1902 Encyclopedia > Pigments


PIGMENTS are coloured powders which, when mixed with oil, water, or other fluids, in which they are in-soluble, form paints. They are distinguished from dyes and washes by their entire insolubility in the irfedia in which they are mixed, whereas dye-stuffs are tinctorial substances applied in solution. Insoluble colours, when used in printing textile fabrics, are distinguished as pigment colours. The sources of materials available as pigments are numerous; many are native coloured earths, others are separated from native metallic compounds and other mineral substances; a large number are artificially pre-pared from inorganic-—principally metallic—sources ; an important class consist of animal and vegetable colouring principles, forming with earthy bodies insoluble powders called lakes; and the dye-stuffs artificially obtained from organic sources are also similarly utilized. In fact all substances coloured or neutral, capable of being presented in the form of impalpable powder, which at the same time are insoluble and unalterable under ordinary atmospheric influences, may be regarded as possible pigments. But there are many qualities practically essential in a pigment which limit the range of available substances. A con-sideration of the first importance is the " body" or cover-ing power of a pigment,—that is, the property of fully covering and concealing with an opaque coating the surface over which it is spread. It is also important that the material should work well in, and be unaffected in appear-ance and constitution by the medium with which it is made into a paint, and that it should spread in an even uniform coat, which should dry well and quickly in the air and adhere firmly to the surface to which it is applied. When dry it should possess durability and resist change under the action of weather and other influences to which paint is exposed. These are the principal qualities re-quisite in paints in their important function of preserva-tive coatings for the surfaces to which they are applied. On their artistic side, as decorative and pictorial materials, pigments should possess purity and brightness of colour with intensity of tinting power, capacity for mixing or coming into contact with other colours without injuriously affecting these or being themselves deteriorated, and permanence and unalterability of tone after long exposure.

Pigments being so numerous and so diverse in their origin, the industries connected with their production and preparation are of necessity varied in character. Many of the substances employed being used in large quantities in other important industrial relations, as well as for paints, are manufactured on a large scale and constitute the basis of considerable chemical industries, as, for example, the manufacture of white lead, Prussian blue, ultramarine, the chrome materials, &c. In other cases the materials require no preparation other than that given to them by the paint-grinder or the artists' colourman, according to the purpose for which the substances are to be prepared.

The colour trade embraces two distinct departments:— ; that of the paint-grinder, who manufactures and com- : pounds the pigments used by artisans, house-painters, and paper-stainers; and that of the artists' colourman, who prepares and supplies the finer, more brilliant, and exten-sive assortment of pigments used for artistic purposes. | The pigments employed for pottery painting and glass and enamel work are a special class of preparations to suit the requirements of these trades. Leaving out of account the chemical reactions involved in preparing raw materials, the ordinary manufacturing operations connected with the preparation of painters' colours are simple, and consist essentially of a careful system of grinding. Formerly, when painters ground their own colours, a stone slab and muller formed the entire apparatus ; but now, when paint-grinding has become a separate industry, efficient machinery has been devised for grinding and its collateral operations. Bulky and rough colours such as whiting and common ochres are dry-ground under heavy edge stones which revolve in a strong iron bed. Ordinary dry colours requiring to be pulverized with more care are mixed to a thin cream with water, which is fed into and ground principally between a pair of millstones dressed and mounted like the ordinary horizontal stones of a flour mill, but smaller in diameter. For fine colours the pigment so ground is levigated, or floated into a vat in which the heavier particles sink, and the lighter, more finely divided portion is run into another vessel at a lower level, where it is deposited as a fine sediment. The sediment is dried in a uniformly heated stove, and when thoroughly dry is again pulverized under a pair of edge stones, and sifted or winnowed; so treated it is ready for use as dry colour. The greater proportion of the white lead and the other common oil paints are ground in oil. For this purpose the raw material is mixed in a machine with oil (sometimes boiled) to the consistence of a stiffish paste, and in this state it is ground in horizontal paint millstones, after which it requires no further preparation than the necessary thinning with oil when to be used for painting. There are many varieties of apparatus used for grinding both dry and oil colours.

