshall in Prussia, and at Kalusz in Galicia. Both potash and soda are frequently constituents of the same glass; but glass made from potash is free from the decided sea-green tinge which invariably is seen in soda glass, although the latter is the more brilliant in lustre. (5.) Lead is the characteristic ingredient of a distinct class of glass of which ordinary flint glass is the type. It is usually employed in the form of minium or red lead (2PbO,Pb02), partly on account of its fine state of division and partly because by giving off oxygen it helps to purify the metal. (6.) Baryta and witherite or baric carbonate have been introduced with much success as a partial substitute for alkali in soda or potash glass, and for a part of the lead in ordinary flint glass, and in all probability barium compounds are destined to occupy a much more important place in glass manufactures than hitherto they have done. (7.) Cnllet or waste and broken fragments of the special kinds of glass to be made is an important and essential ingredient, being added to the extent of about one-third of the whole charge in the melting and preparation of glass. These materials constitute the essbatial ingredients which go to the formation of glass. In coarse varieties, such as bottle glass, alumina and iron are present, but their presence simply results from the inferior and impure nature of the raw materials employed, and are neither essential nor desirable. Some portion of alumina too is taken up from the pots in which the materials are melted. Bleaching or oxidizing agents are also employed to produce a high degree of colourlessness in clear glass, and for this purpose peroxide of manganese, arsenious acid, and nitrate of potash are the materials generally used. These bodies oxidize carbon compounds which may be present, and neutralize to a large extent the colour yielded by iron by converting its protoxide into peroxide. Too much manganese, however, gives the glass a reddish tinge, and excess of arsenic produces a milky cloudiness. The various substances employed to produce coloured and opaque varieties of glass will be enumerated when these special kinds are described. The requisite proportions of the raw materials ground and prepared are intimately mixed with the aid of a mixing apparatus, and in this form con-stitute the "batch." Formerly it was the habit to frit or partially decompose and fuse the ingredients in a form of reverberatory furnace called a calcar arch, but since the use of kelp was abandoned that operation is no longer essential, and generally the well-mixed batch is placed at once in the melting pots, or the tank in the case of tank furnaces.
Melting Pots.These pots or crucibles are made of the finest fire-clay, that from Stourbridge in Worcestershire being exclusively used for glass pots in Great Britain. Great care is requisite in the selection, and in cleansing the clay from extraneous particles, the presence of which, even in the smallest degree, will injure the pot. A fine powder procured by grinding old crucibles is generally mixed, in a proportion seldom larger than a fourth, with what is termed the virgin clay. This mixture dries more rapidly, contracts less while drying, and presents a firmer resistance to the action of the fire and alkali used in the composition of glass than the simple unmixed clay. These ingredients, having been mixed, are wrought into a paste in a large trough, and carried to the pot loft, covered in such a way as to ex-clude dust and other minute particles. Here a workman kneads the paste by trampling it with his naked feet, turning it from time to time until it becomes as tough as putty. It is then made into rolls, and wrought, layer upon layer, into a solid and compact body, every care being taken to keep it free of air cavities, which would, by their expansion in the furnace, cause an immediate rupture of the pots. After pots are made, very great care is necessary to bring them to the proper state of dryness before taking them to the annealing or pot arch. In drying they com-monly shrink about 2 inches in the circumference. When pots are made during summer, the natural temperature is sufficient for drying them ; but in winter they are kept in a temperature of from 60° to 70° Fahr. They remain in the room where they are made for a period varying from nine to twelve mouths. Being afterwards removed to another apartment, where the heat is from 80° to 90° Fahr., they are kept there for abou^ four weeks. They are then removed for four or five d:iys, more or less, according to their previous state of dryness, to the annealing arch, which is gradually and cautiously heated up till it reaches the temperature of the working furnace, whither, after being sufficiently annealed, they are carried as quickly as possible. Pots last upon an average from eight to ten weeks, and they form a costly item in the manufacturing operations, as each pot is worth on an average about £10; and many of them, notwithstanding all care, crack and give way as soon as they are placed in the melting furnace. For all varieties of glass, excepting lead glass, open pots in the form of a truncated cone, as represented in fig. 1, are em-ployed ; but for flint glass a covered pot with an opening at the side, as shown in fig. 2, is essential. Dr Siemens proposed a form of melting pot divided into three compart-ments, the materials being melted in the first, and passing into the second by an opening at the lower part of the partition, where the metal was to be fined and freed from included air-bubbles, and afterwards to pass by a like
opening to the third compartment, whence it was to be drawn for working. The specific gravity of the charge in the first compartment would rise in proportion as the materials melted and became homogeneous in structure. Therefore the metal would sink in proportion as it melted ;
FIG. 2.Flint-Glass Pot.
and the best melted portions pass into the second compartment, in which, under the influence of the direct fur-nace heat, it would be cleared. There, similarly, the per-fectly fined glass falling to the bottom would pass into the cooler working compartment, which is protected by a cover-ing cap. Dr Siemens's idea has been practically developed in his continuous tank referred to below,
Furnaces.A glass-melting furnace or oven is a modified form of reverberatory furnace, which assumes many different shapes and arrangements according to the kind of glass to the manufacture of which it is devoted, and the nature of the fuel used. As regards the latter cause of difference it may be noted that, while coal is the principal fuel em-ployed in Great Britain, dried wood and peat are extensively consumed in Germany, and in modern times gas furnaces on the Siemens and other principles are being freely intro-duced. In the construction of a furnace the principal objects to be kept in view are not only the production and maintenance of an intense heat, but its uniform distribution throughout the furnace, and the bringing of the charges of glass materials directly under its fusiug influence. The form assumed by melting furnaces is, in general, square or oblong for sheet and plate-glass making, and circular in English flint-glass making. The fire-space or grate occupies the centre of the furnace, and the fire, when fuel is used for direct heating, is either fed or stoked from both ends, or raised from under the bars by a patented method. The fire-grate is usually on a level with the floor of the house in which it is erected, but under it ii an arched subterranean passage forming the " cave " or ash-pit, both ends of which extend to the open air outside the glass-house. The fire-grate bars are placed in the top of this arched passage, which thus serves as a canal for the atmospheric air required to main-tain combustion within the furnace ; and for regulating the admission of air, and so controlling the heat, there are doors at both ends of the archway. In some cases two such arched passages at right angles to each other, and intersect-ing at the fire-bars, are constructed, so that either can be used according to the prevailing direction of the wind, &c. In general no flue or chimney is directly connected with the furnace, the only exit for the products of combustion beiug the working holes, and thus the heat is directed around and over each pot placed opposite a working hole in the furnace. Within the furnace, around the grate space in the case of circular furnaces, or on both sides of it in quadrangular furnaces, is a raised bank or narrow platform termed the " siege," on which the melting pots are placed. The number of pots arranged in a furnace vary from four to ten, and each is reached, either for charging or for work-ing off the prepared metal, by means of " working holes " in the side of the furnace situated directly over the pots. The general form and construction of a six-pot crown-glass furnace, which also may be taken as the type of sheet and plate-glass furnaces, is shown in Plate V., where fig. 3 is a ground plan at the level of the siege of a common form of furnace, while in fig. 1 is seen a front elevation of the same furnace, 1, 2, and 3 being the working holes, 4, 5, 6, and 7 pipe-holes for heating the blowing pipes, and 8, 9, and 10 foot-holes for mending the pots and sieges. The furnace are large
FIG. 3.Section of Flint-G-lass Furnace, is covered with a low-roofed crown or dome, and the whole structure is bound together with a system of iron bars. The materials used in the construction and lining of all furnaces must be selected with the utmost care, and built with special regard to the enormous temperature to which they are subjected. Formerly a fine-grained purely siliceous sandstone was much used, but now the principal materials
cone or chimney. Of course in cases where such separate small chimneys are provided no second or outer vault is required.
In the year 1861 Dr C. W. Siemens introduced a form of furnace in which the use of melting pots was altogether abandoned, and the batch was introduced into, melted in, and worked from a tank which occupied the whole bed of the furnace. This furnace he heated from the sides by means of his well-known regenerative gas system described under moulded bricks or blocks of fire-clay of the most infusible and refractory description. For the crown of the furnaces used in plate-glass melting Dinas silica blocks are employed. In laying the blocks and throwing the arches no mixture containing lime can be used, but only fire-clay or Dinas sand, in as small quantity as possible. Should any of the materials of the crown of the furnace gradually fuse under the influence of the heat, the dropping of the molten matter into the glass-pots is the cause of most serious annoyance and loss to the manufacturer.
An English flint-glass furnace furnishes the type of cir-cular furnaces. Usually a large number of pots, sometimes ten, are provided for in such a furnace, because, the objects made in flint glass being in general of small size, the metal is worked off only slowly, and a large number of glass-blowers can be accommodated at the separate work-holes. The arrangements of the cave and fire-grate are the same as in the case of square or oblong furnaces, but flint-glass furnaces differ from the prevailing rule in others by being provided with a system of flues and chimneys, one flue being placed between each pair of pots. The general appearance presented within a flint-glass house is illustrated in Plate Plate VI. fig. 1; and the accompanying woodcut (fig. 3) is a sec- vl-tional illustration showing the construction and internal appearance of a seven-pot furnace. The furnace is com-posed of a double arch or vault springing from strong pillars or abutments bb. The space c, between the outer arch and the vault proper of the furnace d, is a common receptacle for the flues ff led from within the furnace, and the products of combustion escape by the chimney i. The work-holes are at h, and at that place the furnace wall is taken down when a pot requires to be removed and renewed. The "cave" or air canal is seen at k; n is the fire-grate, stoked in this case from one side only ; I shows openings at which the blowing tubes are heated; in is an opening for cleaning the flues; and a is the bank or siege with the position of the pots indicated. Fre-quently instead of being arched the outer portion of the furnace is carried up in the form of a wide truncated cone or open chimney stalk, and in other cases short separate chimney stalks are built for each flue terminating within the glass-house, which itself forms such an open-topped
FURNACE and IRON. In 1872 he effected a further development of the tank furnace by dividing the tank, on the principle of his melting pot, by means of two floating bridges or partitions into three compartments, and thus he elaborated what is termed Siemens's patent con-tinuous melting furnace. Of this improved furnace fig. 4 shows a longitudinal section, and fig. 5 is a transverse section. A is the melting compartment, B the refining compartment, and C the working-out compartment. The compartment A is fed with raw material (or batch) through the door D at the back end of the furnace; it is separated from the compartment B by the floating bridge E, under which the partially melted glass passes to reach the latter. In the compartment B the metal, by the influence of the higher temperature maintained on its surface, is com pletely purified, and sinks to flow under the bridge F in a complete workable condition. Suitable provision is made, by means of air-passages, to keep the sides of the tank of the requisite temperature to prevent any egress of glass through them, and the floating bridges E and F are renewed as often as necessary. The regenerative gas furnace is em-ployed, and the gas and air ports H H', leading from the regenerators I F respectively, are arranged along each side of the tank, so as to cause the flames to play across the furnace. The temperature of the different parts is regulated according to the various stages of preparation of the glass in the several compartments, this regulation being effected by constructing the gas and air ports of larger dimensions, or increasing their number, where the greatest heat is re-quired ; it is also facilitated by means of division walls (not shown in the figures) which may be built over the floating bridges to separate the compartments. The temperature of the working-out compartment C is controlled by regulat-ing the draught of the furnace chimney, by diminishing which more or less flame must necessarily pass from B over the floating bridge F into C, and through the working holes M. The principal advantages resulting from the use of the continuous melting furnace are the following:
1. An increased power of production, as the full melting heat may be employed without interruption, whilst with the old method of melting nearly half time is lost by cooling and settling the metal, the working it out, and the re-heating of the furnace.
2. An economy in working, as only half the number of men are required for the melting operations.
3. A greater durability of the tank and furnace, owing to the uniform temperature to which they are subjected.
4. A much greater regularity of working, and more uniform quality of the product than in other furnaces.
5. For the manufacture of window glass, the compartment C may be so arranged that the blowers can work without interfering with the gatherers; this would do away with the separate blowing fur-nace now in use.
Although the Siemens regenerative gas firing is generally used with tank furnaces, that system is not essential to the successful working of tanks. Mr Archibald Stevenson of Glasgow has patented a tank furnace fired by common coal from one end, with working holes on the other three sides, and furnaces on this principle are worked in a perfectly satisfactory manner with much economy of coal and working room. Tank furnaces are used principally in bottle works and in the manufacture of rolled plate. The following statement shows the extent to which Siemens furnaces and tanks have been introduced by manufacturers.
== TABLE ==
Formerly it was the habit that to the sides or wings of the main melting furnace there were attached calcar or fritting arches, annealing ovens, pot-firing arches, and other subsidiary furnaces required in certain stages of glass manufacture. These were heated by flues leading from the fire-space of the principal furnace; but such a practice is now generally abandoned, and distinct furnaces or ovens, arranged and fired according to the necessities of the case, are provided instead. In the manufacture of common bottle glass, however, for which highly impure materials are used, it is still the practice to prepare a frit in a side arch occasionally attached to the melting furnace.
