a salt very much employed by Dyers and other artists, in their different processes. It has a white colour, an astringent and acid taste, and crystallizes in regular octahedrons. Its specific gravity is 1.731, according to a mean of the experiments of Fahrenheit, Wallerius, Watson, and Hassenfratz. Water at the temperature of 60° dissolves about one fifteenth of its weight of alum; while boiling water dissolves about three-fourths of its weight. When alum is exposed to a dry atmosphere, it effloresces slightly. When heated, it speedily becomes liquid. If the heat be continued, its water is driven off, and it loses about 44 per cent. of its weight. What remains, is a vitreous looking substance, known by the name of burnt alum. When alum is exposed to a strong red heat, it gives off a quantity of oxygen gas, as Dr Milner first ascertained. The same fact was afterwards pointed out by Gay-Lussac.
Alum is one of those substances which chemists have distinguished by the name of triple salts. It is composed of sulphuric acid, potash, alumina and water. That it contained sulphuric acid as a constituent, was known even to the Alchemists. Pott and Margraaf demonstrated, that alumina was another constituent. Mr 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. Margraaf went much farther. He 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. Margraaf's Opusc. II. 111.) Margraaf 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. But, when a solution of potash or ammonia is dropped into this liquid, it immediately deposits perfect crystals of alum. (Sur la regeneration de l'alun. Margraaf's Opusc. II. 86.) He mentions likewise, that manufacturers of alum, in general, were unable to procure the salt without a similar addition; and 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 Margraaf; they still continued, without any rigid examination, to consider alum as a sulphate of alumina.
Bergman 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. Bergman's Opusc. I. 225.) He had observed likewise, that sulphate of potash is frequently found in alum. He decomposed the solution of alum by means of ammonia, evaporated the filtered 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.) He drew, as a conclusion from these facts, that sulphate of potash readily combines with sulphate of alumina. Yet, it is obvious from the whole of his dissertation, that he had no conception that alum is a triple salt. He ascribed the difficulty of crystallization, to the excess of sulphuric acid present. He thought that the only use of the potash was to saturate this excess; and advises manufacturers to substitute clay instead of potash, as a method which would not only saturate the excess of acid, but increase the quantity of alum. This very bad advice was not, we presume, followed by any alum makers. If they tried it, they would soon be convinced of its injurious consequences. Instead of alum, they would have obtained an insoluble tasteless powder, well known by the name of alum saturated with its earth.
After Klaproth had discovered the existence of potash as an ingredient in leucite and lepidolite, it occurred to Vauquelin, that it was probably an ingredient likewise in many other minerals. He collected that alum crystals often make their 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 triple 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 a triple salt; and various analyses of it have been made to determine its constituents. Vauquelin, (Ann. de Chim. I. 167), Thenard and Roard, (Ibid. Tom. LIX. 72), Curandou, (Journal de Physique, LXVII. 1.) and Berzelius, (Ann. de Chim. LXXXII. 258.) successively published the results of their experiments. They differ but little from each other. We shall therefore satisfy ourselves with giving the analysis of Berzelius, which is the latest, and in all probability the most accurate. His result was as follows:
| Substance | Weight | |--------------------|--------| | Sulphuric acid | 34.23 | | Alumina | 10.86 | | Potash | 9.81 | | Water | 45.00 | | Loss | 0.10 | | **Total** | **100.00** |
Or
| Substance | Weight | |--------------------|--------| | Sulphate of alumina| 36.85 | | Sulphate of potash | 18.15 | | Water | 45.00 | | **Total** | **100.00** |
Now, as we shall show in a subsequent article, (See Atomic philosophy), an integrant particle of sulphuric acid weighs 5, an integrant particle of alumina 2.136, and an integrant particle of potash 6. We shall show likewise in the same article, that sulphate of potash is composed of one integrant particle of sulphuric acid, united with one integrant particle of potash; and sulphate of alumina of one integrant particle of sulphuric acid, united to one integrant particle of alumina; therefore, the relative weight of an integrant particle of each of these salts, is as follows:
| Substance | Weight | |--------------------|--------| | Sulphate of potash | 11.000 | | Sulphate of alumina| 7.136 |
But the quantities of each of these salts in alum, according to the preceding analysis, are as the numbers 18.15 : 36.85. Now, the ratio 18.15 : 36.85, is very nearly the same as 11 : 7.136 × 5. Hence it follows, that alum contains 1 integrant particle of sulphate of potash, and 5 integrant particles of sulphate of alumina. An integrant particle of water, as will be shown likewise in the article Atomic philosophy, weighs 1.132. Now, one integrant particle of sulphate of potash, × 5 integrant particles of sulphate of alumina, weigh 46.680. But alum con- tains 45 per cent. of water. To find how many integrant particles of water these amount to, we have this proportion; \( \frac{56}{45} : \frac{46,680}{38,213} \). Now, 38,213, is very nearly the weight of 34 integrant particles of water. For \( 1.132 \times 34 = 38,488 \).