The artists' colourman grinds his pigments with much greater labour, and selects his materials in a more care-ful manner, than is necessary in the case of the ordinary paint-grinder. Pigments for artistic painting in oil are ground in that medium to a definite consistency, and are put up for use in convenient compressible tubes of tin. For water colours the pigments are prepared principally in the form of small indurated cakes or as " moist colours " contained in small porcelain dishes. Water colours may also be obtained thin in tin tubes like oil colours, or as "pastilles," which are thin round cakes intermediate in condition between cake and moist colours.

In enumerating the principal commercial pigments it is usual and convenient to classify them according to their tints. They are not, as a rule, definite chemical com-pounds : many indeed are mixed substances prepared by processes and according to recipes known only to their makers; and, while the same commercial name is fre-quently given to substances quite dissimilar in character, the confusion is further increased by applying many different titles to substances which are practically identi. cal. Thus white lead is known by at least a dozen names, and distinct and even conflicting qualities are by autho-rities attributed to this one substance under its various aliases.

It would be impossible to catalogue all the paints met with in commercial lists, and it would serve no good purpose to enumerate the whole of the pigments which might be and are occasionally used. Premising that details regarding many of the substances will be found under the heading of the metals, etc, whence they are derived, we shall here simply classify, according to their colour, the principal well-recognized pigments of commerce, adding brief remarks regarding each class.

WHITE PIGMENTS.—The whites are the most important pigments used by painters, forming as they do the basis or body of nearly all paints, excepting only certain dark hues. Good available whites are limited in number, and all of real importance are included in the following list:—white lead, a carbonate of lead (chiefly); zinc white, oxide of zinc, called also Chinese white; antimony white, oxide of antimony; fixed white, sulphate of baryta; " silicate " white, sulphate of baryta, or strontia and sulphide of zinc; mineral white, powdered gypsum (with alumina it forms satin white); chalk or whiting carbonate of lime- and china clay, silicate of alumina.

White Lead (see LEAD, vol. xiv. p. 378) is the most important of all pigments, and forms the basis of nearly all ordinary oil paints, which, when coloured, consist of white lead tinted with the necessary coloured pigments. It possesses the greatest amount of body or covering power, and works beautifully in oil, with which it partially combines, drying as a hard homogeneous adherent plaster. On the other hand it is a most poisonous body, very injurious to the persons connected with many of the processes by which it is prepared. As an oil colour it darkens gradually in an atmosphere containing traces of sulphur; it cannot be used at all as a water or distemper colour; and it acts injuriously on the colour of several important pigments. Notwithstanding these drawbacks no white has yet been made that can compete with white lead, although paint manufacturers go far to provide a substitute by adulterating it to such an extent that the white lead frequently bears only a small ratio to the adulterant. Baryta white is the ordinary adulterant, and among respectable manufacturers the intermixture is a well-understood fact, and the relative proportions of white lead and baryta are regulated by a series of grades passing from " genuine " to No. 5 or No. 6 white lead. Many efforts have been made to substitute for ordinary white lead lead carbonates made by other processes, and other lead salts such as the oxychloride (Pattinson's), sulphate, tungstate, antimoniate, &c. ; but none of these has proved permanently successful.

Zinc White.—Next in importance to white lead, is an oxide of zinc prepared by the sublimation and combustion of metallic zinc. The pigment is deficient in covering power and it dries but slowly when mixed with oil. On the other hand it is not injurious to health, its purity of tone is not affected by sulphurous air, it does not affect tints added to it or with which it comes in contact, and it can be used in water as well as in oil. Like white lead it is very much adulterated, and generally with the same agent—baryta white.

Baryta White plays an important independent part as well as acting so extensively as a sophisticator of other pigments. It is prepared by grinding to a fine powder the pure white native sulphate of baryta (heavy spar), and the same substance artificially prepared is known as permanent white or Mane fixd. The artificial preparation is much superior, as a pigment, to the powdered spar ; but both are deficient in body, notwithstanding which they are of great value to paper-stainers and for distemper painting.

Under the name of Charlton White or silicate paints, Mr J. B. Orr prepares a range of white paints which have come into exten-sive use. The pigment as originally prepared under Mr Orr's patent of 1874 consisted of an intimate mixture of artificial sul-phate of baryta and sulphide of zinc in certain proportions, made by the double decomposition of solutions of barium sulphide and sulphate of zinc. In 1881 a patent was secured by Mr Orr for a combination in which strontia takes the place of baryta. It is claimed for these pigments that they possess body greater than white lead, that they are non-poisonous, and that with certain modifications in the manufacture they can be made quite as valuable for distemper painting as for oil colours.