The wdiole of the pots in a common furnace are charged or filled with the prepared " batch " at the same time. Immediately the heat is forced, and the stoking must thereafter be carefully regulated to maintain the high temperature. As the mass begins to fuse it settles down and occupies considerably less space in the pot, to which thereon a second quantity of material is added, and generally a third portion is subsequently filled in so as to have at the close of the melting process as large a quantity of metal as possible. When the fusion is complete a scum composed of uncombined salts, and known as glass gall or sandiver, rises and collects on the surface. It consists almost entirely of sulphate of soda, with sulphate of lime, and a small percentage of chloride of sodium. This scum of glass-gall is carefully removed with a perforated scoop, and the heat of the furnace is then forced to the most intense degree with the view of rendering the metal as fluid and limpid as, possible, so as to free it from all included gaseous bubbles which it still contains. This process of " fining," " refining," or hot-stoking, as it is indifferently termed, involves a temperature which is esti-mated in certain cases to reach from 10,000° to 12,000° Fahr.; and the operation is sometimes assisted by stirring the molten mass with a pole of wood, in a manner analogous to the poling of copper in the refining of that metal. Throughout the operation of melting, test pieces are periodi-cally withdrawn from the pots for the purpose of observing the progress and condition of the glass. When it is found that the vitrification is complete and the object of refining fully accomplished, the heat of the furnace is considerably reduced, so that the glass may be brought into that condition of viscosity in which it is capable of being worked, Tn contradistinction to the refining or hot-stoking period, this is known as cold-stoking.
Glass Working.The means by which melted glass is caused to assume its varied forms for use are (I) by blow-ing ; (2) by casting; and (3) by pressing in mouldsan operation in which the other two processes may be partly combined. Minor manipulative processes which do not fall under any of these heads are called into action; but these are for the most part merely subsidiary to the others, which really comprehend all the lines along which the formation of glass proceeds.
Having regard principally to the forms into which glass is worked and the uses to which it may be applied, the following classification embraces the principal departments of the glass-making industry,
I. Flat glass.
Crown glass. Sheet glass. Plate glass.
II. Hollow glass.
Flint glass, blown. Bohemian glass. Venetian glass. Bottle glass. Slag glass (Britten's). Tube and gauge glass.
III. Pressed and massive glass. Flint glass. Optical glass. Ptrass.
Rod glass, marbles, and beads. IV Coloured, opaque, and enamel glass, including glass mosaics and hot cast porcelain, &c
So far as they involve distinct manufacturing processes, these varieties of glass will be here noticed in the above order. Such of the divisions as result from the application of special methods of ornamentation, and as come under the head of art glass, do not fall within the scope of this article.
CROWN GLASS.This, with sheet or cylinder glass, forms all ordinary blown window glass. Both varieties are precisely the same in composition, being a mixture of sodic and calcic silicates, and differ only in the manner in which the sheets of finished glass are produced. The raw materials employed for this and all other kinds of glass vary within rather wide limits, and, as already explained, the form in which the sodic and calcic compounds are used may also be varied. The following composition of batch for window glass must therefore be regarded as only one out of very many mixtures in use :
Sand, purified 100 parts.
Chalk, or limestone 35 to 40 ,,
Sulphate of soda 40 to 45
Cutlet 50 to 150
To these materials a minute proportion of white arsenic and peroxide of manganese, as bleaching agents, may also be added.
Crown glass was, in the early part of the present century, the only form of window glass made in Great Britain, and consequently it was generally recognized as English window glass, having been manufactured only on a very limited scale in any other country. Since the introduction of sheet-glass making, the crown-glass industry has steadily declined, and now its manufacture may be regarded as practically a thing of the past, not more than one or two crown furnaces being in operation. Seeing that it possesses little more than an historical interest, it is now unnecessary to enter into much detail as to the processes employed in the manufacture of crown glass.
The metal being brought to a proper condition for working, the "gatherer" dips into the pot of metal an iron pipe or tnbe, 6 or 7 feet in length, of the shape shown in fig. 6, heated at that end which takes up the glass, and, by turning it gently round, gathers about l^lh
FIG. 6.Blowing Tube.
of liquid glass on the end of it. Having allowed this to cool for a little, he again dips the rod into the pot, and gathers an additional quantity of from 2 J to 3 lb. This is also permitted to cool as before, when the operation of dipping is again repeated, and a sufficient quantity of metal, from 9 to 10 fb weight, is "gathered," to form what is technically called a table or sheet of glass. The rod, thus loaded, is held for a few seconds in a, perpendicular position, that the metal may distri-bute itself equally on all sides, and that it may, by its own weight, be lengthened out beyond the rod. The operator then moulds the metal into a regular form, by rolling it on a smooth iron plate, called the " marver," a term corrupted from the French word marbre. He then blows strongly through the tube, and thus causes the red-hot mass of glass to swell out into a hollow pear-shaped vessel. The tube with the elongated sphere of glass at the end of it is then handed to the '' blower," who heats it a second and third time at the furnace, press-ing the end, between each blowing, against the bullion bar, so called from the part thus pressed forming the centre of the sheet or " bull's eye," and by the dexterous management of this operation, the glass is brought into a somewhat spherical form. The blower now heats a third time at the "bottominghole," and blows the metal into a full-sized flattened spheroid. When this part of the process has been completed, and the glass has been allowed to cool a little, it is rested on the " casher box," and an iron rod, called a " pontil" or punty rod, on which a little hot metal has been previously gathered, is applied to the flattened side, exactly opposite the tnbe, which is de-tached by touching it with a piece of iron, dipped beforehand in cold water, leaving a circular hole in the glass of about 2 inches diameter. Taking hold of the punty rod, the workman presents the glass to another part of the furnace called the " nose hole," where the aper-ture made by its separation from the tube is now presented and kept until it has become sufficiently ductile to fit it for the operation of the flashing furnace. Whilst here, it is turned dexterously round, slowly at first, and afterwards with increasing rapidity; and the glass yielding to the centrifugal force, the aperture just mentioned becomes enlarged. The workman, taking great care to preserve, by a regu-lar motion, the circular figure of the glass, proceeds to whirl it round with increasing velocity, until the aperture suddenly flies open with a loud ruffling noise, which has been aptly compared to the unfurl-ing of a flag in a strong breeze ; and the glass becomes a circular plane or sheet, of 4J feet diameter, of equal thickness through-out, except at the point called the bullion or bull's eye, where it is attached to the iron rod. The sheet of glass, now fully ex-panded, is moved round with a moderate velocity until it is sufficiently cool to retain its form. It is carried to the mouth of the kiln or annealing, arch, where it is rested on a bed of sand and de-tached from the punty rod by a shears. The sheet or table is then lifted on a wide pronged fork, caUed a faucet, and put into the arch to be tempered, where it is ranged with many others set up edgewise, and supported by iron frames to prevent their bending. From 400 to o'OO tables are placed in one kiln. A sketch of the interior of a crown-glass house, during the progress of these operations, has been given in Plate VI., fig 2. The kiln having been clayed up, the fire is permitted to die out, and the heat diminished as gradually as possible. When the glass is properly annealed, and sufficiently cold to admit of its being handled, it is withdrawn from the oven after the removal of the wall built into the front of the arch, and is then quite ready for use. The largest sized tables of crown glass made will cut into slabs 30 inches across, from which squared pieces measuring 38 by 24 or 35 by 25 inches may be obtained.
SHEET GLASS, as already mentioned, is the same in composition as crown glass, which it has now entirely supplanted. The success of sheet glass is due principally to the fact that it can be produced in sheets of much greater dimensions than is possible in the case of crown glass ; it is free from the sharp distorting striae and waves common in crown class ; there is no loss of glass as there is with the bull's eye of crown ; and modern improvements effected in the manufacturing process leave little distinction in brilliancy of surface between the two qualities. Sheet glass is made on the greatest scale in Austria, Germany, and Belgium, and it was long distinguished in the British market as German sheet glass. In 1832 Chance of Birmingham and subsequently Hartley & Co. of Sunderland introduced the manufacture into England, and in the hands of these firms, as well as of others who followed in their footsteps, the industry prospered and developed, till it has now attained dimensions equal to those it has reached in most of the Continental nations, where the art was long established before it came into use in England.
Sheet-glass making involves two principal operations,(1) the blowing of the cylinder, and (2) the opening, Rattening, or spread-ing of the glass. The structure and internal arrangements of the melting furnace is practically the same as in the case of crown glass. The ordinary type of oblong furnace usually contains 10 pots5 in each side of the fire-grateeach pot being of a capacity of about 1 ton or 22 cwt. of metal. Radiating from the work-holes, and raised about 7 feet above the floor level, or a correspondingly deep sunk pit, are ten long stages with an open space between each sufficient to allow the workman to swing about his long tube freely in forming the elongated cylinder of glass. Fig. 7 is a ground plan of a common sheet-glass furnace and staging of planks c, at the extremities d of which are placed a tub of water and a wooden moulding-block. In-stead, however, of having these stages erected in front of the melting furnace, it is now a common practice to gather and block the glass at the melting furnace, and to blow it in front of a separate ij oblong reheating or blowing | furnace, from each opening J of which the wooden stage lj runs out over a pit excavated r to the depth of 7 feet or \ thereby. Common bricks may I be used for the construction of this reheating furnace, as the heat required in it is by no means intense.
Blowing.The charge or batch requires about 16 hours to melt, and other 8 hours are consumed in cooling it to FlQ_ 7._Plan of sheet-Glass Furnace, the working consistency.
When the metal is ready for working, the workmen take their stations, each having his own pot and stage and also an ant, and commence making the cylinders. After gathering the quantity of metal required (which on an average amounts to 20 tb), the workman places it in a horizontal position in the large hollow of a wooden block (fig. 8), which has been hollowed so that, when the workman turns the metal, it sball form it into a solid cylindrical mass. In the meantime, the assistant, with a sponge in his hand, and a bucket of water by his side, lets a fine stream of water run into the block, which keeps the wood from burning, and also gives a brilliancy to the surface of the glass. The water, the moment it comes in contact with the glass, is raised to the boiling point, and in that state does no injury to the metal ; but it is only when the metal is at a high temperature that such is the case; for, whenever the glass is cooled to a certain degree, it immediately cracks upon coming in contact with water. When the workman perceives that the mass of metal is sufficiently formed and cooled (fig. 9), he raises the pipe to his mouth at an angle of about 75 degrees, and commences blowing it, at the same time continuing to turn it in the wood block, till he perceives the diameter to be of the requisite dimen-sions (fig. 10), which are usually from 11 to 16 inches. The workman then reheats this cylindrical mass, and, when it is sufficiently softened, commences swinging it over his head, continuing to reheat and swing till he has made it the desired length, which is commonly about 45 inches. It is now a cylinder of say 45 inches long by 12 inches in diameter, one end being closed, and the other having the pipe attached to it. The workman begins to open the end which is closed, for which purpose he encloses the air in the cylinder, by stopping the aperture of the pipe with his finger ; and then placing the close end of the cylinder towards the fire, it becomes softened, while at the same time the air within is expanding, and, in about thirty seconds, the softened glass at the extremity of the cylinder gives way, forming an aperture as in fig. 12. The workman then turns the cylinder round very quickly, and, by keeping the opened extremity warm at the same time flashes it out perfectly straight as at g, fig. 13. If the burst edges are ragged in appearance they are trimmed by cutting with a pair of scissors before finally expanding. The bursting of the end of a thick heavy cylinder has to be determined by allowing a glowing drop of glass to fall on the spot to be burst before presenting it to the fire.
The other end, which is attached to the pipe, has now to be cutoff, and is done in the following manner. The workman, having gathered a small quantity of metal on the pontil, draws it out into a thread of about one-eighth of an inch in diameter, laps it round the pipe end of the cylinder, and, after letting it remain there for about five seconds, withdraws it suddenly, and immediately applies a cold iron to the heated part, which occasions such a sudden contraction, that it cracks off where the hot string of glass has been placed round it. Another method is to draw a semi-cylindrical rod of iron e (fig. 13) heated to redness around the line d of desired fracture, and a drop of water then allowed to fall on the line so drawn determines its fracture. The stages in the development of a cylinder to its full length are illustrated in figs. 9 to 13.