Hence, it follows, that alum is a compound of
\[ \begin{align*} \text{Sulphate of potash} & - 1 \text{ integrant particle.} \\ \text{Sulphate of alumina} & - 5 \\ \text{Water} & - 34 \\ \end{align*} \]
Total 40
Making altogether no fewer than 40 integrant particles united together. It is not easy to see how these 40 particles, by their symmetrical union with each other, can form a tetrahedron, which Haury conceives to be the primitive form of the integrant particle of alum. The octahedron is a much more probable primitive form. It might be conceived to be formed by the six integrant particles of sulphate of potash, and sulphate of alumina, and the thirty four particles of water, which are probably much smaller, may fill up the intervals between them. The objection to this supposition is, that in that case the atom of sulphate of potash cannot be surrounded by the atoms of sulphate of alumina, as one would naturally expect, from their mutual affinity for each other. The most probable supposition of the arrangement of the integrant particles of alum is, that the atom of sulphate of potash occupies the centre; that it is surrounded at equal distances by the five atoms of sulphate of alumina, each surrounded by seven atoms of water. But, we do not readily see how such an arrangement would either form a tetrahedron or octahedron. This symmetrical arrangement of the different integrant particles that compose a crystal deserves to be studied with care, because it is probable, that it will throw light on the nature of affinity, and the way in which atoms combine together.
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 ἀλύτρωσις given by Dioscorides in the 123d chapter of his 5th book, it becomes quite obvious, that he alludes to the same substance. Hence it follows, that ἀλύτρωσις is the Greek name for alumen. Pliny informs us, that alumen was found naturally in the earth. He calls it salugatoriae. Different substances he informs us, were distinguished by the name of alumen; but they were all characterized by a certain degree of astrin gency, and were all employed in dyeing and in medicine. The light-coloured alumen was useful in brilliant dyes; the dark-coloured only in dyeing black, or very dark colours. One species of alumen 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 colours. Several other species of alumen are described by Pliny, but we are unable to make out to what minerals he 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 misty, sory, chalcanthum, (Plini, xxxiv. 12.) As alum or 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 salts accurately from each other. In the writings of the Alchemists we find the words misty, sory, chalcanthum, applied to the alum as well as 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. Beckman devoted a good deal of time to trace 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 there are others that pretend that the term originated at Civita Vecchia, where alum is made from a yellow mineral in the state of a hard rock.
Different alum works existed in the neighbourhood 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 neighbourhood 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 neighbourhood of Genoa, and the manufactory flourished in different parts of Italy. To this country, it was confined for the greatest part of a century. Various manufactories of it were established in Germany, by the year 1544. In the time of Agricola, there was a manufactory of it at Commoton in Bohemia. About the same time, an alum work was established near Carthagena in Spain, at Alcmaron.