The oxides of antimony, tin, bismuth, &e., form white pigments ; but these possess no peculiarities which render them valuable for painters' use. The carbonate of lime, more or less pure and in various degrees of pulverulence under several names, such as Chalk White, Paris White, Whiting, &c, is very extensively used in distemper work for walls, roofs, &c., and in paper-staining, occupy-ing in these relations the important place held by white lead in oil painting. Mineral white or satin white consists of powdered gypsum and alumina, a preparation very largely used by paper-stainers for their glossy satin bodies. There are several other white earths of relatively little importance as pigments.

BLUE PIGMENTS.—The list of blue colours of real importance is not extensive, comprising, as principal items, ultramarine, Prussian blue, the cobalt blues, and indigo. The following list embraces the names and varieties ordinarily recognized in commerce :—ultra-marine (native), powdered lapis lazuli ; ultramarine (artificial), silicates of alumina and soda with sulphide of sodium ; Prussian blue, cyanide of iron ; Paris blue, modified Prussian blue ; Antwerp blue, fine Prussian blue ; smalts, a cobalt glass ; azure blue, a preparation of smalts ; cobalt or Thenard's blue, sub-phosphate of cobalt ; cau-uleum, stannate of cobalt and sulphate of lime ; mountain blue, native carbonate of copper ; lime blue, carbonate of copper and lime ; Verditer or Bremen blue, hydrated oxide of copper ; indigo from species of Indigo/era ; indigo carmine, prepara-tion of indigo.

Apart from the important colours ULTRAMARINE, PRUSSIAN BLUE, and INDIGO, separately noticed, these blues, which are not of much value for painters, owe their colour principally to cobalt and copper. The principal cobalt colour is Smalts, called also strewing smalts, cobalt glass, zaffre, Saxony blue, &c. It is prepared by smelting together the mineral arsenide of cobalt, pure sand, and carbonate of potash into a glass. The molten glass is cast into cold water, then ground fine and levigated. Smalts is chiefly available for distemper and fresco painting, and is not much used as an oil colour. Azure Blue is generally recognized as a preparation of smalts, but the name is given to several compounds. Cxruleum is a light blue colour of durable quality with a greenish tinge, consisting of a combination of cobalt oxide with stannic acid ; and Cobalt Blue, the subphosphate of cobalt, a colour discovered by Thenard, possesses a purple tinge. Carbonate of copper, either in the form of the mineral azurite or artificially prepared, is a principal source of the copper blues, which, however, possess little value as pigments owing to their tendency to blacken under exposure. Blue Verditer, a greenish blue which passes into green verditer, is a hydrated oxide of copper.

YELLOW PIGMENTS.—The following list includes the ordinary yellow colours of commerce :—ochres and sienna earth, native earths tinted with iron ; Mars yellow, hydrated ferric oxide ; chromes, chromâtes of lead and other metals ; massicot, protoxide of lead ; Naples yellow, antimoniate of lead ; mineral yellow, basic chloride of lead ; aureolin, nitrate of potassium and cobalt ; cadmium yellow, sulphide of cadmium ; orpiment, trisulphide of arsenic ; Indian yellow, urio-phosphate of calcium ; gamboge, resin of Garcinia ; Dutch pink, a vegetable lake ; yellow lakes.