Flattening.The cylinder so blown and detached is now allowed to cool; and, previous to its flattening, the burst extremity being thinner than the remainder, and slightly contracted at its edge, has to be removed to the depth of about 2 inches. For this purpose the cylinder is placed vertically in the jaws of a cutting instrument, having a diamond cutter, pressing by a spring, inside the glass (fig. 14). The cutter moves by small wheels on the table on which it is placed, and being pushed around the cylinder it makes an accurate cut of uniform height. The cylinder has then to be split longitudinally to allow it to be opened out to a flat sheet. To ac-complish this the practice was formerly to lay the
tening kiln, which consists of two chambers built together, the one for flattening the cylinders, the other for annealing the sheets, the former being kept at a much higher temperature than the latter.
shows a section of a flattening (L) and annealing kiln (M) in common use. The split cylinder 0 is introduced and gradually pushed for-ward so as to beuniformly heated till it reaches P, the flattening stone or table, mounted on a movable waggon N. On this waggon after it has been flattened it is carried into the annealing arch M, as shown by the dotted outline. Here in a less heat it gradually stiffens, till it is ready to be moved by a forked tool to a horizontal position on the bed of the annealing oven. The waggon then goes back to the flattening arch, and when it is reintroduced with another sheet, that previously flattened is ready to be piled up on edge at R, and thus the work proceeds till the annealing oven is filled, when it is closed up and allowed to cool down by slow degrees. Chance Brothers & Co. of Birmingham are the introducers of a system of continu-ous flattening and annealing furnaces. This they accomplish by means of two contiguous circular kilns having revolving soles, the
The cylinder, after being gradually reheated, is placed in the centre of the flattening oven, upon a smooth stone, with the split side upwards. In a short time it becomes softened with the heat, and
cylinder horizon-tally on a bench, and draw a red-hot iron two or three times along the inner surface at the line of desired fracture. Now the splitting is done with a diamond cutter fixed in the cleft of a stick and (fig. 15) guided from end to end of the cylinder by a straight-edge K laid within it. The cylinder is now ready tr be taken to the flat-by its own weight falls out into a flat square sheet of 45 inches by 36. The flattener, with a piece of charred wood, rubs it quite smooth, and then places it on edge in the annealing arch, where it remains about three days to be annealed.
shows a section of a flattening (L) and annealing kiln (M) in common use. The split cylinder 0 is introduced and gradually pushed for-ward so as to beuniformly heated till it reaches P, the flattening stone or table, mounted on a movable waggon N. On this waggon after it has been flattened it is carried into the annealing arch M, as shown by the dotted outline. Here in a less heat it gradually stiffens, till it is ready to be moved by a forked tool to a horizontal position on the bed of the annealing oven. The waggon then goes back to the flattening arch, and when it is reintroduced with another sheet, that previously flattened is ready to be piled up on edge at R, and thus the work proceeds till the annealing oven is filled, when it is closed up and allowed to cool down by slow degrees. Chance Brothers & Co. of Birmingham are the introducers of a system of continu-ous flattening and annealing furnaces. This they accomplish by means of two contiguous circular kilns having revolving soles, the
In the arrangement of the flattening and annealing ovens numerous improvements have been effected, which have resulted in greatly increased smoothness and uniformity of the glass, and in considerable economy of time and labour in the operations. Fig. 16
size. In these troughs the emery powder is deposited in increasingly fine division, according to the growing capacity of the trough and the consequent slow replacement of its contents. The last touches of the smoothing process can only be given by the hand, which at once detects any appearance of grittiness. Both sides of the glass are in succession submitted to these operations, after which it is again bedded in plaster and fixed on the polishing table. The polishing is done with reciprocating rubbers, covered with fine, felt, and supplied with rouge (peroxide of iron) in a liquid state. While a reciprocating motion is communicated to the rubbers, the table itself moves backward and forward in a transverse direction, so that all parts of the plate are equally brought under the polishing influence of the rubbers. About 40 per cent, of the weight of the rough plate is removed in the three polishing operations.
Ordinary finished plates vary in thickness from about £th to fth inch, and the largest sizes measure about 17 feet by 9 feet 6 inches. The great St Gobain Company of France quotes regular prices up to 324c. (10 feet 8 inches) by 204c. (6 feet 8 inches), beyond which size the price becomes special. In the Paris Exhibition of 1878 that company showed a silvered plate 24 feet by 14 feet, the largest piece of plate glass which has hitherto been polished.
Rolled Plate.A form in which unpolished plate glass is extensively employed is the patent rolled plate, originally made by Hartley & Co. of Sunderland. The surface of the casting table on which the rolled plate is spread, instead of being smooth and plain, is engraved or otherwise indented with fine lines, grooves, or flutes, or it may be with small squares, lozenges, or even ornamental patterns, and the glass, of course, takes on its lower surface an accurate impression of any such pattern. Boiled plate is now very largely used for partitions, and in places where obscure lights are required. Such plates are always cast comparatively thin, and of moderate size, so that a large number may be piled in the annealing oven. They, as well as certain qualities of coloured glass, are cast by ladling the molten metal from huge pots which may contain about 2J tons of material. By this ladling numerous " air bells " are enclosed in the glass, but the circumstance does not affect the durability and usefulness of the glass.
FLINT GLASS OR CRYSTAL. The name flint glass originated in the circumstance that at first the silica used in the manufacture of this variety of glass was in the form of ground flints. The industry belongs characteristically to the United Kingdom, where it was first established on a large scale, and to the present day flint glass is much more extensively manufactured in England than in any other country.
Flint glass is a compound entirely different from those above described, consisting as it does of a silicate of potassium and lead. As is the ease with all kinds of glass, its composition and the pro-portion of ingredients used in its preparation vary widely. The average composition of a batch has been stated thus:
Fine white sand 100 parts.
Minium (red lead) 66
Refined potash 33
Cullet CO to 100
Small portions of bleaching ingredients, as white arsenic or manganese, are also sometimes added. Purity of materials is of the utmost consequence 'n the manufacture of flint glass, which is prized in proportion to the transparency, uniformity, sparkle, and freedom from colour of the metal; and, as finished objects are in many cases massive, defects of colour are very obvious. Flint glass is much more fusible than the kinds destitute of lead; it possesses great brilliancy, owing to its high refractive and dispersive power, but being comparatively soft its surface is easily scratched and dulled. It is also slowly corroded by alkalies, and contact with solutions of sulphides blacken it. Its specific gravity varies according to the proportion of lead it contains ; and, as the silicate of lead tends to sink when the glass is in the molten state, Faraday found glass from one melting to vary from 3'28 to 3'85, and in another instance from 3-81 to 4'75. The higher the specific gravity of the glass the greater is its refractive power and consequent brilliancy.
Flint glass is in no case used for architectural or structural purposes, but its purity and lustre peculiarly fit it for table glass, ornamental objects, glass globes and lustres, and for imitations of gems and precious stones. For the latter purpose a dense glass called strass, appropriately coloured when necessary, is employed, and a glass of still greater density and refractive power is used for optical puposes. The softness of flint glass adapts it for engraving, cutting, and polishing ; and these methods of ornamenting the finished glass are very much employed.
The special covered form of pot (fig. 2, supra) and the arrangement of the furnace have already been alluded to. Plate VI., fig. 1, represents the various arrangements, tools, and processes connected with a flint-glass house, the building in the centre being the large cone or chimney built over the furnace, which is seen through the arches a. At 5 and 6 men are seen at the working holes with-drawing metal from the pots on their long iron tubes ; 7 is the marver on which the gathering is rolled till it acquires a circular uluvpe ; at 8 a blower is seen in process of expanding a gathering of glass by blowing ; and at 9 a servitor or second man is attaching a post or lump of metal he has gathered on a pontil or punty to the end. of a blown globe of glass. The two masses of glass are thus united together, and that attached to the hollow tube is separated by touching it, near to where the tube enters the globe, with a small piece of iron wetted with water. By this means the glass cracks, and a smart blow on the iron tube completes the disunion. The workman now takes the punty from his assistant, and laying it on his chair arm, rolls it backward and forward with his left arm, while with his right he moulds it into the various shapes required, by means of a very few simple instru-ments. By one of these, called a pucellas, the blades of which are attached by an elastic bow like a pair of sugar-tongs, the dimen-sions of the vessel can be enlarged or contracted at pleasure. Any superfluous matter is cut away by a pair of scissors. For smoothing and equalizing the sides of the vessel a piece of wood is used. After the article is finished it is detached from the punty and carried on a pronged stick to the annealing oven.
The annealing oven or leer for flint glass is a low arched furnace, generally of considerable length, with several openings at each end between wdiich narrow lines of rails run. On these rails, small waggons, or trays mounted on four wheels, are placed,and the articles to be annealed are filled into such waggons. They are slowly pushed to the hottest part of the chamber, and passing that point they very gradually approach the cold end of the oven, from which they are withdrawn fully annealed. As each waggon is withdrawn at one end, another is entered at the other so that the line from end to end is kept constantly full. The ordinary method of gradually decreas-ing the temperature around the articles stationary in the leer is also practised.
Flint-Glass Cutting, Engraving, and Etching.The sparkle and brilliancy of flint glass is developed by the process of grinding and polishing technically called glass-cutting. In fig. 18 is seen a representation of a glass-cutter's mill, a being the pulley and band communicating motion to the mill 6, which is made of wrought or cast iron. Over it is suspended a wooden trough or cistern c, containing a mixture of sand and water, which is fed on the wheel as required for the operation of grinding. Smoothing is done on a wheel of fine sandstone to which water alone is applied, and for polishing, a wooden wheel supplied with emery, and finally with putty powder (oxide of tin), is employed. The trough d under the wheel receives the detritus of the grinding and other operations. The articles are held in the hand, and applied to the mill while rotating. The punty marks are ground off tumblers, wine-glasses, and the like, by boys holding them on small stone mills. Ground or obscured glass is made by rubbing the surface with sand and water. Iron tools fixed on a lathe and moistened with sand and water are used to rough out the stoppers and necks of bottles, which are completed by hand with emery and water. Engraving is the production of ornamental surfaces by a fine kind of grinding mostly done with copper discs revolving in a lathe. Etching is variously' done by submitting the portions to be etched or bitten to the influence of hydrofluoric acid, the remainder of the glass being stopped off or protected by a coating of wax or some pitchy compound.
PRESSED GLASS.The most brilliant effect is produced by cutting, but moulding or pressing is much cheaper, and this branch of the art has now reached a high state of excellence. Glass formed by pressing in moulds, known distinctively as pressed glass, is peculiarly an English industry, principally because it is only flint glass, or glass possessed of similar properties, that can with advantage be formed in that way. Although moulded glass has ex-isted from early Roman times, it is only within the present century that the modern industry has been developed, and of late years the trade has assumed important dimensions. A metal that melts at a comparatively moderate heat, and does not quickly pass from the plastic state, is essential for success in pressed glass making, because it has not only accurately to fill all the intricacies of the mould, but it must also be susceptible of fire-polishing. This operation consists of a reheating sufficient to melt a thin superficial stratum of the glass, whereby the roughness and obscurity of surface incidental to moulding is removed, and a smooth brilliant effect brought out, inferior only to the sparkling appearance produced by cutting. The moulds for pressed glass are made of iron or bronze ; with great accuracy of surface ; and they are, in use, kept a little under a red heat. The various segments of the mould are so hinged or connected as to close and leave internally a space representing the form and size of the article to be made, the internal hollow not being pro-duced by blowdng but by the plunger of the press under which the mould is placed. The required quantity of metal being dropped into the mould, the plunger descends and forces it into all parts of the cavity, completing immediately the formation of the article, which is then fire-polished by reheating, and afterwards annealed. In this way glass with elaborate facets, bosses, flutings, or other bold orna-ments can be produced with rapidity and ease ; and the only bar to great cheapness is the heavy cost of the lead and potash in flint glass. Several manufacturers both in England and on the Con-tinent, where the pressed glass industry is extending, now partially supply the place of these costly materials by lime and baryta ; and indeed English pressed glass of excellent quality is now in the market containing neither lead nor potash to any appreciable extent.
BARYTA GLASS.The high price of red lead, and various disadvantages connected with its use, have given rise to many efforts to find an efficient substitute for it in the manufacture of table and ornamental glass. Barium compounds, principally the native sul-phate (common baryta or heavy spar) and the artificially prepared carbonate, have been more or less experimentally tried ever since 1830 ; but of late years the use of baryta has attracted much atten-tion, and in several French and Belgian glass-works it is under-stood to have taken its place as a raw material, without, however, much being publicly said regarding the subject. H. E. Benrath, the scientific director of the Lisette glass-works near Dorpat has in-vestigated the application of baryta in glass-making with great ful-ness. Baryta, it appears, can be used as a partial substitute for the alkalies in glass-making ; and indeed it was affirmed by Peligot that carbonate of baryta could altogether supplant either potash or soda, and yield a glass perfectly free of alkali. Such a glass is, how-ever, shown by Benrath to be without practical value ; but he has demonstrated that baryta may be used in the place of either lead or lime, to produce an easily fused dense glass much more brilliant than common glass, and in appearance and properties intermediate between that and flint glass. The qualities of the glass and its usefulness for various purposes can be modified by using both baryta and lime in varying proportions. There seems little doubt that baryta will occupy an important place in the future of the glass industry.
BOTTLE GLASS.This department of glass manufacture is of importance on account of its enormous extent ; and although the raw materials employed in the trade are coarse and impure, and though the finished product has little appearance of excellence, the quality of the glass is in the highest degree important. Glass bottles, for example, are used for storing and preserving ail manner of liquid substances for food, some of which undergo active chemical change, throughout a period of many years. In such a case it is of the highest consequence that the glass should be capable of resisting the solvent and corrosive action of acids and other substances which may be imprisoned or generated within the bottle, and such an ob-ject is attained by the high proportion of alumina which is found in bottle glass. Bottle glass varies in tint from the dark-green, almost black, semi-transparent claret bottles to clear and transparent qualities such as are employed for bottling aerated waters. The difference in colour is partially due to the varying purity of the materials used, and partly to the action of bleaching or oxidizing agents. The materials ordinarily employed are common sand, gas-lime, brick-clay, common salt, and soap-boilers' waste ; but local circumstances have much influence in determining the class of materials used. In Continental bottle works lava, basalt, and simi-lar rocks of volcanic origin were formerly employed; and in Den-mark and Sweden fluoride of calcium, left as a waste product of the manufacture of soda from cryolite, is used with marked advantage.
