1902 Encyclopedia > Alum


ALUM, a compound salt employed in dyeing and various other industrial processes. It is soluble in water, has an astringent, acid, and sweetish taste; reddens vegetable blues, and crystallizes in regular octahedrons. When heated, it liquefies; and if the heat be continued, the water of crystallization is driven off, the salt frothes and swells, and at last a white matter remains, known by the name of burnt alum.

Its constituents are sulphuric acid, alumina, an alkali and water. The alkali may be either potash, soda, or ammonia. Hence there are three district species of slum, depending upon the nature of the alkali which each contains. Potash alum (in which the alkali is potash) is the common alum of this country, although both soda alum and ammoniacal alum are manufactured. The term alum is now used in chemistry as a generic one, and is applied to the class of double slats formed by the union of the sulphates of alumina, chromium, or iron with the sulphates of the alkalies. The composition of the ordinary potash alum is represented by the formula AIK(SO4)2 12H2O.

The progress made by chemists in the discovery of the constitution of alum was very slow. The species first investigated was potash alum. That it contained sulphuric acid as a constituent was known even to the alchemist. Pott and Marggraff demonstrated that alumina was another constituent. Pott, in his Lithogeognosia, showed that the earth of alum, or the precipitate obtained when an alkali is poured into a solution of alum, is quite different from lime and chalk, with which it had been confounded by Stahl. Marggraff went much farther. Her not only showed that alumina is one of the constituents of alum, but that this earth possesses peculiar properties, is different from every other substance, and is one of the ingredients in common clay ("Experiences faites sur la Terre d’Alun," Marggraff’s Opusc. Ii. 111). Marggraff showed likewise, by many experiments, that crystals of alum cannot be obtained by dissolving alumina in sulphuric acid, and evaporating the solutions. The crystals formed are always soft, and quite different in their appearance from alum crystals of alum ("Sur la Regeneration de A’Alun," Marggraff’s Opusc. Ii. 86). He mentions likewise that manufacturers of alum is general were unable to procure the salt without a similar addition, that at first it had been customary to add a quantity of putrid urine, and that afterwards a solution of carbonate of potash was substituted in its place. But subsequent chemists do not seem to have paid much attention to these important observations of Marggraff; they still continued, without any rigid examination, to consider alum as a Sulphate of alumina.

Bergmann indeed had observed that the addition of potash or ammonia made the alum crystallize, but that the same effect was not produced by the addition of soda or of lime ("De Confectione Aluminis," Bergmann’s Opusc. i. 225). He had observed likewise that Sulphate of potash id frequently found in alum. He decomposed the solution of alum by means of ammonia, evaporated the filleted liquid to dryness, and exposed the residue to a red heat. A quantity of Sulphate of potash often remained behind in the crucible (ibid, p. 326). From these facts be drew the conclusion that Sulphate of potash readily combines with Sulphate of alumina.

After Klaproth had discovered the existence of potash as an ingredient in luecite and lepidolite, it occurred to Vauquelin thatit was probably an ingredient appearance during the analysis of stony bodies; and, considering that alum cannot be obtained in crystals without the addition of potash, he began to suspect that this alkali constituted an essential ingredient in the salt. A set of experiments, undertaken on purpose to elucidate this important point, soon satisfied him that his conjecture was well-founded. Accordingly, in the year 1797 he published a dissertation demonstrating that alum is a double salt, composed of sulphuric acid, alumina, and potash (Annales de Chimie, xxii. 258). Soon after, Chaptal published the analysis of four different kinds of alum namely, Roman alum, Levant alum, British alum, and alum manufactured by himself. This analysis led to the same result as that of Vauquelin (Ann. De Chim. xxii. 280).

Since that time alum has been admitted by chemists to be triple salt, and various analyses of it have been made to determine its constituents. Vauquelin (Ann. De Chim. xxii. 258), successively published the results of their experiments. These analyses gradually led to an accurate knowledge of the remarkable of the composition of this slat.