Of these colours the more important are the ochres and the various combinations containing chromium. The Yellow Ochres are native earths coloured with hydrated ferric oxide, the brownish yellow substance that colours, and is deposited from, highly ferruginous water. These ochres are of two kinds—one having an argillaceous basis, while the other is a calcareous earth, the argillaceous variety being in general the richer and more pure in colour of the two. Both kinds are widely distributed, fine qualities being found in Oxford-shire, the Isle of Wight, near Jena and Nuremberg in Germany, and in France in the departments of Yonne, Cher, and Nièvre. The original colour of these ochres can be modified and varied into browns and reds of more or less intensity by calcination. The high heat expels the water of hydration from the iron oxide, changing it into red ferric oxide. The nature of the associated earth also influences the colour assumed by an ochre under calcina-tion, aluminous ochres developing red and violet tints, while the calcareous varieties take brownish red and dark brown hues. The well-known ochre Terra da Sienna which in its raw state is a dull-coloured ochre, becomes when burnt a fine warm mahogany brown hue highly valued for artistic purposes. Yellow ochres are also artificially prepared—Mars Yellow being either pure hydrated ferric oxide or an intimate mixture of that substance with an argillaceous or calcareous earth, and such compounds by careful calcination can be transformed into Mars Orange, Violet, or Red, all highly important, stable, and reliable tints. The metal chromium owes its name to the intense coloration produced by the combination of its oxide, chromic acid, with various metals and alkaline earths. Several of these salts are soluble, but those which form pigments are insoluble compounds. The principal chrome pigments—the various shades of lemon and yellow chrome deepen-ing to orange tints—are composed of the neutral chromate of lead, the difference of hue depending on the greater or smaller proportion of lead used in the preparation. The basic chromate of lead has a deep orange colour passing into the minium-red-like hue of chrome red. Strontia Chrome, the chromate of strontium, is a pale lemon pigment of fine quality and permanence. With zinc, chromic acid forms two combinations, neutral and basic, both possessed of an intense yellow colour ; and chromate of barium also furnishes a useful yellow colour. Lead itself, without chromium, is the basis of several valuable yellows. Massicot, the protoxide of lead, is a clear yellow pigment deficient in body. Naples Yellow, a colour highly esteemed by early artists, is an antimoniate of lead which in early times was obtained from native sources; and Mineral Yellow is an oxychloride of lead. The sulphide of cadmium forms the fine durable Cadmium Yellov), a colour now much appreciated for artistic use. The arsenical yellow, Orpiment, is now little used as a pigment, although formerly, under such names as King's Yellow, Imperial Yellow, and Chinese Yellow, it was held in high esteem by artists. Aureolin, a nitrate of potassium and cobalt, is a colour of recent origin which has come into high favour among artists. Indian Yellow is a colour of animal origin of no perma-nence, and Gamboge is a gum resin yielded by trees of the genus Garcinia, principally employed as a water colour. The yellow lakes are comparatively unimportant, but some, known, rather absurdly, as Dutch, English, or Italian Pink, are largely used in paper-staining.

RED PIGMENTS embrace two distinct series of substances—the reds of inorganic origin, and red lakes obtained from animal and vegetable colours. The principal commercial varieties are as follows :—rouge, Turkey red, and Indian red, red ferric oxide ; Venetian red, ochreous ferric oxide ; ochres, earths coloured by ferric oxide; vermilion and cinnabar, sulphide of mercury; antimony vermilion, red sulphide of antimony ; Derby red, a form of chrome red ; red lead or minium, red oxide of lead ; chrome red, basic chromate of lead ; realgar, bisulphide of arsenic ; madder lake, alizarin and alumina; madder carmine, preparation of aliza-rin ; carmine lake, cochineal red and alumina; carmine, preparation of cochineal; wood lakes, from various red dyewoods.