For bottle-making the tank furnace with or without compart-ments as already described is much used ; but pot furnaces also continue in use. The arrangements of a common bottle house are seen in fig. 19, which is a ground-plan indicating a bilateral
FlG. 19.Plan of Buttle House.
arrangement of a double bottle house, with the complete plan of a four-pot furnace and ash arches. The furnace is oblong, similar to the crown furnace, but arched over in a barrel shape. It is erected in the centre of the brick cone, above a cave, which admits the atmosphere to the grating. The working holes of this furnace, opposite each pot, for putting in the materials and taking out the liquid glass, are each about 1 foot in diameter. At each angle of the furnace there is also a hole about the same size communicating with the calcining arch, and admitting the flame from the main furnace, which reverberates on and calcines the materials in the arch. In the figure, 1 shows the main furnace ; 2, 3, 4, 5, the ash arches for calcining the materials ; 6, 7, 8, 9, 10, 11, annealing arches ; 12, two-pot arches ; 14, clay-house for picking, grinding, sifting, and afterwards working the clay into paste for the pur-pose of manufacturing pots ; 15, mill house for grinding clay ; 16, a building containing a calcar furnace for experiments, or for pre-paring the materials, when the ash arch attached to the main fur-nace is under repair, including 1, a sand crib, and 2, an ash crib for sifting and mixing the materials, sufficient for two houses.
The following is an outline of the process of making a common bottle. After the metal has been skimmed, the person wdio begins the work is the gatherer, who, heating the pipe, gathers on it a small quantity of metal. After allowing this to cool a little, he again gathers such a quantity as he conceives to be sufficient to make a bottle. This is then handed to the blower, who, while blowing through the tube, rolls the metal upon a stone, at the same time forming the neck of the bottle. He then puts the metal into a brass or cast-iron mould of the shape of the bottle wanted, and, continuing to blow through the tube, brings it to the desired form. The patent mould now in use is made of brass, the inside finely polished, divided into two pieces, which the workman, by pressing a spring with his foot, opens and shuts at pleasure. The blower then hands it to the finisher, who touches the neck of the bottle with a small piece of iron dipt in water, which cuts it completely off from the pipe. He next attaches the punty, on which is a little metal gathered from the pot, to the bottom of the bottle, and thereby gives it the shape which it usually presents. This punty may be used for from 18 to 24 dozen of bottles. It is occasionally dipped into sand to prevent its adhering to the bottle. The finisher then warms the bottle at the furnace, and taking out a small quantity of metal on what is termed a ring iron, he turns it once round the mouth, forming the ring seen at the mouth of bottles. He then employs the shears to give shape to the neck. One of the blades of the shears has a piece of brass in the centre, tapered like a common cork, which forms the inside mouth ; to the other blade is attached a piece of brass, used to form the ring. The bottle is then lifted by the neck on a fork by a boy, and carried to tho
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annealing arch, where the bottles are placed in bins above one another. This arch is kept a little below melting heat, till the whole quantity, which amounts to 10 or 12 gross in each arch, is deposited, when the fire is allowed to die out.
SLAG GLASS. Under a patent obtained by Mr Bashley Britten, a manufacture of bottles has recently been established with every prospect of success, the leading peculiarity of the process being the use of blast-furnace slag, molten as it issues from the furnace, as a principal ingredient. The use of slag in bottle-making is by no means new, but the catching of the intensely hot Uquid mass and its immediate use for glass-making had not before been attempted ; and therein results the great saving in fuel and consequent economy of the manufacture. A company formed to work the process has erected glass-works in Northamptonshire in the immediate neigh-bourhood of a set of blast furnaces, and these works are now in con-stant and successful operation. A regenerative gas furnace applied to a glass tank working on Dr Siemens s continuous principle is used, and in it the ingredients of the glass are fed at one end of the tank, where they are fused and fined, and the fused "metal" flows through a bridge to the other end of the tank, wdience it is worked out, blown into bottles, and annealed in the usual way. The tank is from time to time fed with fused slag taken as it flows from the blast furnaces, and with it is introduced the required proportion of the other in-gredients. The slag furnishes more than half the total material of the glass, and, as it is already melted, its use effects a saving of about half the heat or fuel and also half of the time necessary for the production of the "metal." Thus the prime cost of the glass as it is worked out is considerably less than that of glass made in the ordinary way. The natural tint of the glass thus produced is greenish, but it can be coloured to any required tint, and by careful fining and bleaching it can be produced almost as colourless as com-mon window glass. The working qualities of the glass are excellent; it comes from the furnace in that beautifully plastic condition which renders it capable of being blown, cast, pressed, or otherwise moulded into any desired form, and the company expects to manufacture other articles besides bottles from a material so cheaply produced.
OPTICAL GLASS is of two principal kindsflint and crownthe combination of these two, with their different refractive powers, being necessary to produce perfect achromatism in the lenses of telescopes. For astronomical telescopes, formicroscopes, and for all delicate scien-tific instruments in which optical glass occupies a place, glass of the utmost purity, transparency, freedom from colour, streaks, and strife is of the highest importance ; and to secure these qualities to the fullest extent much care, trouble, and expense are requisite. The first really successful maker of optical flint glass was M. Guinand of Solothurn in Switzerland who succeeded in making discs 9 inches in diameter free from stria?. Guinand died in 1823, and fromhis son, M. George Bontemps learned his secret, and at Choisy-le-Boi, near Paris, further improved the manufacture. In 1848 M. Bcntemps was induced by Messrs Chance of Birmingham to establish the art in their great works. In the hands of that eminent firm the preparation of optical glass has attained a perfection not approached by any other glass workers, and. the chef d'ceuvre of optical glass hitherto made consists of a pair of flint and crown glass discs, 29 inches in diameter, exhibited by Chance Brothers at Paris in 1855. Regarding these Sir David Brewster said, "I have entertained the hope that the English Government would purchase these discs and construct with them the grandest achromatic telescope that ever was contemplated by the most sanguine astronomer." They were, however, purchased by the French Government in 1867.
Optical flint glass contains more lead, and is consequently heavier and more refractive, than the quality used for common purposes. It is made in a furnace having a single covered pot, and Guinand's secret consisted in constantly stirring the mass while it is in a mol-ten condition so as to keep the heavier lead silicate from falling to the bottom. For the very highest qualities of optical glass, the contents of the pot are most scrupulously cleared, and the stirring is continued after the heat is lowered till the contents are cooled down to little more than a red heat. The furnace is then closed and the metal is allowed to cool and anneal gradually in the pot within the furnace. When withdrawn the pot is broken, and the mass of glass is polished on two opposite sides so that any imperfections may be detected by examination. From the mass, cut horizontally, perfect discs of such size as can be formed are then obtained. Optical glass is also blown into thick cylinders, and cast in slabs from £ inch to 1 inch in thickness.
Chance Brothers make six kinds of optical glass, of which the average densities and refractive indices for the three hydrogen lines and for the sodium line are given in the following table :
== TABLE ==
In 1830 Faraday proposed the use of a com pound silicate and borate of lead glass, with a density of 5'44, for optical use ; and Maezand Clemandot have successfully introduced a boro-silicate of zinc. An optical glass of higher refractive and dispersive power than any previously known has been made by Lamy from a mixture of silica, red lead, and carbonate of thallium. The glass is perfectly homo-genous, but of a yellow tintan evil, however, said to be avoidable by the use of the sulphate instead of the carbonate of thallium. The extraordinary refractive power of the glass renders it peculiarly adapted for the fabrication of imitation precious stones.
STRASS.Remarkably faithful imitations of every kind of precious stone can be made from suitably prepared and, when necessary, coloured glass. The transparent basis from which arti-ficial precious stones is prepared i3 called strass or paste, a material which must of necessity be the purest, most transparent, and most highly refractive glass that can be prepared. These qualities are combined in the highest degree in a flint glass of unusual density from the very large percentage of lead it contains. Among various mixtures given by Donault-Wieland as suitable for strass the following is an example:powdered quartz 300 parts, red lead 470, potash (purified by alcohol) 163, borax 22, and white arsenic 1 part by weight. Special precautions are adopted in the melting of the materials, and the finished colourless glass is used for imitating diamonds. When employed to imitate coloured precious stones the strass is melted up with various metallic oxides, to which refer-ence will be made under coloured glass. Artificial precious stones are, of course, easily distinguished from real stones by their inferior hardness, and by chemical tests. They may also be generally detected by a comparatively soft warm sensation they communi-cate when applied to the tongue.
GLASS TUBES, used for gauge glass for steam boilers and for many other purposes, are made by a very simple process; but as the manufacture is a separate department of industry it demands some notice. A gathering of glass is made on a blow-pipe, which is marvered, and slightly blown, so as to form a thick-walled elongated globe. To the end of this globe opposite the blowing pipe a pontil-rod is attached by an assistant, and the two workmen move back-wards the one from the other, drawing out the tube as they recede. One or two boys watch the process of elongation, and when the tube reaches the desired gauge they fan its surface so as to " set" the glass, and thus prevent further attenuation at that particular point. The relation of the mass of metal to the original cavity determines the comparative stoutness and bore or internal diameter of the tube, and it requires much dexterity to make a tube at once straight and of uniform gauge and diameter. In drawing out tubes of large gauge the operatives recede from each other at a slow rate ; and in proportion as the size of tube decreases the rate of drawing out increases. In the Venetian factories, where small tubes for beads are made, the workmen move at a smart walking pace.
MASSIVE GLASS.Under this term may be conveniently noticed the manufacture of various familiar solid glass objects which do not acquire their form either by blowing or pressing in the ordinary sense. Glass Hods form the basis of many of these objects ; and the formation of a plain rod of glass is accomplished by a manipulation in all respects similar to that described under the head of glass tubes, the only difference being that the rod is drawn from a solid instead of from a hollow or blown gathering as in the case with tube drawing. From solid rod glass, glass buttons of various forms are "pinched" by heating the rod till it softens, and immediately pinching it in heated moulds made and worked like ordinary pincers, but having moulds of suitable form in place of the gripping surface of the pincers. The small facets of glass lustres and girandoles and glass marbles, are made by an analogous process.
SPUN GLASS.Certain qualities of glass in the plastic condition are capable of being drawn out to threads of great tenuity, which, wdiile possessing much brilliancy and beauty of colour, are perfectly flexible and elastic, and feel to the touch soft and smooth like fine wool. A good deal of attention has been given to glass spinning in Vienna and in the Bohemian glass-works, the thread produced being woven into many textiles for upholstery and wearing purposes. The material is specially useful in millinery ornaments owing to the fine colours in which it may be produced, and to the fact that it is unalterable in and unaffected by all kinds of weather.
A remarkable and novel application of glass was patented in 1878 by Mr J. B. Hannay. It consists in making glass a cementing or binding substance in the manufacture of emery wheels, now so much used instead of files. For preparing the wheels any broken fragments of glass are utilized. These are reduced to powder, mixed with proportions of powdered flints and emery, and in the form of a cake introduced on a layer of paper into a furnace where the material is submitted to a heat sufficient to fuse it into a compact ringing mass. The resulting cake is of intense hardness and dura-bility, and cuts through ordinary emery wheels with ease. As the glassy emery wheels can be made very much cheaper than those at present in use, there is little doubt that this material will come into extensive use, in the rapidly increasing applications of emery to grinding, smoothing, and cutting.
COLOURED GLASS.When to the ordinary materials in the melting pot small quantities of various metallic oxides and other mineral substances are added, coloured but still transparent glass is produced. The colours yielded vary in intensity according to the proportion of oxides used ; and the temperature at which the fusion is effected, the length of time the molten glass remains in the melting pot, not only modify, but actually change altogether, the resulting colour. Indeed, it has been asserted by M. Bontemps that all the colours of the spectrum may be obtained by the use of one oxide alone, if employed in varying proportions and at different temperatures. The materials, temperatures, and other conditions employed by manufacturers for producing certain of their colour effects in glass are kept as trade secrets; although, in a general way, the substances which produce particular colours are perfectly well known. Blue is obtained by the use of cobalt,the ordinary blue pigment smalt being a powdered cobalt glass. Yellow glass can be prepared from several sources: uranium yields a beautiful opalescent yellow ; salts of silver are the source of hue shades of yellow; a different tone of the same colour may also be obtained from oxide of antimony; and a dull yellow is produced from powdered charcoal. Green was at one time prepared chiefly by the use of cupric oxide and of ferrous oxide; now oxide of chromiumwhich produces a beautiful emerald greenis much employed, that substance being mixed with other oxides when modified colours are desired. For red glass, cuprous oxide is employed to produce an intense ruby tint; and the purple of Cassiusa compound of gold with tin oxide yields magnificent shades of ruby, carmine, and pink, while oxide of iron also is the source of a brownish-red colour. For the produc-tion of violet tints the black oxide of manganese is depended on, and a mixture of the oxides of manganese and cobalt is employed for black. The deep-black glass prepared at Venice for making glass beads contains a large percentageabout 11 "40of manganese. Avanturine an imitation in glass of the mineral bearing that name, is a warm, brown, opaque glassy body, studded with innumerable minute spangles having a metallic lustre. It was originally made in the Venetian glass-houses, but can now be manufactured gene-rally throughout the Continent.' According to Von Pettenkofer, the metallic spangles consist of cuprous oxide, and one means of preparing the glass consists of melting equal parts of cuprous oxide and ferrous oxide with the glassy mass. The. other opaque varieties of coloured glass are obtained by using the various metallic oxides with the compounds that produce milky or white glass.