One of the most remarkable differences between the three species of alum is the solubility of each in water. At the temperature of 60°, 100 parts of water dissolve---
9°37 parts of ammoniscal alum,
14°79 parts of potash alum,
327°6 parts of soda alum.

This great solubility of soda alum renders the manufacture of it very difficult. It does not easily crystallize; indeed, when the weather is not, crystals of it can hardly be venient and more economical for dyers and caliso-printers, provided it could be furnished at the same rate with common alum. But the greater difficulty attending the making of it would probably prevent it from being saleable at a price sufficiently low to make it available as a mordant.

Soda alum was first mentioned by Mr. Winter in 1810, in his account of the Whitby alum processes (Nicholson’s Jour. xxv. Pp. 254, 255); but before that time it had been made by Mr. Charles Macintosk of Crossbasket. Mr. William Wilson, at Hurlet, near Glasgrow, afterwards made it in considerable quantities. Specimens of it have been sent by Dr. Gillies from the neighborhood of Mendoza, in South America, where it occurs native in considerable quantity.

These three different species of alum differ also somewhat from each other in their specific gravities, which are as follows: ---

Ammoniacal alum ------------------------------- 1°56
Potash alum--------------------------------------- 1°75
Soda alum----------------------------------------- 1°88

The word alumen, which we translate alum, occurs in Pliny’s Natural History. In the 15th chapter of his 35th book he gives us a detailed description of it. By comparing this with the account of (GREEK WORD) given by Dioscorides in the 123d chapter of his 5th book, it is obvious that the two are identical. Pliny informs us that alumen was found naturally in the earth. He calls it salsogoterræ. Different substances , he informs us, were distinguished by the name of alumin; but they were all characterized by a certain degree of astringency, and were all employed in dyeing and medicine. The light-colored alumen was useful in brilliant dyes, the dark-colored only is dyeing black or very dark colors. One species was a liquid which was apt to be adulterated; but when pure it had the property of striking a black with the juice of the pomegranate. This property seems to characterize a solution of Sulphate of iron in water. It is quite obvious that a solution of our alum would possess no such property. Pliny says that there is another kind of alum which the Greeks call schistos. It forms in white threads upon the surface of certain stones. From the name schistos, and the mode of formation , there can be little doubt that this species was the salt which forms spontaneously on certain slaty minerals, as alum slate and bituminous shale, and which consists chiefly of sulphate of iron and sulphate of alumina. Possibly in certain places the sulphate of iron may have been nearly wanting, and then the salt would be white, and would answer, as Pliny says it did, for dyeing bright colors. Several other species of alumen are described by Pliny, but we are unable to make out to what minerals be alludes.

The alumen of the ancients, then, was not the same with the alum of the moderns. It was most commonly a sulphate of iron, sometimes probably a sulphate of alumina, and usually a mixture of the two. But the ancients were unacquainted with our alum. They were acquainted with sulphate of iron in a crystallized state, and distinguished it by the names of misy, sory, chalcanthum (Pliny, xxxiv. 12). As alum and green vitriol were applied to a variety of purposes in common, and as both are distinguished by a sweetish and astringent taste, writers, even after the discovery of alum, do not seem to have discriminated the two slats accurately from each other. In the writings of the alchemists we find the words misy, sory, chalcanthum, applied to alum as well as to sulphate of iron; and the name atramentum sutorium, which ought to belong, one would suppose, exclusively to green vitriol, applied indifferently to both.

When our alum was discovered is entirely unknown. Beckmann devoted a good deal of attention to the history of this salt, and published a curious dissertation on the subject; but his attempts to trace its origin were unsuccessful. The manufacture of it was discovered in the East, but at what time or place is totally unknown. It would appear that, about four or five hundred years ago, there was a manufactory of it at Edessa in Syria, at that time called Rocca, --- hence, it is supposed, the origin of the term rock alum, commonly employed in Europe; though others allege that the term originated at Civita Vecchia, where alum is made from a yellow mineral which occurs in the state of a hard rock.