The principal mineral reds owe their colour to oxides of iron and to compounds of mercury. The reds due to iron are closely allied to the yellow ochres and other ferruginous pigments. As already explained in connexion with these yellows, tints passing through orange to deep purple reds are obtained by calcination of yellow hydrated ferric oxide, and in this way a great variety of ruddy and red tints are prepared. The proportion of ferric oxide in these compounds ranges from pure oxide to combinations in natural ochres containing not more than 2 or 3 per cent, of iron. Rouge or Mars Red, Crocus, Indian Red, and Turkey Red are all pure ferric oxide, varying in depth of tint from having undergone different degrees of calcination, or from being made from different artificial or natural sources. The other iron reds are all of the nature of ochres—some of them, such as Venetian Red, being artificial compounds. These reds form exceedingly useful durable colours which do not injuri-ously affect the tints with which they are associated. Of red colours from mercury, Cinnabar and Vermilion are the most import-ant, the former being the native and the latter an artificial sul-phide of mercury (see MERCURY, vol. xvi. p. 34). Vermilion is one of the most pure, brilliant, solid, and durable of all colours. Its beauty is largely affected by the smoothness of the powder to which it is reduced, and in this respect that obtained from China is of the highest excellence. Being a costly pigment, vermilion is freely adulterated with other reds, a fraud easily detected by the perfect volatility of the genuine substance. From mercury com-bined with iodine is prepared a pigment of unequalled vivacity and brilliance, Iodine Scarlet, but unfortunately as fugitive as it is bright, and consequently not available for work requiring perma-nence. The principal red colour from lead is Minium or Red Lead, a pigment of great antiquity obtained as a product of the oxidation of massicot, or by the calcination and oxidation of white lead. It is orange red in colour, of good opacity and body, but it has the fault of white lead and lead colours generally, blackening in con-taminated air and injuring colours with which it comes in contact. By itself it is a valuable paint for first coating exposed iron surfaces to prevent their oxidation, and it is an excellent dryer, on which account it is much used in preparing boiled oil for painters. Chrome Red, a basic chromate of lead known also as Persian or Derby Red, is a brilliant pigment ranging in tone from orange to a deep vermilion hue. It is obtained by precipitating a solution of acetate of lead with bichromate of potash, with the addition of more or less of caustic potash or soda,—the proportion of the latter addition determining the depth of resultant tone. Antimony Vermilion is the red variety of the sulphide of antimony which, as found in nature (stibnite), is a dark grey body with metallic lustre. This, when fused and kept some time at a high heat and suddenly cooled, by allotropic modification becomes a fine vermilion red. The colour is artificially prepared by acting on solutions of the butter of antimony (antimony chloride) with hyposulphite of soda or lime. It is a colour of excellent purity and body as a water colour, but unfortunately it becomes brown by exposure. The lakes form a numerous and important class of red pigments. A lake is a com-bination of a colour of organic origin with a metallic oxide or salt, commonly with alumina. Originally all lakes were red colours, trie name being derived from the lac insect Coccus lacca, the colouring matter of which forms the lake now known as Lac Lake. But lakes of any colour or tint are now made. The most important lake pigment is Madder Lake, a compound of alumina and the tinctorial principle of madder root, Bubia officinalis, but now made with artificial alizarin. Scarlet or Carmine Lake has cochineal for its colour basis, and there are corresponding lakes from lac, kermes, &c. Wood Lakes coloured with several of the red dyewoods have little durability, but they are nevertheless largely used by paper-stainers. Carmine, a colouring matter from cochineal, and Madder Carmine or Field's Carmine, from madder, are exceedingly brilliant colours ; but the first of them is of a fugitive character.

GREEN PIGMENTS form an extensive group embracing two sections :—(1) simple greens, in which green is a primary inherent or natural colour ; and (2) compound greens, made up of intimate mixtures of blue and yellow pigments. The latter class it is obvious are capable of indefinite modification by simply varying the proportions of the compound ingredients. The following list embraces the principal commercial greens :—Brunswick green, oxychloride of copper ; malachite green or mountain green, hydra ted carbonate of copper ; verdigris, sub-acetate of copper; verditer or Bremen green, hydrated oxide of copper ; Seheele's green, arsenite of copper ; Schweinfurt green, mixed acetate and arsenite of copper; emerald green, a variety of Schweinfurt green ; mineral green, mixed copper oxide and arsenite ; chrome green, oxide of chromium; Guignet green or veridian, hydrated oxide of chromium; Cassei green, manganate of baryta; cobalt green, oxides of cobalt and zinc ; ultramarine green, modified artificial ultramarine ; Veronese earth or terra verde, a form of ochre ; green lakes.

The greater proportion of these greens are copper compounds— the most brilliant of them containing also arsenic. They are all poisonous colours, the latter especially being dangerous poison; and there can be no doubt that their free use in wall papers, the colour-ing of toys, artificial flowers, &c, is frequently the source of dangerous disease and even death. Brunswick Green, the most important non-arsenical green, is an oxychloride of copper, but factitious Brunswick greens are not uncommon. Seheele's Green, the arsenite of copper, and Schweinfurt Green, mixed arsenite and acetate of copper, are very powerful and brilliant colours. These copper greens all blacken in foul gases and when mixed with oil, and thus, although they possess great body, they are much more useful to the paper-stainer than the painter. The sesquioxide of chromium both water-free and hydrated, prepared in various ways, forms important stable green colours which resist atmospheric influences ; and chromium is further the basis of several other green colours, which, however, are not of importance. Cobalt Green, a mixed oxide of cobalt and zinc, discovered by the Swedish chemist Rinman, is a valuable and durable but expensive colour. Cassei Green, called also RosenstiehVs Green, is a fine innocuous pigment made by melting together sulphate of baryta and oxide of manganese, and carefully washing the resulting mass in water. Verona Green or Terra Verde, a natural celadon green highly valued by artists for permanence, is a mixed earthy body coloured by ferrous oxide, and Ultramarine Green, also a stable body, is an intermediate pro-duet of the manufacture of ultramarine blue.

