The varieties of aluminous slate which, by calcina tion, and subsequent exposure to air and humidity, af ford alum, appear to consist of the usual earthy ingre dients of slate,—silex, argil, and perhaps magnesia, with sulphate of iron; or it may be regarded as a slate impregnated with pyrites, or sulphuret of iron. The calcination is of utility in disposing the sulphuret of iron to oxygenation from the action of the atmospheric air, and of water ; by the exposure, oxygen is absorbed, and the sulphur is converted into sulphuric acid; this may in part combine with the argil, but it is not impro bable, that its combination is at first principally with the oxide of iron. The sulphate of iron, by long exposure, absorbs oxygen ; the oxide, which is its base, passes to a higher state ol oxidation, and in this state exerts a less energetic action on the sulphuric acid ; it is there fore more easily attracted by the argil ; and by these successive changes, the whole of it, or nearly so, may at length be combined with that earth. In the natural formation of alum, which takes place slowly in some fos sils of this kind, we accordingly find the alum in various degrees of intermixture with sulphate of iron, that in which the process is farthest advanced being probably most free from it. These slates must also contain a quantity of potash ; a substance, which it is now suffi ciently established, is frequently found in the mineral kingdom. From the practice, however, in the alum manufacture, of adding a proportion of potash to the li quor previous to crystallization, it appears not to exist in the slate in the full proportion necessary' to convert the whole sulphate of argil into the triple salt which forms alum. The manufacturers have found, that it is equally advantageous to employ sulphate of potash, or n.uriate of potash, to contribute to the formation ol the: alum; and these salts tieing products of various cal processes, and being scarcely applicable to any other purpose, are economically employed. The one. the sulphate of potash, is obtained in the manmacture of aqualortis ; the other, the mui iate of potash, from the residual liquor in the neanufacture of soap. The mann lacturers prefer the muriate to the sulphate of potash, lor which a very good reason has been assigned. The principal impurity to which alum is liable, and winch it is an object of importance to avoid as much as possible, is hum the presence of iron, in the state of suipnate. of iron. When muriate of potash is added, the potash, uniting with the sulphuric acid and argil, enters into the composition of the alum; while We muriatic acid unites with the oxide of iron, retains it in solution, and forms an uncrystallizable salt, which remains in the mo ther water. The kelp employed at Wii d)). cannot be considered as operating by the soda which it contains ; for this alkali Bergman found to be ol no utility in pro moting the crystallization of alum, but probably by' the sulphate and muriate of potash which exist in it. The urine which is used must furnish that variety of alum which contains ammonia.
It appears that when ircumstances are favourable, a natural formation of alum takes place, from sonic varie ties of aluminous or bituminous slate. A remarkable example of this kind is to be found in the Hulett mine, or pit, near Glasgow, from which within these few years a very large quantity' of alum has been procured. A brief account is given of it in the 16th volume of Nichol son's Journal. In this coal mine, a stratum ol slate or shale, about ten inches thick, placed above the coal, has been exposed to the action of the atmosphere above 200 years, the mine having been open during that pe riod; it is protected by situation from the infiltration of much water. Under these favourable circumstances,
the natural formation of alum has taken place to a con siderable extent, and it is found disseminated in silky like filaments, and even in congeries oi' small crystals, in the interstices of the shale, accompanied with sul phate of iron. It is extracted by' lixiviation, along with a portion of sulphate of iron, which has been formed by the same natural process. The ley is evaporated, and the sulphate of iron is removed as much as possible, by priority of crystallization; muriate of potash is then added, probably with the view of preventing the impu rity of the alum from the presence of iron, by the ope ration already explained. The Icy is lastly evaporated to the specific gravity' 1.450, and is set aside to crystallize.
The presence of sulphate of iron in this natural for mation, renders it probable, according to the view stated above, that alum has been formed from the oxygenation, by the gradual action of the atmosphere, of the sulphu ret of iron or pyrites contained in the shale, the sulphu ric acid of which has been transferred to the ?agil. And as the salt exists ready formed and crystallized, a por tion of potash must also be contained in this particular variety of slate.
A manufacture of alum was established by Chaptal in France, in which it was formed by the direct combina tion of its principles, pure clay being exposed to the va pours of sulphuric acid ; and sulphate of potash being added to the ley obtained by washing the materials after a certain period. This may' afford a pure alum, but the process must be more expensive than the indirect modes by which the alum is usually procured.
We have entered on these details with regard to the extraction and manufacture of this salt, as it is one of considerable importance in some of the manulactures of this country ; and there is reason to believe, that there arc many situations in which the preparation of it might be successfully conducted. Its particular chemi cal properties, will, with more propriety, be considered under its history in the article CHEMISTRY.
There arc several varieties of alum, it has been men tioned, to be found in commerce, some of which arc pre ferred to others for particular purposes. The English alum is in large irregular masses, of a crystalline struc ture, transparent and colourless. The rock alum is in small pieces, covered with an efflorescence or a reddish colour. The Roman alum is in fragments, having an irregular crystalline form, opake from efflorescence, and has been considered the purest of any. The differ ences among these have been very imperfectly investi gated. The English alum has been said to have a more sensible impregnation of iron than Lie others; and hence its inferiority in the art of dyeing in the composition of some of the more delicate colours. The rock alum, from its colour, may be suspected to contain a slight im pregnation of cobalt or manganese.
The principal consumption of alum is in the arts of dyeing and calico printing. It is one of the most power ful of the mordants, or substances which either modify the tint from colouring materials, or render the colour more permanent. It also enters into the composition of lakes or pigments. It is used in the art of tanning; is added to tallow to give it greater hardness in the manu facture of candles; and is employed for various purposes in the practice of medicine. (b)