Different alum works existed in the neighborhood of Constantinople. About the time of the fall of the Grecian empire the art of making alum was transported into Italy, at that period the richest and most manufacturing country in Europe. Bartholomew Pernix, a Genoese merchant, discovered alum ore in the island of Ischia, about the year 1459. Nearly at the same time John di Castro, who was well acquainted with the alum works in the neighborhood of Constantinople, suspected that a mineral fit for yielding alum existed at Tolfa, because it was covered with the same trees that grew on the alum mineral near Constantinople. His conjecture was verified by trials, and the celebrated manufactory at Tolfa established. Another was begun in the neighborhood of Genoa,; and the manufacture flourished in different parts of Italy. To this country it was confined for the greater parts of a century. Various manufactories of it were established Germany by the year 1544.

England possesses no alum works till the reign of Charles I. Thomas Chaloner, son of Dr. Chaloner who had been tutor to Charles, while hunting on a common in Yorkshire, took notice of the soil and herbage, and tasted the water. He found them similar to what he had seen in Germany where alum works were established. In consequence of this he got a patent from Charles for an alum work. Since that time various alum works have been established in different parts of Great Britian, --- the most important now in operation being the Whitby works, originally established by Mr. Chaloner; and the works at Pendleton, near Manchester, and Goole, Yorkshire, and at Hurbet and Campsie, both in the neighborhood of Glasgrow.

Several alum works exist in Sweden, particularly in West Gothland. There is one, for example, at Hænsæter, near the borders of the Wener Lake. But for a description of the Swedish works we refer to Bergmann’s Opuscula, i. 284, or English translation, i. 342.
Various minerals are employed in the manufacture of alum, but by far the most important of them are the following three: alum-stone, alum-slate, bituminous shale.

Alum-stone or Alunite was first observed at Tolfa, near Rome, in the 15th century, and afterwards in Hungary and several other places, chiefly in trachyte or other volcanic rocks. It appears to be produced by the action of sulphureous vapors on the feldspars they contain, and generally occurs in compact, granular, or earthly masses, mixed with quarts or feldspar. Small crystals are found in cavities, and are either rhombohedrons with angles of 89°10`, and thus nearly cubes, or these with the polar angles replaced by the basal plane. The specific gravity ranges from 2`58 to 2`752, the compact varieties being the lighter. Its hardness is 3-5 to 4, or rather softer than flour spar. It has a distinct cleavage perpendicular to the axis of the rhombohedron, and conchoidal fracture in other directions. The pure varieties are white and colorless, but it is often colored grayish, yellowish,, or reddish. The crystals decrepitated before the blowpipe, but are infusible, as well as the compact alunite. The alum is extracted from this mineral by repeated roasting and treating with water. The absence of iron accounts for the superior purity for which the Roman alum was long celebrated.

Alum-slate is a far more abundant substance, occurring in beds in different formations. Thus it is common in the older Palæozonic or Silurian strata of Scandinavia and Scotland. Generally it is distinctly slaty, but sometimes forms rounded balls or concretions. It contains much carbonaceous matters, and hence its color is grayish or bluish-black. It has a dull luster, is soft and sectile. It contains much disseminated iron pyrites, and on decomposition in the air yield sulphate of iron, and alum as an efflorescence on the surface.

Many of the shales or slate clays in the coal formation also produce alum when acted on the atmosphere. Such are those used for manufacturing alum at Campsie and other places near Glasgrow. Where they contain much bituminous matter they show a shining resinous streak and grayish-black color, and are named bituminous shales. These burn when heated, with a pale flame and sulphureous odor.

The alum slates at Whitby in Yorkshire belong to the Lias, and are used in the alum works in the neighborhood, In other places, as in many parts of Germany, similar beds are founded in Tertiary formations, particularly in connection with the brown coal deposits. When fresh dug they often show no trace of alum, which only appears after exposure to the air, or when the decomposition of the iron pyrites is assisted by the action of heat.