The uses of coloured glass are various, the most obvious and usual being for ornamental windows, for signal lights, for imitation precious stones and ornamental table glass, &c. For the imitation of precious stones the strass already alluded to forms the basis, and both flint glass and Bohemian (potash) glass are much used for coloured domestic glass. Window and signal coloured glass are made both as rough plate and as sheet glass. In the case of plate glass the metal is, of course, uniformly coloured throughout, but coloured sheet glass may either be composed of " pot metal " or it may be " flashed colours." Pot metal consists of glass uniformly coloured throughout; but in flashed colours the body of the glass is transparent sheet metal covered on one surface only with coloured glass. It is very simply made: the workman, taking up on the end of his blowing tube a gathering of clear metal from one pot, dips this into a pot of coloured metal, thus gathering over the transparent mass a uniform stratum of coloured glass. The whole is then blown in the ordinary manner, and according to the original relative proportions of clear and coloured metal will be the thickness of each in the finished sheet. It is obvious that this process admits of many variations, such as gathering the coloured metal first, or making alternate gatherings of coloured and clear metal so as to have clear within coloured, coloured within clear, and so on.
IRIDESCENT GLASS.Ancient glass, which has for ages been submitted to the slow disintegrating influence of the damp of the earth and other gently operating agencies, in many instances dis-plays an iridescent play of colours of a most magnificent description. The iridescence thus shown, it has been long known, is due to a pro-cess of decomposition resulting in the formation of excessively thin scales of glass. Numerous attempts have been made to imitate by artificial means the gorgeous display of colours thus produced by the slowly acting influences of many centuries, and a certain amount of success has attended some of these efforts. The Venetian glass workers possess the means of giving the surface of their glass a kind of metallic iridescence; and in certain Hungarian glass houses iridescent glass has been made for at least about 20 years. But in 1873, at the Vienna Exhibition, iridescent glass formed a prominent feature, and since that time it has become very common. The iridescent glass now generally seen is a plain flint glass having a slightly metallic tinge and a play of colours like a soap bubble. It is probable that several methods of producing iridescence in glass are practised, as the nacreous lustre in different examples varies considerably. The subject was investigated by MM. Fremy and Clemandot; and under a patent obtained by the latter gentle-man, one method, commonly practised, has been made public. It consists in submitting the object to be iridized to the influence of a weak acid solutionsuch as water with 15 per cent, of hydrochloric acidunder the combined influence of heat and pressure. The effect certainly falls immensely short of the iridescence of ancient glass, but the glass assumes permanently a pearly iridescence, and, though the effect is tiresome, the process will doubtless continue to occupy a place among the methods of ornamenting table glass, &c.
OPAQUE GLASS.Absence of transparency in glass may be due to any of three causes(1) to the grinding, or roughening by other means, of the surface of ordinary clear glass; (2) to devitrifica-tion or crystallization of the substance; and (3) to the mechanical intermixture of an opaque substance in the glassy mass. Obscured glass was formerly principally prepared by a process of grinding the surface,the means employed in the case of sheet and plate glass being simply the smoothing process, which forms an intermediate stage in the operation of polishing plate glass. Now the greater part of ordinary obscured glass is prepared by the agency of Tilghman's sand blast, an apparatus by which a fine stream of sand is blown with great violence against the glassy surface. The impinging grains of sand abrade the surface with extraordinary rapidity, and by protecting certain portions with suitable stencils, elaborate patterns in clear glass are produced in a very simple manner. Alabaster glass, so called on account of its resemblance to that substance, is an opaque variety of glass which has been long known and used. Its opacity is due to a process of devitrification it readily undergoes, favoured by the excess of uncombined silica used in its preparation. The materi a,l is prepared from a combination such as 100 parts of sand, 40 of potash, 5 of borax, and 5 of talc (silicate of magnesia). Into the composition of this glass it will be observed no lime enters, although sometimes bone-earth is added to the materials. For enamel glass a mixture of varying proportions of lead and tin oxides is prepared by calcining the two metals together, and using the compound in the preparation of a mixture for fusing, of which the following is an example :sand 100 parts, pure potash 80, and mixed oxides 200 parts. The proportion of tin varies within wide limits, and oxide of antimony may be substituted for the tin. Bone glass, milk glass, and opal glass, differing in degree of opacity, are made by adding to the materials of clear glass large proportions of bone-ash, or of oxide of tin, or both together, and with these substances wdiite arsenic may also be combined. The following is an example of a batch for opal-flint glass :sand 100 parts, bone-ash 30, potash 30, borax 5, and "red lead 5 parts. Such a glass was formerly in extensive use for the opal shades of gas lights and moderator lamps, &c.; but the ruddy glow possessed by the rays passing through the imperfectly opaque glass was an objectionable feature in the resulting material. During recent years an opal or milk glass free from such a defect has been introduced, and it is now in extensive use for globes. This preparation, which diffuses light from its surface in a clear pure white glow, owes its milky opacity to the use of cryolitea mineral substance consisting of a double fluoride of aluminium and sodium (A12F6, 6NaF) obtained from Greenland. The cryolite glass was first brought prominently into public notice by the Hot-Cast Porcelain Company of Philadelphia, by whom it was made on a large scale, although the material had been in use in Bohemian and Silesian glass-works for some j'ears previous to the commencement of the manufacture in America. For milk white glass the materials used aresand 100 parts, cryolite 40, and zinc oxide 10 parts. The finished glass, wdiich is remarkably strong, hard, and indifferent to acids contains about 15 per cent, of undecomposed cryolite, to wdiich its opacity is due. The copious evolution of fumes of hydrofluoric acid during the melting, -which continues even in the working, is the source of serious difficulty in the manufacture of cryolite glass.
TOUGHENED OR HARDENED GLASS.In the year 1875 the announcement that a French gentleman, M. de la Bastie, had discovered a means of rendering glass practically unbreakable attracted a great amount of attention ; and his statements were immediately made the subject of practical investigation throughout the glass-making community. All the experiments made in connexion with M. de la Bastie's process tended to confirm his claim to have dis-covered a method of rendering glass capable of bearing a shock or strain variously estimated at from 30 to 100 times greater than the same material annealed in the common way. De la Bastie was led to undertake the prolonged series of experiments, which ultimately resulted in his discovery, by the consideration that the brittleness of glass arises from weak cohesion of its molecules; and his efforts were first directed to improving its molecular arrangement, by sub-mitting glass, in a molten state, to forcible compression. This series of experiments led to no practical result; and the line of investigation he ultimately pursued, as well as the merits and defects of his process, are thus succinctly stated by Mr H. J. Powell of Whitefriars Glass Works, with whom M. de la Bastie carried out his first practical experiments with hollow glass. Mr Powell, writing in August 1875, says of the process:
" That it consists in plunging glass heated to the melting point into a bath containing an oleaginous mixture, at a high temperature, but considerably cooler than the glass ii self; and that this, according to the specification of the patentee, is effected by re-heating already manufactured and annealed glass in a kiln, and passing it thence into the bath. After a rough trial of this process, which cer-tainly answers well for flat or solid glass, we decided that it is defective, for hollow flint glass, as hollow vessels, left to themselves in a kiln, are almost certain to collapse on reaching the required heat. To avoid this difficulty, and knowing that a vessel in course of manufacture, however hot, is always under control whilst it remains on the workman's rod, we placed a bath as near the mouth of the working-pot as possible, and directed the workman, instead of sending the finished vessel to the annealing oven, to drop it into the bath. The vessel is caught in a wire net, and is ready for removal as soon as it has acquired the temperature of the bath. For all vessels made in one piece, e.g., tumblers, finger basins, &c, this process answers well; and It is obvious that if It proves to be the best way of treating hollow flint glass, the use, for this description of glass, of the complicated machinery described in M. de la Bastie's specification will be done away with, and the glass will be tempered in the course of manufacture, instead of being re-heated and tempered after it has been already manufactured and annealed. We ascertained, with M. de la Bastie's aid, the right constituents and right temperature of a bath for flint glass; for although the conditions for sheet, plate, and flint glass are nearly the same, there is a difference, and it seems probable that every chemically different glass, and even every different thickness of glass, may require certain variations. In our experiments as to the hardness of the glass, we found that it could be marked, but not cut, with the diamond, and, although it could be smoothed and engraved in the ordi-nary way, that the disturbance caused by the wheel, when penetrating to any appreciable depth, tended to weaken, or even to cause the destruction of the entire mass. The value of the invention, as far as it concerns flint glass, is at present somewhat modified by difficulties in manipulation. 1. It seems to be im-possible to heat a vessel made up of different pieces and of various thicknesses to an absolutely equal temperature throughout, so that the whole may be equally tempered. 2. It seems also impossible to displace the air from a narrow-mouthed vessel quick enough for the inside and outside to be tempered simultaneously. However, setting aside these difficulties, we come to a point which applies equally to all sorts of hardened glass. Hardened glass is not 'unbreakable'; it is only harder than ordinary glass, and though it undoubtedly stands rough usage better, it has the disadvantage of being utterly disintegrated as soon as it receives the slightest fracture, and ux> to the present, until broken, of being undistinguishabie from ordinary glass. This glass is known as ' toughened 1 glass, and we have seen the terms 'malleable' :ind 'annealed' applied to it. Nothing can be more misleading than these unfortunate epithets. The glass is hard, and not tough or malleable, and is the very opposite to annealed glass. Annealed glass is that the molecules of which have been allowed to settle them-selves; the molecules of hardened glass have been tortured into their position, and until the glass is broken are subject to an extreme tension It is the sudden change of temperature that ' hardens'; glass heated up together with the oil may be annealed, but decidedly is not hardened. A piece of hardened glass is only a modified Rupert's drop, i.e., it is case-hardened; the fracture of botii is identical, both resist the diamond, and both can be annealed. Moreover, in the middle of imperfectly hardened glass a line is plainly visible, whicii seems to mark the extent of the case-hardening. This line resolves iiself under the microscope into a mass of bubbles and strise; it seems to be the nucleus of breakage, and consequently as soon as the cutting wheel approaches it, utter destruction ensues."
The great anticipations which at first were formed as to the extended use of hardened glass have not been realized. M. de la Bastie has improved several of his processes, but the demand for his productions, at no time great, is understood to decrease rather than to increase. For a short time the process was worked by both Messrs Powell and Messrs Pellatt in London, but both these eminent firms have given it entirely up. Sheet glass hardened by the process does not appear ever to have come generally into the market, the most serious obstacle to its introduction being the im-possibility of cutting it with the diamond, after which the utter destruction resulting from fracture is a serious defect. For laboratory purposesas flasks and beakers, &cit has been suggested that the glass has great advantages, but experiments have proved that its great resistance is not absolutely reliable, and that hardened vessels sub-mitted to a high heat lose their distinguishing peculiarities and become as common glass. Thus a glass, partially fill, d with water and heated considerably above the boiling point at the parts un-covered with water, broke, the bottom of the glass showing the fracture peculiar to hardened glass, wdrile the upper uncovered part was broken into large sharp-edged fragments like common glass. A modified process of hardening, patented by Herr F. Siemens, consists in pressing and suddenly cooling the glass in moulds specially constructed to conduct away the heat with the various degrees of rapidity found to produce the best results.
Plate glass , 1,156,435 sq. ft. £106,S
Flint glass 93,112 cwts
Common bottles 575,160
Other glass manufactures 72^209
STATISTICS OF THE GLASS TRADE.According to a factory report of 1871, there were in
that year 240 glass-works in the United Kingdom, employing, in addition to steam-power, 21,434 operatives, of whom 2116 were females. Of these works 213 were in England, 19 in Scotland, and 8 in Ireland. Further, there were at the same date 37 glass-cutting factories, employing 500 people, principally situated in the county of Warwick. The quantities and value of glass manufactures exported were as under in the year 1878 :
== TABLE ==
The following table shows the comparative imports and exports of glass in the ten years ended 1877 :
== TABLE ==
In the Bulletin de la Société a l'Encouragement pour l'industrie nationale for 1877 there is an elaborate statistical computation of the extent and value of the glass manufacture throughout the world, based chiefly on returns applicable to 1874. The writer, M. Henry de Fontaine, arrives at the conclusion that the annual production of glass has almost doubled in the past twenty years, and estimates the total yearly production throughout the world at a value of six hundred millions of francs.
Bibliography.The literature of glass-making of English origin is scanty and imperfect. In France and Germany the subject has received much fuller attention. The following list embraces the principal works:Antonio Neri, Ars Vitraria, cum Merritti observationibus, Amst., 1668 (Neri's work was translated into English by C. Merritt in 1662, and the translation, The Art of making Glass, was privately reprinted by Sir T. Phillipps, Bart., in 1826) ; Johann Kunkel, Vollständige Glasmacher-Kunst, Nuremberg, 1785; Apsley Pellatt, Curiosities of Glass-making, London, 1849; A. Sauzay, Marvels of Glass-making (from the French), London, 1869; G. Bontemps, Guide du Verrier, Paris, 1868; E. Peligot, Le Verre, son histoire, sa fabrication, Paris, 1878 ; W. Stein, Die Glasfabrikation (in Bolley's Technologie, vol. iii. ), Brunswick, 1862; H. E. Benrath, Die Glasfabrikation, Brunswick, 1875 ; J. Falck and L. Lobmeyr, Die Glasindustrie, Vienna, 1875. (J. PA.)