England possessed no alum works till the reign of Charles I. Thomas Chaloner, Esq., 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. This manufactory was worth two thousand a-year, or perhaps more. But some of the Courtiers thinking this too much for him; prevailed with the King, notwithstanding the patent, to grant a moiety of it to another person. This was the reason why Mr Chaloner was such a partisan of the Parliament, and such an enemy of the King, that, at the end of the civil war, he was one of those who sat in judgment upon his Majesty and condemned him.* Since that time, various alum works have been established in different parts of England and Wales. But none at present exist, except the Whitby works, originally established by Mr Chaloner, and two works at the Hurlet and Campsie, both in the neighbourhood of Glasgow.
Several alum works likewise exist in Sweden, particularly in West Gothland. There is one for example at Hanseter, near the borders of the Wene lake, on the west side of the mountain called Kinnekulle. But, for a description of the Swedish works, we refer to Bergman's Opuscula, Vol. I. p. 284, or the English translation, Vol. I. p. 342. We do not know if any alum works exist in Poland or Russia; but, as the greatest part of these extensive countries consists of alluvial soil to a great depth, it is probable, that little alum ore will be found in them.
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 is the mineral which occurs at Tolfa, near Rome, from which the celebrated Roman alum is made. Werner also possesses specimens of the same mineral from Hungary. At Tolfa, it is said to constitute a hill. Its colour is greyish white, sometimes yellowish grey, and sometimes, as Gay Lussac informs us (Ann. de Chim. LV. 267.) reddish from the peroxide of iron. It is massive and rather hard. It has no lustre. It is translucent on the edges. Fractures uneven, approaching to the fine earthy. Fragments blunt edged. Does not adhere to the tongue. It is rather heavy; but its specific gravity is not accurately known. This mineral, according to the analysis of Klaproth (Gehlen's Neue Allgemeine Journal der Chemie, VI. 35.) is composed of the following constituents:
| Silica | 56.5 | |--------|------| | Alumina| 19.0 | | Sulphuric Acid | 16.5 | | Potash | 4.0 |
Vauquelin likewise analyzed this mineral, and found the same constituents, though in different proportions. We have no analysis of the varieties of alum stone which contain peroxide of iron. Thus it appears, that alum stone contains all the ingredients of alum ready formed.
Alum slate is a much more abundant mineral than alum stone. It is said to alternate with primitive clay slate. It occurs abundantly along with transition slate; and there can be little doubt that it occurs likewise in the flint formations. In West Gothland in Sweden, it constitutes a part of different hills; as Kinnekulle, Hunneberg, and Halleberg; in all of which it appears to alternate with flint trap rocks. It occurs likewise abundantly in Whitby in Yorkshire. We have never ourselves been upon the spot; but, from the general structure of Yorkshire, and the neighbouring counties, indeed of the whole east coast of England, there can be very little doubt that, in this position, it is also a flint rock.
Alum slate, as the name implies, is a slaty rock, though sometimes it occurs in balls. The colour is blueish-black, with a strong shade of grey. Fracture straight slaty. Fragments tabular. Its internal lustre is glimmering. It retains its colour in the streak; but acquires more lustre. Soft. Not particularly brittle. When exposed to the air it effloresces, and acquires an aluminous taste.
This mineral has never been accurately analyzed. But there can be no doubt that it contains silica, alumina, iron, sulphur, charcoal, and often likewise potash. Probably this was the mineral upon which the alum scissile of the ancients was found.
Bituminous shale, the brandschiefer of the Germans, is a slaty mineral, which almost constantly accompanies beds of coal; and, accordingly, is very common in Great Britain. Its colour is brownish-black. Its fracture is thin slaty. Fragments tabular. Internal lustre glimmering; but the colour is not altered. Very soft. Rather sectile. Feels rather greasy. Easily frangible. When heated, it burns with a pale flame and sulphureous odour, and becomes white. It has never been accurately analyzed; but it is probably nothing more than slate clay, which occurs so abundantly in the independent coal formation, impregnated with the matter of coal. Its other principal constituents must be silica, alumina, and iron pyrites.