Several native varieties of sulphate of alumina and soda alum occur in South America, some of the most remarkable of which it may be proper to specify.
1. Sulphate of alumina, or Alunogene, was first found at Rio Saldanha, but is now obtained from several places in Europe and America. The color is white, here and there tinged yellow, obviously from external impurities. It occurs in fine crystalline needles; luster silky; taste that of alum, but stronger’ specific gravity, 1-6 to 1-7; soft; before the blowpipe behaves like alum.
2. Soda-alum. It occurs natives in the province of St. Juan, situated to the north of Mendoza, on the east side of the Chilian Andes, at about 30° S. lat. The alum is white, and composed of fibres adhering longitudinally, and having a certain breath, but very thin. It bears some resemblance to fibrous gypsum, but it is harder, not being scratched by the nail, though the knife scratches it with great case. It is sectile. The outer fibres are white and only slightly translucent, as if they had lost a portion of their water; but the internal fibres are transparent, and have a silky aspect. It tastes precisely like alum, and is very soluble, water at the temperature of 62° dissolving 3-773 parts of it, and boiling water dissolving any quantity whatever. When exposed to heat, it behaves very nearly as common alum.
3. There is a mineral called aluminite, which was observed in the environs of Halle many years ago, and which was afterwards detected by Mr. Webster in clay resting on chalk at Newhaven in Sussex. This, if it were sufficiently abundant, would constitute on excellent materials for the manufacture of alum. Its color is snow-white. It occurs in reniform pieces of greater or smaller size. Fracture fine-earthy; dull; streak glistening; opaque, adheres feebly to the tongue; soils very slightly; very soft; feels fine; but meager; specific gravity, 1 7054. It consists of alumina, sulphuric acid. And water.

Four different processes are employed in the manufacture of alum, according to the nature of the mineral from which the alum is to be extracted.

The process employed at Tolfa is the simplest of all. If the Tolfa stone be kept constantly moistened with water for about two months, it falls to powder of itself, and yield alum by lixiviation. But this is not the process employed by the manufacturers. The alum-stone is broken into small pieces, and piled on the top of a perorated dome, in which a wood penetrate through the pieces of alum-stone, and a sulphureous odor is disengaged, owing to the decomposition of a portion of the sulphuric acid in the stone. This roasting is twice performed; the pieces of ore which the first time was at the edge of the dome, being the second time put in the middle. The process of roasting this stone required consideration attention. If the heat be too great, the quality of yielding alum is destroyed; if the heat be too small, the stone does not readily fall to powder. There can be little doubt that the unroasted stone would yield more alum than the roasted; but probably the additional labour requisite in the latter case would more than swallow up the increase of product.

The roasted stone, which has now acquired a reddish color, is placed in row between trenches filled with water. This liquid is so frequently sprinkled on it hat the stone is always moist. In two or three days it falls to powder, like slacked quicklime; but the daily watering is continued for a month. The success of this part of the operation is said to depend very much on the weather. When the weather is rainy, the alum is all washed out, and little or nothing left for the manufacturer to extract.
When the stone has by this process been reduced to a sufficiently fine powder, it is thrown into a leaden boiler filled two-thirds with water. During the boiling the powder is frequently stirred up, and the water that evaporated is replaced. When the boiling has been continued for a sufficient time, the fire us withdrawn, and time allowed for the earthly matter to subside to the bottom. A cock is then opened, which allows the clear liquor to flow out into deep wooden square vessels, so made that they can be easily taken to pieces. Here the alum gradually crystallizes, and attached itself to the sides and bottom of the vessel. The mother liquid is then drawn off into shallower wooden troughs, where more alum crystals are deposited. The liquid has now a red color, and is muddy; and the last alum crystals are mixed with this red matter. They are washed clean in the mother liquor, which is finally pumped into a trough, and used in subsequent processes.

The alum obtained at Tolfa, is known by the name of Roman alum, and is in very high estimation. It is always mixed with a little reddish powdery matter, which is easily separated from it.

Alum-slate, being very different in its compositions, requires a different treatment to fit it for yielding alum. If the alum-slate contains a notable quantity of lime or magnesia, it does not answer the purposes of the manufacturer so well. The essential ingredients in alum-slate, for the alum-makers, are alumina and iron pyrites.