The manufacture of coloured glass, which is the basis of the beautiful and interesting art of glass painting, originated at a period of remote antiquity, and the use of enamels, to vary or ornament its surface, was known to the ancient Egyptians ; but the formation of windows of mosaics of coloured glass upon which the shapes of figures and orna-ments are painted with an enamel fixed by fire is mediaeval, and emphatically a Christian art. In all probability it was suggested by the mosaic pictures with which churches were adorned from an early period for the instruction of the illiterate, as was shown by the inscription which they bore, "sanctas plebi Dei." The step from mosaic pictures to glass mosaic windows wras merely a question of time; it is not known when the step was taken, but coloured windows existed in St Sophia at Constantinople in the 6 th century, whilst the basilicas of St John Lateran and of St Peter at Eome were adorned about the same time in the same manner. In the year 709 Wilfrid, bishop of York, invited workers in glass from France ("artifices lapidearum et vitrearum fenestrarum primus in Angliam ascivit "). The French claim the honour of having invented the process of painting upon the mosaic windows of coloured glass, and of thus transforming them into works of art, and also of teaching this to the English, who in their turn instructed the Germans ; but Muratori, in the second volume of his Antichità Italiche of the Middle Ages, printed a treatise on mosaic and painted glass written by an anonymous Italian in the 8th century, and probably not later than the 11th was written the interesting essay Diversarum Artiurn Schedula Theophili Presbyteri et Monachi, which details with minute accuracy the process of painted glass as it has been practised with some additions and modifications, throughout the best periods of the art ; it may reasonably be assumed that Theophilus describes methods invented before his time. Probably the oldest specimen of glass painting now existing is a window of the 11th century in a church at Neuwiller, in Alsace, representing St Timothy. The figure is rudely designed, but, with the rich border of ornament, shows that the executant knew his art, which in the following century is further illustrated by windows in St Denis, near Paris, erected by the abbot Suger, which are still preserved. It was however in the 13th century, that great age of the revival of art, that glass painting attained its first great development, and notwith-standing the claims advanced by France, it is most probable that as all art radiated from Italy as a general centre of invention and progress, as well as of faith and dogma, so glass painting partook of the general impulse. It has indeed been asserted that glass manufactured in the north was superior to that produced in Italy, and this is admitted by Vasari, but an examination of old Italian windows throws doubt upon this statement, for the Italian glass will be found to be more even in texture, more diaphanous, and certainly not inferior in colour, whilst the beautiful, pearly, white glass of the earliest date in Italy is superior to the pale green representing white in northern glass, and assorts much more harmoniously with the glowing coloured glass with which it is associated. Considered as a branch of fine art, Italian painted glass occupied a very high posi-tion at all periods of its history, for the designs were fre-quently made by some of the most famous of that long roll of immortal artists who have had so few equals elsewhere. In Germany the family of Hirschvogel of Nuremberg and other eminent artists, including, it is popularly believed, Albert Diirer, and in France Jean Cousin, Bernard Palissy, Louis Fauconnier, and others, equalled the Italian glass-painters, whilst both German and French artists excelled them in technical processes.
The late Mr Charles Winston, author of An Inquiry into tlie difference of style observable in Ancient Glass Painting, especially in England, with his usual accuracy and profound knowledge of his subject, thus classifies the consecutive styles:the Early English, from the date of the earliest specimens extend to the year 1280: the Decorated, which prevailed from 1280 to 1380; the Perpendicular from 1380 to 1530; and the Cinquecento from 1500 to 1550. The styles successively prevalent in Italy, although they have an affinity with those following each other in northern countries, cannot be accurately designated by the titles selected in England, the last excepted. Like other branches of painting they are most readily divided and understood by centuries; as the 13th century, a style principally influenced by Giunta Pisano and Cimabue; the 14th century, in which the spirit of Giotto and Orcagna and their followers prevailed; the 15th century, the first period of which was transitional, and the second early revival, as illustrated by the designs for windows of Lorenzo Ghiberti, Donatello, Paolo TJcello, Pietro Perugino, Andrea delta Robbia, and other great artists of the time. The last age, called by Mr Winston the Cinquecento, lasted in Italy beyond the period assigned by him for its termination, and was characterized by florid magnificence of design and splendour of colour, imitative of the pictorial art of that age. and the ornament resembled that prevalent amongst the ornamentists of the schools of Raphael and Michelangelo.
In this brief analysis of the history and practice of glass painting, the Italian examples of the art are selected for description as being less known than those existing in other parts of Europe, which have been minutely and ably illustrated. Besides being classified by centuries, Italian glass painting may be appropriately arranged under the following heads or schools :the Pisan, Florentine, Sienese, Umbrian, Lucchese, Bolognese, Lombard, and Venetian ; for notwithstanding the lamentable destruction of painted glass in Italy even now in progress, specimens by artists belonging to these schools still exist, and the names of a numerous array of glass-painters of these provinces are preserved. The best examples extant of this art in Italy of the first half of the 13th century are two couplets in the apse of the famous basilica of St Francis at Assisi. In general arrangement and design they resemble windows of the same age in other parts of Europe classed by Mr Winston under the general head of Early English, being divided into panels of varied and admirably designed geometrical forms surrounded by diapers and borders of rich fancy and glowing colour, which, however, are rather more confused than similar details in contemporary glass elsewhere. The panels are filled with scripture subjects, and Italian skill and refine-ment are obvious in the design of the figures ; this may be readily understood when it is considered that Italian artists of this time rendered the Greek art, uni-versally followed, with more sentiment and power than any other people. The ornament shows the influence of Byzantine conventions, but the ornamentists imitated natural forms of foliage sooner than northern artists. A remarkable peculiarity of the early painted windows at Assisi, which are here taken as the best and almost the only specimens of 13th-century glass left in Italy, is that throughout the couplets the backgrounds in each light differ in colour. The subjects for instance in the right light are on a blue ground, on the left on a red ground, in other examples the grounds are alternately blue and green. It might be supposed that all unity of effect must have been destroyed in this way; but such is the skill with which the general harmony of colour is arranged, that the counter-changes are less objectionable than might appear possible. Early Italian glass painting, like that of other parts of Europe, is characterized by an obvious ignorance of perspec-tive on the part of the designers, but there are manifest indications of attempts to represent retreating surfaces and lines; that these have not the effect intended was not due, as some suppose, to the maintenance of a principle appropriate to glass painting, but simply to a want of knowledge of perspective laws common to all, even the greatest artists of the time.
Besides executing panel windows with small figures, the artists of the 13th century painted figures of comparatively large proportions under canopies of simple and primitive forms. These figure windows were placed in positions at a distance from the eye, as above in the clerestory, or at the ends of aisles. At Assisi such figures are painted at the bases of some of the windows irrespective of the idea of distance, the upper portions being filled with panels and small figures. This irrational system gave way to the entire window being filled with large figures placed over each other within canopies or geometric borders, a method of design which survived in Italy to the close of the 15th century.
The coloured windows of the upper and of the lower church are associated with mural paintings which cover every available space on the walls and vaults. It might be supposed that the refulgent painted glass would either obscure the frescos or diminish their effect by contrast; but in the first place the glass is not so thick as that of northern manufacture, nor is it made opaque by the effect of climate, whilst, as already noticed, the white glass is purer, and the brilliant sun of a southern climate illumines the frescos sufficiently, notwithstanding the coloured medium through which the rays are transmitted. Such is the case at Assisi; but at Florence the coloured glass in the cathedral darkens the church too much, from causes which will be explained afterwards. The true method of combining coloured glass in a building with the presence of pictures and sculpture in a less sunny climate will be illustrated in describing glass painting of the 16th century, the subject being im-portant at the present time.
In the next great age of art, that which commences with the triumphs of the genius of Giotto, glass painting evi-dently shared in the general progress. No windows remain which are associated with his name as designer, but in Santa Croce at Florence, by the will of Count Alberto di Lapo, dated the 9th of July 1348, money was provided for adorn-ing the apse of the church with frescos and painted windows to be completed in three years. The frescos were paiuted by Angelo di Taddeo Gaddi, and it seems reason-able to suppose that he designed two of the three windows, that in the centre being of later date. The following extract from the archives of the cathedral of Florence shows that Angelo Gaddi designed for glass-painters: "A window in Santa Beparata"the ancient name of the cathedral " over the door towards the street of the Cassettai is com-missioned of Antonio of Pisa, master glass-painter, and the design is by Angelo Gaddi." The windows illustrate the system of ranging single figures under canopies over each other. The colouring is harsh; there is too great a preval-ence of dark green; and the general design of ornament is meagre and confused. This is increased by the capricious changes of the colours of the canopies, which are not white, these bsins rare in Italian glass painting. Thus in one of the windows the first two canopies are respectively red and green, which colours are counterchanged in those immedi-ately over; the next two in ascent are green and yellow, the next pair brown and yellow, the fifth order shows both yellow. Thus that variety of colour prevalent in the back-grounds of Italian 13th century work is found in the canopies of windows of the next century, a custom limited to glass painting, and not found as a rule either in mural or other pictures. There are in Santa Croce several win-dows of the 14th century, but they are generally inferior to those of the same period extant in St Francis of Assisi, where there are important examples of rare beauty of design and workmanship, more harmoniously coloured than those at Florence, and suggesting that the Umbrian excels the Florentine school of glass painting.
Throughout the whole of the painted windows existing in Florence, of the fully developed style of the 14th century, and for a considerable portion of the 15th, the influence of the architectural design of Giotto and Orcagna is very perceptible. The graceful twisted shafts common to the works of both architects, the richly adorned niches and gablets, the dome-like covering of the famous baldacchino in the church of Or San Michele, the work of Orcagna, are features which are imitated in various ways by Italian glass-painters. The colour is especially noteworthy; the canopy, somewhat squat in form, is adorned in every part with rich and diversified colours evidently imitative of the varied marbles and the infinity of marble inlay and mosaics, characteristic of so much of the mediaeval architecture of Italy; whilst in northern countries the canopies in windows, with their beautiful details of form showing such rich fancy and such graceful lines, are chiefly white, not that they are altogether colourless, for it is of the perfection of mediaeval architecture to associate colour with form. That which in Italy was done by the help of rich stores of marbles of many hues, was effected in the north, where these were not avail-able, by means of polychromatic painting, which was imi-tated in window design by the glass-painters. Thus the canopies in Italian windows differ as much from those pre-valent at the same period in the rest of Europe as the campanile of Giotto differs from the spires raised by the genius of northern architecture.
In the history of painted glass in Italy during the 15th century, the windows of the cathedral of Florence, dating from 1390 to 1503, occupy an important position, not only by reason of their interest as connected with that cele-brated church, but also because they were designed and executed by artists of the highest reputation. The cathe-dral was founded on the 8th of September 1298, the architect being Arnolfo di Cambio di Colle di Valdelsa. In 1334 Maestro Giotto was architect, and commenced the famous belfry. In 1364 the church was vaulted over at its eastern end, and in 1420 Filippo Brunelleschi and Lorenzo Ghiberti, who built the clerestory of the nave with its round windows and Renaissance cornice, were appointed joint architects. Painted glass was introduced into the windows thirty-six years before the completion of the cupola, and thirty before that of the clerestory. These dates are an interesting testimony to the importance attached at the time to painted windows as portions of the design of so great a church. They were erected in the aisles, before the nave was finished, by Don Lionardo di Simone, monk of Vallombrosa, and Niccolo di Pietro della Magna, so early as 1390, and when the nave was roofed over by its architects, Fra Bernardino di Stefano executed the two first windows of the clerestory from designs by Lorenzo Ghiberti. Ghiberti is also alleged to have de-signed many of the painted windows at the east end of the church; but those now existing, judging by the design and colour, as well as by the technical execution, cannot be his, for they are manifestly of earlier date, whilst the authorship of some of them is recorded in the archives without reference to Ghiberti. One only, on the north side of the apse and in the lower row, suggests the design of this great artist, the suggestion being strengthened by the fact that the diapered ornament on the ruby dress of the figure is made by the wheel, which brings this window within the 15th century, whilst the diapers in the other figures of the same series are executed in an older style. It has been stated that Ghiberti advised the muni-cipality of Florence to invite a celebrated glass-painter of Liibeck, Francesco di Domenico Lievi da Gambassi, by letters, the second being dated October 15, 1436, to settle in Florence with special privileges; he came, and it is assumed that he painted Ghiberti's designs for glass, but of this there seems to be no satisfactory evidence. We find that in 1434, before his arrival, Maestro Domenico di Pisa painted the east window of the drum, representing the coronation of the Virgin, which was designed by Donatello in competition with Ghiberti, and preferred. As it wa3 on the 12th of January 1434 that Brunelleschi completed the dome, evidently no time was lost in com-mencing the painted windows. Bernardo di Francesco del Boni is recorded in the archives as having executed in 1442 the following windows in the drum, called in Italian
the tribune :the Resurrection, designed by Paolo Uccello, and the Ascension and the Prayer in the Garden, by Lorenzo Ghiberti, although by this time Francesco di Lievi da Gambassi had been some time in Florence. Bernardo del Boni is also recorded to have painted the Annunciation designed by Uccello, removed some years ago, and the Ascension described as being either by Ghi-berti or Uccello; the uncertainty is curious. In 1448 the same artist painted the Presentation in the Temple, which was designed by Ghiberti. What windows then did the artist from Liibeck really execute? Born an Italian, he was instructed from his youth in Germany, and it is reasonable to suppose that his style would be German. Now several of the upper windows of the transepts contain figures which have a decided affinity with German ideas, and it is not unreasonable to suppose that these were by the new comer. He established furnaces by special permis-sion, and it has therefore been hastily assumed that he manufactured coloured glass; but these no doubt were to burn his painted glass. The letter inviting " Francescho Dominici Livi de Ghambasso comitatus Flor., magistro in omni et quocunque genere vitreorum de musayco et de quodam alio colore vitreorum," dated 15th October 1436, was preceded by another written on the 26th August 1434, which is thus recorded:" Letter written to the master glass-painter Gambassi, then in Scotland, and who made works in glass of various kinds, and was held to be the best glass-painter in the world." It is now vain to express the feeling with which Scotch people must regret the destruction of the works of this excellent artist in Holyrood Chapel; the blow then inflicted upon ecclesiastical art never has been remedied.