Slate clay itself, at least not sufficiently impregnated with coaly matter to deserve the name of bituminous shale, is frequently employed in the making of alum. This is the case in the neighbourhood of Glasgow.
Four different processes are employed by the alum manufacturers, according to the nature of the mineral from which the alum is to be extracted.
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* Letters written by Eminent Persons, in the Seventeenth and Eighteenth Centuries; and Lives of Eminent Men, by John Aubrey, Esq.; Vol. II. p. 281. The process employed at Tofia is the simplest of all. If the Tofia stone be kept constantly moistened with water for about two months, it falls to powder of itself, and yields alum by elixivation. 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 perforated dome, in which a wood fire is kindled. The smoke and flame of the wood penetrate through the pieces of alum stone, and a sulphurous odour is disengaged, owing to the decomposition of a portion of the sulphuric acid in the stone. This roasting is twice repeated; the pieces of ore which, the first time, were at the edge of the dome, being the second time put in the middle. The process of roasting this stone requires considerable 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 colour, is placed in rows between trenches filled with water. This liquid is so frequently sprinkled on it, that the stone is always moist. In two or three days it falls to powder, like slackened 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. In such cases, it is obvious that the alum stone should be covered by a shade from the rain.
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 evaporates is replaced. When the boiling has been continued for a sufficient time, the fire is withdrawn, and time allowed for the earthy 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 attaches itself to the sides and bottom of the vessel. The mother liquor is now drawn off into shallower wooden troughs, where more alum crystals are deposited. The liquid has now a red colour, 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 Tofia is the purest of all. It 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. What this red matter is, has not been ascertained; but it is not peroxide of iron. To the eye it has very much the appearance of a vegetable matter. We have some notion that it is added artificially by the sellers of Roman alum. Probably Roman alum at first had a red tinge, in consequence of the red matter in the mother liquor remaining partially attached to it. The goodness of the alum may have given the purchasers a partiality to the red colour, and induced the sellers to add a red powder artificially. We have never had an opportunity ourselves of examining this matter; but have been informed by those that have, that it contains no iron.
It is not improbable that this process would be improved by grinding the Tofia stone to a fine powder in a mill, without any previous roasting, and then keeping the powder moistened with water for a considerable time. If the residual earth, after the alum is extracted, be boiled with sulphuric acid, the liquid yields alum crystals by evaporation. This is a demonstration that neither the alumina nor the potash is exhausted, and that the sulphuric acid, driven off by the roasting, is so much alum lost to the manufacturer. Indeed, the quantity of sulphuric acid in the alum stone is not sufficient to occupy the whole of the potash in the formation of alum. It would be necessary to add about one-tenth of the weight of the alum stone of sulphuric acid, if it were wanted to employ the whole potash present in the stone. The consequence of this addition (supposing no loss) would be an additional quantity of alum, amounting to rather more than one-fifth of the weight of the alum stone employed.
Alum slate being very different in its composition, requires a different treatment to fit it for yielding alum. If the alum slate contain a notable quantity of lime or magnesia, it does not answer the purposes of the manufacturer so well. Indeed, the proportion of lime present may be conceived to be such, that no alum would be obtained whatever. As alum slate has never been subjected to accurate analysis, we do not know in what proportion these two earths exist in it, or whether they may not, in many cases, be absent altogether. 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 use the alum slate itself as fuel for roasting the ore. 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 usual to place alternate layers of roasted and unroasted 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 coloured 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 it is employed as fuel for burning limestone. Great quantities of limestone are burnt in this manner at Humeberg, near the south side of the lake Wener. The roasted ore has usually a brown colour. When it is red, the quantity of alum which it yields is considerably diminished.
By this roasting the pyrites is decomposed. The sulphur is converted into sulphuric acid, while the iron is oxidized. In what manner this change is produced it is not easy to say. Indeed, it does not seem certain that pyrites is a constant ingredient in alum slate. We have never been able to detect any, by the eye, in any specimens of Whitby alum slate which we have examined. At Hansacter in Sweden, no sulphate of iron crystallizes when the liquid is evaporated; yet, if pyrites had been present, it is difficult to see any reason that should prevent this salt from being formed. Hence it is probable, that, in alum slate, the sulphur is sometimes at least combined with other substances than iron. It must always be in a state of combination; for, if it were in a loose state, it would be driven off by the roasting. This point deserves to be elucidated, by analyzing different varieties of alum slate.