BROWN PIGMENTS.—Many of the painters' browns are simply tints obtained by mixture. In the case of simple pigments the shades pass by fine gradations into yellows and reds, so that the limits of classification are not well defined. The following are generally classed as pure browns :—umber, silicate of iron and man-ganese ; brown ochres, called Mars brown, iron brown, &c, native and artificial earths ; Vandyke brown and Cologne or Cassei brown, peaty ochres ; purple brown, ferric oxide ; Spanish brown or tiver, a brown iron ochre; bistre, washed beechwood soot; sepia, secre-tion of cuttle-fish ; brown lake ; asphaltum, natural and artificial pitch.

Iron and manganese, separately or combined, earthy or pure, are the sources of the principal brown pigments. Some of them are intermediate products between yellow ochres and red ochres by calcination of the yellow, and, as they are ochreous in their nature, their colours may be heightened or otherwise modified by calcining. Thus Umber, which properly is a hydrated silicate of manganese and iron, is brightened in colour by calcination into Burnt Umber. The finest umber comes from the island of Cyprus, and is known as Turkey umber. Large quantities also of "English" umber are mined in Devonshire and Cornwall. Real Vandyke Brown, a very celebrated pigment, ought to be a kind of bituminous peaty earth of a fine rich semi-transparent colour, allied to which are Cologne and Cassei Earth. But under the name Vandyke brown pure ferric oxide and ferruginous earths of a clear brown hue are also sold. Cappagh Brown is a peaty earth coloured by manganese, found at Cappagh near Cork, Ireland, and is a valuable artists' colour, as is also Bistre, a brown washed from the soot of beechwood. Sepia, a much valued warm brown, is a substance secreted by the cuttle-fish, Sepia officinalis, which emits it to cloud the water for conceal-ing its whereabouts when alarmed.

BLACK PIGMENTS form a numerous class of bodies, though those in common use are easily enumerated. They appear in commerce principally under these names :—vegetable black, carbonized vegetable matter ; lamp black, soot of oils and fats ; Indian ink, preparation of lamp black ; ivory black, carbonized ivory and bone ; bone black, carbonized bone ; blue black, washed wood charcoal; charcoal black, carbonized wood ; black wad, a native oxide of manganese ; black lead, a form of carbon ; tar, from distillation of organic substances.

Most of these blacks owe their colour to carbon. From the charring of vegetable substances are prepared Charcoal Black, Blue Black, and Vegetable Black, but these take many names according as they are prepared from carbonized wood, twigs of the grape vine, peach and other fruit stones, cork, the lees of wine, &c. Bone and Ivory Blacks again are carbonized animal substances, principally bones, which when skilfully burned yield dense durable blacks. Lamp Black of the best quality is the soot deposited from the imperfect combustion of oils and fats, and the soots of resin and tar are also collected and used under this name. Indian Ink (see vol. xiii. p. 80) is a form under wdrich lamp black of the finest quality occupies an important position among pigments. Of the other blacks Tar is the most important owing to its extensive use as a preservative and antiseptic coating.

Several pigments are prepared on account of special properties apart from the protective and decorative purposes for which ordinary paints are applied. Among such may be mentioned Balmain's luminous paint, a preparation in oil or water of certain of the phosphorescent sulphides. Objects coated with this material have the property of continuing to emit light in dark situations for some time after they have been exposed in daylight or to high artificial light. The luminous paint has been proposed for coating buoys, signals, public notice boards, clock and watch dials, playing balls, match boxes, &c, but it has not come into extensive use. Powdered asbestos has been introduced as a fire-proof paint for wood; but all common paints applied as distemper colour are equally fire-proof in the sense that they themselves are incombustible, and when they coat wood thickly they offer great resistance to an incipient fire, and even retard combustion under very high heat. Numerous anti-fouling compositions for the painting of ships' sides and bottoms and anti-corrosive, inoxidizable, damp-proof, and water-proof paints have been patented, some of which are in extensive use. (J. PA.)

The above article was written by: James Paton.

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