The first process is to roast the ore. In Sweden, where the fuel is wood, and consequently expensive, it is customary to used the alum-slate itself as fuel for roasting the one. For this purpose a small layer of brushwood is covered with pieces of alum-slate, and set on fire; and as the combustion proceeds, new layers of alum-slate are added. It is usually to place alternate layers of roasted and unroasted the alum-slate. The combustion continues for a month or six weeks. At Whitby, coal is employed for roasting the alum-slate. Indeed the alum-slate of Whitby is lighter colored than that of Sweden, and probably would not burn of itself. So great is the quantity of combustible matter in the Swedish alum-slate that is employed as fuel for burning limestone. Great quantities of limestone are burnt in this manner at Hunneberg, near the south side of the lake Wener. The roasted ore has usually a brown color. When it is red the quantity of alum which it yields is considerably diminished.

By this roasting the pyrites is oxidized into sulphate of iron and sulphuric acid, thus : ---
FeS2 + O7 + H2 O = FeSO4 + H2 SO4

The sulphuric acid as it is produced is, however, at once neutralized by the large excess of alumina producing Sulphate, so that the result of the action is to produce a mixture of the sulphates of iron and alumina.

The roasted ore has an astringent taste, owing to the sulphate of iron and sulphate of alumina which it contains. The next process is to lixiviate it with water, in order to dissolve these slats. For this purpose it is put into reservoirs made of wood or masonry, with a stopcock at the bottom to draw off the water. The usual method is to keep the water for twelve hours in contact with ore that has been twice lixiviated; then to draw it off, and allow it to remain for an equal period on ore that has been once lixiviated. Lastly, it is run upon fresh ore, and allowed to remain on it for twelve hours longer. If the specific gravity of the liquid thus treated be 1-25 at the temperature of 55°, it may be considered as saturated with sulphate of alumina and sulphate of iron; but probably this specific gravity is not often obtained.
The liquid, thus impregnated with salt, is now boiled down in leaden vessels to the proper consistency for crystallization. In Sweden the fuel employed for this purpose is alum-slate. By this means a double effect is produced --- the liquid is evaporated, and the alum-slate is roasted. During the boiling abundance of oxide of iron falls, mixed with selenite, if lime be one of the constituents of the alum-slate. When the liquid is sufficiently concentrated it is let into a square reservoir, in order to crystallize. Great quantities of sulphate of iron crystals are usually deposited in this vessel. These are collected by drawing the liquid off into another reservoir. When all the sulphate of iron that can be obtained has been separated, a quantity of sulphate of potash or ammonia, muriate of potash, or potash is procured from the sulphuric acid makers, and the muriate of potash from the soap-makers. By this addition alum is formed in the liquid, and it gradually deposited itself in crystals on the sides of the vessel. These crystals are collected, and dissolved in the smallest quantity of boiling water that will take them up. This solution is poured into large wooden casks. In a fortnight or three weeks the alum crystallizes, and covers the sides and bottom of the cask. The hoops are now taken off, and the staves of the cask removed. A mass of alum crystals, having the shape of the cask, remains. This mass is pierced, the mother liquor allowed to run out, and preserved for a subsequent process. The alum, being now broken is pieces, is fit for sale.

The manufacture of alum from bituminous shale and stale-clay bears a considerable resemblance to the manufacture from alum-slate, but differs in several particulars. We shall give a sketch of the processes followed in two works of this kind that are in operation in the neighborhood of Glasgrow. The bituminous shale and slate-clay employed are obtained from old coal-pits, which are very extensive near Glasgrow. The air in these coal-pits is moist, and its average temperature about 62°. The shale having been exposed for many years, has gradually opened in the direction of its slaty fracture, so as to resemble in some respects a half-shut fan; and all the chinks in it are filled with a saline efflorescence in threads. This salt is white, with a shade of green, has a sweetish astringent taste, and consists of a mixture of sulphate of iron and sulphate of alumina. In order to obtain these salts in a state of solution, nothing more is requisite than to lixiviate this shale with water. The lixiviated ore being left exposed to the weather, forms more salt, which is gradually washed out of it by the rain-water, and this water is collected and preserved for use.