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 out these salts. For this purpose, it is put into reservoirs made of wood or masonry; with a stop-cock 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 we presume that 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 gelinite, 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, muriate of potash, or putrid urine, is mixed with the liquid. The sulphate of 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 deposits 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 in pieces, is fit for sale.
The manufacture of alum from bituminous shale, and slate clay, bears a considerable resemblance to the manufacture from alum slate; but differs in several particulars. There are two works of this kind in the neighbourhood of Glasgow, managed with great skill, and excellent in every respect. We shall give a sketch of the processes followed in these works.
The bituminous shale and slate clay employed, are obtained from old coal pits, which are very extensive in the neighbourhood of Glasgow. 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 chunks 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. Nothing more is requisite than to lixiviate this shale with water, in order to obtain these salts in a state of solution. 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 these boilers are composed of stone, and the heat applied by means of steam. This is a great saving, 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 it is continually stirred about. This addition converts the sulphate of alumina into alum. The liquid is now let into another trough, and allowed to remain till it crystallizes. There are two salts contained in solution in this liquid, namely, 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 a yellow colour, 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 repeatedly 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. We have seen the liquid in a cask that had stood eleven days in summer, still more than blood-hot. The hoops are then removed, precisely as in the manufacture of alum from 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 dried. 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 salt, 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 mixed with urine or potash. This method being found 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 sulphur and nitre. This residue is little else than sulphate of potash. The mixture was formed into balls about five 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 vapour 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 has been considerably improved by Berard, the present proprietor of the Montpellier alum work. Instead of exposing the calcined balls to the fumes of sulphuric acid, he sprinkles 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 clay. 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 neighbourhood of Paris, and is still followed, or was at least followed some years ago by M. Bouvier. A mixture is made of 100 parts of clay, 50 parts of nitro, 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.
We may mention another process described by Caraudan, and certainly practicable, and even easy, though we do not believe that it would be attended with profit. He forms 100 parts of clay into a paste with water, holding 5 parts of common salt in solution. This paste is formed into cakes, and calcined in a reverberatory furnace. The calcined mass is reduced to powder, and well mixed with the fourth part of its weight of concentrated sulphuric acid. When the muriatic acid vapours are dissipated, as much water is added as there had been employed of acid, and the mass is kneaded with care. A strong heat is produced; the composition swells; more water is added; and at last a solution of potash, in which the alkali amounts to one fourth of the acid employed. The liquor is now drawn off, and, on cooling, it yields a copious deposit of alum crystals.
In the preceding sketch of the manufacture of alum, no notice has been taken of alum earth as an alum ore, because the writer of this article has never had an opportunity of seeing any manufactory of the salt from this material. We conceive the process to be nearly the same as that followed, when bituminous shale or slate clay is the alum ore. Alum earth seems to be a vegetable substance, or rather to be derived from the vegetable kingdom. It is connected with coal, and may, without impropriety, be considered as a variety of brown coal. There is a manufacture of alum from this substance at Frienwalde, in Germany. Klaproth subjected this variety of alum earth to analysis, and from 1000 parts of it obtained the following constituents:
| Substance | Percentage | |--------------------|------------| | Sulphur | 28.5 | | Charcoal | 196.5 | | Alumina | 160.0 | | Silica | 400.0 | | Black oxide of iron| 64.0 | | Sulphate of iron | 18.0 | | Gypsum | 15.0 | | Magnesia | 5.0 | | Sulphate of potash | 15.0 | | Muriate of potash | 5.0 | | Water | 107.5 |
The excess of one and a half per cent. obviously proceeded from the water adhering to the salts. (J.)