Many other glass-painters executed windows in the cathedral, the last of whom, Sandro di Giovanni Agolanti, in 14S6 glazed the windows of the lantern, showing that it was finished, and in 1503 inserted glass "in an aperture for the astrologers to see the sun through." The windows of the cathedral of Florence are of rich deep colours, but unfortunately they are incrusted with dirt inside and outside. Many of them which have been broken have been wretchedly repaired, and the modern glass is toned with glazings of oil paint. They darken the church to such an extent that works of art within it are seen to disadvan-tage. Michelangelo's group of the Pieta is merely a dark mass, and it may be mentioned that his statue of Christ in Santa Maria sopra Minerva is similarly sacrificed to the painted glass lately introduced.
The history of glass painting is illustrated by windows in other churches in Florence ; those in Santa Croce have been alluded to, with the exception of the magnificent circular window of the fagade designed by Ghiberti. It is of special interest, as showing this great sculptor's ideas of colour as well as of form. The subject is the Descent from the Cross, and the colour strongly recalls the bright clear system of Fra Beato Angelico. In Santa Maria Novella there is another very fine circular window of the 14th cen-tury, the history of which seems to be forgotten ; but in design it recalls the style of Andrea Orcagna, to whom also may be attributed the fragments of a window in the Strozzi chapel, which he partly painted in fresco, and for which he also provided the admirable altarpiece. In another Strozzi chapel in the same church, painted by Filippino Lippi in fresco, there is a very rich window which is obviously also of his design, but so completely has all critical knowledge of glass painting disappeared in Florence, that this and other noble works excite no attention and no remark, and are in danger either from removal or from the clumsiest renovation. The great window of the chancel, a large triplet, is so excellent that it places Sandro di Giovanni Agolauti in the foremost rank as a designer and glass-painter of the 15 th century. He was born in 1446, and of his numerous works this superb triplet, which was erected in 1491-2, is the only example now remaining, whilst there are twenty-one records of his productions, evidently a small portion of them ; the last describes him as decrepit in 1515, and mentions that Niccolo di Giovanni di Paolo, master glass-painter, took his place.
A window in St Francis at Assisi by the Umbrian Angioletto da Gubbio, an artist of the early part of the 15th century, deserves special notice in connexion with the admirable works of this great age of the art. It was erected about a century after the death of Cardinal Gentili (who built the chapel) by his heirs. The window is so full of light, so delicate in its arrangement of colour, and so tenderly and skilfully painted, whilst the prevalent white ornament is so brilliant, that it is perhaps the most perfect type in existence of a painted window suitable for a building containing works of art, or where the climate is unfavourable and consequently the light imperfect. The close of the 15th century was in important respects the greatest epoch of painted glass in Italy : the general style of design of the great quattrocentista masters, their monu-mental and decorative methods of composition, simplicity of attitude and form, and rich unbroken colour, were emi-nently adapted for reproduction in glass painting, whilst the prevalent style of ornament was singularly graceful and pleasing. The influence of the Italian style was felt in France, Germany, and the Low Countries, in which very noble works of glass painting rivalling the Italian were pro-duced by eminent artists, examples of whose works may be seen at Bourges and Rouen, at Antwerp and Brussels, and in various cities in Germany, although singularly enough one of the finest typical examples of this great epoch of German glass painting exists at Assisi,the glazing panels of which the noble window has been composed being dis-persed through two or three windows of the upper and lower church. The technical execution of the windows of this period in all these countries was superb, showing how thoroughly the artists were trained in every process of their art. In England the feeble Perpendicular style was con-temporaneous, entirely devoid of good drawing, pallid in colour and weak in execution, a singular contrast in every respect to the state of the art on the Continent.
Amongst the most active and able glass-painters of the prolific 15th century in Italy the Gesuati monks, whose convent was close to Florence and was razed to the ground during the siege when Michelangelo aided in the defence of the city, were celebrated. They prepared colours for artists, supplying Michelangelo when he painted the Last Judgment, and they executed numerous windows of which there are records from 1467 to 1562. As glass-painters they painted the designs intrusted to them with forcible execution, but their drawing was less happy and their ornamentation heavy. There are several windows by these Brethren in Florence: in Santo Spirito the great circle of the fagade shows every indication of having been de-signed by Pietro Perugino ; in San Salvatore, near San Miniato, the east window, evidently by a Gesuato, has very probably been designed by Andrea della Robbia. In another window in the same church a figure of St John is a very fine specimen of their art. Several windows filled with small round panes of white glass, with richly coloured borders and sacred monograms or heraldry in the centre, are especially noteworthy as admirable types for domestic architecture. There are also works of the same character as the above in the church of Santa Maria Maddalena dei Pazzi.
A glass-painter was now to make his appearance in Italy who was destined, although a foreigner, to eclipse in the opinion of the Italians all previous masters of the art; this was Fra Guglielmo di Marsillat, born in the diocese of Verdun in 1475. It is not known under whom he studied glass painting, but for protection from the consequences of some escapade he entered the Dominican order. Claude, an eminent French glass-painter, being sum-moned to Rome by Julius II. to decorate the Vatican with painted windows, induced Marsillat to accompany him. Of the numerous windows which they must have painted only two remain in the church of Santa Maria del Popolo, so complete has been the destruction of painted windows in the Eternal City. Marsillat executed a number of magnificent windows in his manner for the cathedral of Arezzo The following is a form of contract made with him :" The board of works of the cathedral have commissioned three windows of painted glass for the cathedral from Master William, son of Peter, Frenchman, that is, one window above the chapel of St Francis, one window above the chapel of Matthew, one window above the chapel of St Nicholas, for the price of fifteen livres the square braccio,"nearly 2 square feet," to be burnt in the fire and not painted in oil, and they are to be finished by the end of June 1520." Marsillat, who painted well in oil and fresco, was the designer of his own windows. He was so impressed by the works of Michelangelo that he imitated his manner, and one of his windows at Arezzo, representing the Raising of Lazarus, is almost a transcript of the picture of the same subject designed by Michelangelo, and painted by Sebastian del Piombo, which is now in the National Gallery, London. The design of the Frenchman is rather more crowded, but is characterized by more life and action. He introduced into glass painting the magnificent architectural back-grounds, with figures on balconies and terraces, made familiar by the works of Paul Veronese ; he drew the human figure admirably, and was very successful in the repre-sentation of motion, and his arrangement of colour was perfect in harmony. It is not to be wondered at that his works produced so great an effect upon the minds of his contemporaries. They also show upon his part a consummate knowledge of technical conditions, and his windows are so perfectly executed that except where injured by violence or carelessness they are in admirable condition at the present time. Marsillat also executed some fine glass paintings at Cortona, which have been removed and are now in Florence.
Without entering upon the description of a consider-able number of interesting works of glass painting in various parts of Italy, and of detailed notices of the artists, enough has been done to illustrate the history and pro-gress of the art from the 13th century to nearly the first half of the 16 th. Later much less coloured glass came to be used, and a considerable change in taste took place in this art as in architecture ; it is shadowed forth even as early as 1501 in the following record:"Quod in dicto loco semper et pro omnia tempore in dicto loco et ecclesia sunt media die tenebre et ut vulgo dicitur buio et habeto colloquio cum consolibus Sancter Johannes de vietro faciat sumptibus opere unam finestram ex vitri cum ochi biacchi f regio d'allato et cum arme popoli." The desire for windows admitting more light led to the invention and adoption of a beautiful variety containing much white glass varied with yellow stain, and the introduction of a small quantity of coloured glass. These windows were painted with graceful arabesques executed in enamels and burnt in, and they never have been surpassed at any time for excel lence of drawing and skill of execution. Such are the windows in the Laurentian library at Florence, attributed to Giovanni da Udine, but erected after his death. Other examples might be referred to, and they are now of great value as showing the best description of ornamental window suitable for buildings of the developed Renaissance style, adorned with works of painting and sculpture. They have enough of colour to harmonize them with painted decora-tions, and enough of white glass not to destroy the chiaro-scuro of sculpture. It should be remembered that the 13th century produced beautiful windows possessing similar advantageous qualities suitable for mediaeval buildings con-taining modern sculpture, such as the famous five sisters of York cathedral, or the white windows of tlie same period at Chartres.
The technical execution of the glass paintings still existing in Italy resembles that prevalent elsewhere, and the method of the old masters is clearly described by Theo-philus. The glass used was either pot metal or coated; that is, the colour either pervades the whole body of the metal or is applied as a film over white glass, invariably the case with ruby, which would seem black but for this contrivance. The artist employed to design a window prepared a full size working drawing, which, according to Theophilus, was executed on a whitewashed board in his time ; but later these were made on paper, as is proved by the following contract from the archives of the cathedral of Florence (1437):" To Lorenzo di Bartoluccio (Ghiberti), sculptor, seven florins, being the half of the price of his skill and labour for drawing four figures on paper of bombagia (coarse cotton) for a window to be executed by Bernardo di Francesco, glass-painter, at three livres per figure." Guided by the firmly marked outlines, the glazier cut his mosaic of glass of the requisite colours by the aid of a hot iron, the diamond not being used for this purpose till the 17 th century. The pieces of glass were futher reduced to the shapes and sizes needed by the aid of the grosing iron, still a familiar instrument; thus prepared they were ready for the painter, and at the present time are attached to a transparent easel formed of a large sheet of glass. Theophilus thus describes the enamel colour prepared for painting with:"Take copper beaten small, burn it in a little pipkin until it is entirely pulverized, then take pieces of green glass and sapphire " (a blue paste used in mosaic work), " and pound them separately between two porphyry stones ; mix these ingredients together in the proportions of one-third powder, one-third green glass, and one-third sapphire. Grind them on the same stone with wine or urine, put them into an iron or leaden vessel, and paint the glass with the utmost care. For with the three colours " (that is, shades of one colour), " if you are diligent in the work, you can make the lights and shadows of the draperies in the same manner as in coloured painting. When you have laid on the first touches in the drapery w?ith the aforesaid colour, spread it in such a manner that the glass may be pure " (that is, untouched) " in that part which you are accustomed to make light in a picture, and let the touches be dark in one place and light in another " (that is, graduated), " and again yet lighter and distinguished with much care, that it may appear as if three shades of colour had been applied. This order you should observe below the eyebrows, and round the eyes, and nostrils, and chin, round the naked feet and hands, and other portions of the naked body; and thus let the glass painting have the appearance of a painting composed of many colours." It must be obvious from this description of the most ancient method of glass painting, in important respects the same as that still followed, that the art is regulated by certain necessary conventions distinguishing it from painting upon opaque surfaces. The features and accessories are drawn with sharp black lines making them out clearly to be seen at a distance, and in a very different wayfrom the representation of such details in apicture. The highest lights are the local colour, that is, the pure colour of the glass ; the deepest shadows are solid black produced by laying on the enamel thickly; all this is precisely the reverse of the system pursued at the same early period in painting pictures in which the shadows were the local colour and the lights white. As painting advanced it gradually approached more nearly to the scale of colour of the glass-painter, the lights instead of being white were painted of appropriate shades of the local colour, and the shadows were graduated into black in their deepest parts. It may have been that glass painting suggested this important change in ideas of colour and chiaroscuro. The glass being painted as described must be burnt to fix the enamel thereon with its flux. The separate pieces with the painted side upwards are placed upon the shelves of a kiln or iron box, covered with whiting ; the kiln is placed in a brick oven, and a fire kindled which surrounds it with flames. Much experience is needed to determine when the glass is sufficiently burnt to fix the enamel. When this has been done the fire is withdrawn, the oven is hermetically sealed, and the glass is left to cool gradually, so as to be annealed. From the appearance of many Italian windows as compared with those executed elsewhere, it is reasonable to infer that the Italians were less skilful than their northern contemporaries in firing their glass. The next process is to unite the numerous pieces of mosaic, thus painted and burnt, into panels of a given size, which is done by means of ribbons of lead grooved on both sides, the ribbons being soldered at all their angles, thus firmly tying the mosaic together. When the painted window is erected, the leads are seen internally in contrast with the transparent glass as intensely black lines, but externally, where they catch the light, as meandering lines of metal dividing the surface. Mistaken attempts have repeatedly been made to get rid of these lines, but they are constituent parts of the art, can only be got rid of by changing its true character, and are only disfigur-ing when injudiciously introduced, as for instance in some 14th-century glass in Santa Croce, in which the white of the eyes are fixed with leads of the form of spectacles, so that all the saintly persons represented appear, ridiculously enough, to wear these aids to defective vision. The panels, which are technically called glazing panels, are inserted in their places in the windows by means of grooves prepared in the stone work, into which they are secured with cement and strengthened at intervals with cross bars of iron called saddle bars, to which the glass is made fast by ties of copper wire. Thus