The next step in the process is to boil down the liquid to a sufficient state of concentration. At Campsie all the boilers are composed of stone, and the heat is applied to the surface. This is a great living, as leaden vessels are not only much more expensive, but require more frequent renewal. When the liquid is raised to a sufficiently high temperature in the stone reservoir, pounded sulphate of potash, or muriate of potash, as they can be procured is mixed with it; and there is an agitator in the vessel, by which is continually stirred about. This addition converts the sulphate of alumina into alum. The liquid is now let into trough, and allowed to remain till it crystallizes. In this liquid there are two salts contained in solution --- viz., Sulphate of iron and alum; and it is an object of great consequence to separate them completely from each other. The principal secret consists in drawing off the mother liquor at the proper time; for the alum is much less soluble in water than the sulphate of iron, and therefore crystallizes first. The first crystals of alum formed are very impure. They have yellow color, and seem to be partly impregnated with sulphate of iron. They are dissolved in hot water, and the solution poured into troughs, and allowed to crystallize a second time. These second crystals, though much purer, are not quite free from sulphate of iron; but the separation is accomplished by washing them repeated with cold water; for Sulphate of iron is much more soluble in that liquid than alum. These second crystals are now dissolved in as small a quantity of hot water as possible, and the concentrated liquid poured while hot into large casks, the surface of which is covered with two cross beams. As the liquor cools, a vast number of alum crystals form on the sides and surface. The casks are allowed to remain till the liquid within is supposed to be nearly of the temperature of the atmosphere. This, in winter, requires eleven days; in summer, fourteen or more. The liquid, after standing eleven days in summer, has been observed to be still above blood heat. The hoops are then removed, precisely as in the manufacture of alum-slate.

There always remains in the boilers a yellowish substance, consisting chiefly of peroxide of iron. This is exposed to a strong heat in a reverberatory furnace, and it becomes red. In this state it is washed, and yields more alum. The red residue is ground to a fine powder, and fried. It then answers all the purposes of Venetian red as a pigment. By altering the temperature to which this matter is exposed, a yellow ochre is obtained instead of a red.

In France, where alum ores are by no means abundant, alum is manufactured from clay. This method of making the slats was first put in practice by Chaptal when professor of chemistry at Montpellier. His methods have been since gradually improved and brought to a state of considerable perfection. The first process tried was this: The clay was reduced to a fine powder in a mill, and then mixed with sulphuric acid. After remaining some days, it was exposed for twenty-four hours to a temperature of about 130°. It was then lixiviated, and the liquid inconvenient, was abandoned for the following: --- The clay being well ground, was mixed with half its weight of the saline residue from a mixture of sulphuric and nitre. This residue is little else than sulphate of potash. The mixture was formed into balls about 5 inches in diameter, which were calcined in a potter’s furnace. They were then placed on the floor of a chamber in which sulphuric acid was made. The acid vapor caused them to swell, and to open on all sides. In about a month they were sufficiently penetrated with the acid. They were then exposed to the air, under shades, that the saturation might become more complete. Finally, they were lixiviated, and the liquid being evaporated, yielded pure alum.

This process was considerably improved by Berard, of the Montpellier alum work. Instead of exposing the calcined balls to the fumes of sulphuric acid, her sprinkled them with a quantity of sulphuric acid of the specific gravity 1-367, equal to the weight of the clay employed; but it is obvious that the proportion must vary with the nature of the day. The solution takes place with the greatest facility, and crystals of alum are obtained by evaporating the liquid.

Another process was put in practice by Chaptal, in the neighborhood of Paris. A mixture is made of 100 parts of clay, 50 parts of nitre, and 50 parts of sulphuric acid of the specific gravity 1-367; and this mixture is put into a retort and distilled. Aquafortis comes over, and the residue in the retort being lixiviated with water, yields abundance of excellent alum.
For chemical constitution and relations of the alums, see CHEMISTRY. (--)

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