fixed, experience has shown that painted windows if duly cared for will last for many cen-turies. Regarded in their connexion with past history, no monuments of art surpass painted windows in interest; they are only equalled by the paintings still extant in the tombs of the Egyptians as illustrations of faith, history, and customs. It was almost the universal usage that persons of every rank and position, as well as corporate bodies and guilds, made offerings of painted windows to churches. Whilst they give the most vivid ideas of ancient taste and methods of decoration, the religious opinions of successive ages are interpreted by the manner in which sacred subjects are selected or represented; the actual state of art at different times, from its rudest forms to the most perfect, is admirably exemplified; and, as it was customary to introduce the donors in appropriate parts of their gifts, the priuce or noble in knightly panoply, with his armorials, is seen kneeling, sometimes accompanied by his spouse. In like devout attitude the ecclesiastic appears in the robes and with the insignia of his offics, or the wealthy merchant and his wife in appropriate costume. In other examples trades-men, surrounded with the objects and materials of their commerce, sell to their customers articles of clothing or of food. Every occupation is illustrated in these ancient pictures in glass. How great then has been the loss to art and history caused by the reckless demolition of painted glass which has taken place in all parts of Europe, but especially in Italy, where the clergy have been the chief offenders ! The Italians have not been impelled to this lamentable destruction of these precious records and beauti-ful creations by any of the convictions, changes of faith, or fanaticism, fatal to so many works of art in other countries, but by unreasoning transformations of taste, by ignor-ance, or by the paltry desire of gain. In northern climates ancient glass is found to have been affected by the action of the weather, its outside surface is roughened and pierced to a small depth by little pits, or is so altered as to resemble sheets of zinc ; it is also covered with dirt, especially inter-nally, the enamel rougher than the surface of the glass favouring this incrustation. These injuries and accumulations veil the glass paintings, and produce a mysterious confusion of form and colour, admired by many, who to obtain in modern work such effects of ago, and to conceal the inharmonious crudity of colour, soil it artificially with varnish, paint, or enamel applied externally, or with dirt applied of set purpose, showing thereby their ignorance of art and their bad taste, by having recourse to processes which are outside the domain of art, for perfect harmony of colour and effect of chiaroscuro ought to be the results of the artist's cultivated knowledge and skill, and nothing should be done to hasten the obscuration of the windows or to anticipate the effects of time. Important improvements have been made in ike art of glass painting, as well as in the manufacture of glass, since Theophilus wrote his treatise, but some of these have not been favourable to the art; for instance, old glass is much quieter in colour than modern, and as it wras less diaphanous, and less smooth in texture, it was better adapted to the operations of the glass-painter. This being observed, eminent manufacturers of coloured glass in England and on the Conti-nent have prepared imitations of the soft, pleasing hues of old glass, and of its varied texture, with considerable success. The addition of an enamel brown from iron, and much warmer in tone than that made from copper, has been an important aid to glass painting, whilst about the middle of the 16th century numerous coloured enamels were invented, which, however ingenious and beautiful, subsequently modified the art unfavourably Vehicles composed of spirits of turpentine, fat turpentine, or gum Senegal have advantageously replaced the primitive distempers already alluded to. An important addition was the discovery of the yellow stain, made from silver, the only purely transparent colour applicable to the surface of glass. It has been described as the invention of the Beato Giacomo of Ulm, who practised glass painting at Bologna, and died in the odour of sanctity in 1491 ; but the yellow stain was characteristic of glass paintings for more than a century before his time. The introduction of the system of coating white with coloured glass in the same way as it had been coated with ruby glass, and the invention in the 15th cen-tury of removing portions of the coloured glass by abrasion or with the wheel so as to lay the white bare iu conformity with special designs, together with the practice of staining such white portions yellow, led to important modifications in the art. Embroideries on coloured garments and other ornaments were thus easily represented, and the system is very useful in painting heraldry. Instead of the wheel fluoric acid is now chiefly used to remove the film of coloured glass from its white backing. The leads with which the mosaic of glass is brought together in glazing panels have undergone various changes. Early lead-work is massive; but at the beginning of the 1 oth century it becomes lighter in appearance but without loss of strength, for although the grooves were made shallower, the lead, was increased in thickness and was rounded on the outer sur-faces. Alterations also were made in the method of fixing painted windows : the saddle bars, according to climate, were made more or less robust; and, instead of crossing the windows in straight horizontal lines, they were frequently bent to suit the design. Generally speaking, horizontal bars are not objectionable unless they cross faces, or hands and feet,an arrangement which ought carefully to be avoided. Mosaic windows should be made waterproof, and the saddle bars should be painted at intervals of time, as otherwise the rust injures the glass.
Enamelled glass, that is to say, white glass enamelled with colours, finally took the place of mosaic glass. In England in the last century glass-painters of merit who practised enamel painting have left considerable works, amongst whom Francis Eginton, Forrest, Henry Giles, Robert S. Godfrey, Jarves, and especially Jervais, who in 1717 executed from designs by Sir Joshua Reynolds the great west window of New College, Oxford. Whatever may be thought of their method, these glass-painters were meritorious artists. Reference has been made to some of the works of the admirable glass-painters of other countries besides Italy, but the object of the present paper has been to illustrate the history and practice of the art by Italian specimens hitherto little observed, and very inadequately and generally inaccurately described.
At the present time the art of glass painting is practised in different countries with very divergent views of its character and limits. Some think that the more nearly it can be assimilated to pictures in oil or fresco the better, whilst others maintain that all such resemblance is beyond its distinctive conditions as a branch of decorative art. It is a common but erroneous belief that the art of glass painting was lost, and that it has been revived in the pre-sent century. It survived in its latest form of the enamel method, classed by Mr Winston as the intermediate style, which is still carried out with unsurpassed skill in Germany. Undoubtedly the mosaic system had disappeared, and it has been judiciously and ably restored wherever glass painting is now common, although with different modifications and ideas of its nature. In England glass-painters possessed of much technical skill and cultivated knowledge of old forms of the art, have produced meritorious works within the limits of almost servile imitation, insisted upon by prevalent but mistaken sentiment; and in too many instances this imitation has reduced the art to the state of mere trade, so that at no period of its history have worse specimens been executed, too often found in the windows of the grandest monuments of mediaeval architecture, which ought to have been preserved from such profanation. Of the great value and interest of early painted windows, as well as of those of more matured art, every one who studies them with intelligence must be sensible; and this value and interest are increased by the fact that they illustrate, with perfect truth, the tastes and ideas, the faith and customs, of the periods during which they were created, but modern counterfeits do nothing of the kind, and can convey no such impressions to future times. It is only by the restoration of the old union which existed between the great artist and the glass-painter, dwelt upon in these columns, that the beautiful art of glass painting can be really restored, nor are we without a completely successful instance of the happy results of this union. A window in the parish church at Alnwick, designed by the late William Dyce, R.A., and painted on glass at Munich, is a magnificent specimen of the art, equal in design and execution to the works of its golden age.
In Germany the arts ornamental still flourish as branches of fine art. There, as generally throughout the Continent, the acquirement of a knowledge of ornament forms part of the curriculum of study of most artists; to the entire neglect of this in England, in academies of fine art, may be attributed the low estate into which these branches have fallen. The most eminent German artists of the present century have made designs for painted windows, which have been executed by highly-trained glass-painters, with that care which is so characteristic a national attribute. Such being the case, it remains a source of wonder that artists surrounded by precious remains of ancient genius remarkable for exquisite colour should notwithstanding show so little ability as colourists. They assimilate the coloured glasses of the best qualities, with every attention to the laws of harmony, but they do not bring them into union as the old masters did by forcible painting of the shadows and half tints. The shadows generally are too transparent, and the general effect is weak. The finest work of the Munich school of glass painting, and one of the best windows produced in the present century, is in the Parliament House Edinburgh. It is richer in colour than is usual; and, having been designed by the illus-trious Wilhelm von Kaulbach, the general composition is of a noble character.
In France an imitative school, resembling that which has been dominant in England, has executed skilful mimicries of ancient glass painting, and has restored successfully ancient windows broken or otherwise injured in revolutionary times or by neglect. Glass-painters of this class may be found in France, who dispute the supremacy in bad art of their island rivals. In efforts to escape from this abject imitation, trained artists have produced original works of considerable power of form and colour, but too many aim at a picturesque eccentricity, and an affected design, inconsistent with the grave beauty of the art ; and Jean Cousin and other great masters of the grand period of French glass painting have no successful followers, nor has any painted window been produced in France in the present century which equals that by the Scottish artist, William Dyce, or that by the German, Von Kaulbach.
In Italy there are glass-painters whose merits as draughtsmen, designers, and executants place them in the first rank, but their windows are almost invariably laborious imitations of pictures in oil ; they have undoubtedly lost the methods as well as the ideas and style of their great predecessors, whose windows they not unfrequently injure deplorably by their restorations. It is to be regretted that artists so ad-mirably trained in many respects should so little compre-hend the magnificent works of former times which would be their best models, and which they have skill enough to rival but for their vicious method of execution.
In Belgium the art is practised with considerable skill, and works of merit have been executed, but here, as elsewhere, the modern glass-painter is inferior to his predecessors, although he is surrounded by so many admirable specimens of ancient art.
The following works on glass painting may be advantageously studied:Ferdinand de Lasteyrie, Histoire de la peinture sur verre d'après ses monuments en France, Paris, 2 vols, folio, 1852 ; Id.; Quelques mots sur la Théorie de la peinture sur verre, Paris, 12mo, 1853 ; Id., Notice sur les vitraux de l'abbaye de Rathhausen, canton de Lucerne, Paris, 1856 ; A. Lenoir, Histoire de la peinture sur verre et description des vitraux anciennes et modernes, &c., Paris, 1803 ; Id., Notice historique sur l'ancienne peinture sur verre, sur les moyens pratiques dans cet art depuis l'époque de son invention jusqu'à nos jours, et par suite sur Jean Cousin, qui a excellé dans le même art, Paris; E. H. Langlois, Essai historique et descriptif sur la peinture sur verre ancienne et moderne, Eouen, 1832; Pierre le Vieil, L'art de la peinture sur verre et de la vitrerie, Paris, 1774 ; Glass: the manner home to anneile or paint in glass: the true receptes of the cullers,, 1616 ; Gessert, Rudimentary Treatise on Painting on Glass, London, 1851; Mrs Merrifield, Ancient Practice of Painting in Oil, Miniature, Mosaic, and on Glass, &c., London, 1849 ; Charles Winston, An Enquiry into the difference of style observable in ancient Glass Painting, especially in England, Oxford, 1847 ; Padre L. V. Marehese, Memorie dci pin insigni Pittori, Scultori, e Architetti Domenicani, Florence, 1846 ; G. Gaye, Carteggio inedito d"artisti dei secolixiv., xv., xvi., 3 vols., Florence, 1839 ; Gaetano Milanesi, Le Opère di Georqio Vasari con nuove annotazioni e commente, Florence, 1879. (O. H. W.)
See introduction to Catalogue of Glass Vessels in the South Ken-sington Museum, where an engraving of it is given.
An example connected with the specimens just described is the
cup belonging to Baron Lionel de Rothschild; though externally of an opaque greenish colour, it is by transmitted light of a deep red. On the outside, in very high relief, are figures of Bacchus with vines and panthers, some portions being hollow from within, others fixed on the exterior. The changeability of colour may remind us of the " cálices versicolores " which Hadrian sent to Servianus.
A large proportion of the obscuring of the commoner Tarieties of glass and of the obscured ornamentation on plate and sheet as well as on flint glass is now produced by means of Tilghman's sand Mast described below.
Tins circumstance has led to the erroneous statement, repeated to the present time in every guide-book and by every writer on the cathedral, and to the still prevalent belief, that the three windows in the fagade of the church were designed by Ghiberti, who has himself recorded : '' I designed for the front of Santa Maria del Fiore, for the central round window of the fagade, the Assumption of our Lady, and I designed those on each side." The windows which Ghiberti really designed were the great circle of the facade, a noble work still in its place, and the two of the clerestory on each side of it, long since removed and lost. That they were painted by Fra Bernardino di Stefano in 1423 is thus shown in the archives of the cathedral"Fra Bernardino di Stefano, of the order of the Preachers of Santa Maria Novella, is to execute two round windows in the nave of Sa Maria del Fiore, one to the right and the other to the left,that to the right representing Joachim driven from the Temple, that to the left the Death of the Virgin Mary, and the designs are by Lorenzo di Bartoluccio (Ghiberti)." It is thus evident that the windows of the clerestory of the nave were to he painted with the life of the Virgin Mary, of which these two were the first and last of the series, whilst the Assumption of the Virgin appropriately occupied the centre. Those in the clere-story having disappeared, it has invariably been supposed that the two remaining at the ends of the aisles are those described by Ghiberti, although they differ in subject and entirely in style from his work. They are in fact thus mentioned in the archives :" In 1414 Nicolo di Pietro della Magna painted the two windows on each side of the front of the cathedral," which record settles the question.