CHAPTER XIV.

_ALUM TANNAGE OR TAWING._

We have now followed the raw material up to the final stage of preparation for its actual conversion into leather, and it remains to consider the means by which that important change is produced. Though as yet the vegetable tanning process is most largely used, and possesses the greatest commercial importance, the use of mineral salts has long been known, and, through the advent of chrome tanning, has placed the permanent supremacy of the vegetable tannins in considerable doubt. Not only the importance of mineral tanning processes, but their greater simplicity from the scientific side, justify their consideration before those of vegetable origin.

In the previous chapters it has been shown that to produce a permanent leather, it is not only necessary to dry the fibres in a separate and non-adherent condition, but so to coat them or alter their chemical character that they are no longer capable of being swelled and rendered sticky by water. All salts which produce a contraction or dehydration of the fibre similar to that caused by alcohol are capable of the first effect in a greater or less degree. Many sulphates, and particularly those of sodium and magnesium, though they will not alone produce leather, will so far contract the fibres as to greatly hasten tanning by vegetable tanning materials, and they are therefore capable of useful application in quick tanning processes, especially where tough and light-weighing leathers are aimed at, which may be subsequently weighted and solidified by further treatment. Strong solutions of ammonium sulphate are almost as strongly dehydrating as alcohol, and will produce white leathers very similar to those formed by pickling, a fact which is certainly of considerable commercial importance. None of these salts, however, can form a complete leather in themselves, but require the assistance of metallic salts which will permanently fix themselves in the fibre, and diminish or destroy its attraction for water. Many substances have this power in a greater or less degree, but all those of commercial importance belong to the group of which aluminium, iron and chromium are representative, and which are capable of producing salt-forming oxides of the formula M₂O₃ (e.g. alumina, Al₂O₃). Manganese, of which the salts of this type are very unstable, has very slight tanning power, while titanium, which in many ways is allied to the group, though it does not strictly belong to it, has recently been patented as a tanning agent. For the present, however, we may limit our attention to the three metals first named.

Alumina and its salts demand the first attention, not only as having been used for leather manufacture in very early times, but as being still important commercially. The metal aluminium is now well known, and its oxide, alumina, Al₂O₃ is abundant in nature, combined with silica in the form of clay and bauxite, as fluoride in combination with sodium fluoride in cryolite, and in some cases as a native sulphate. Alum-shale, which was formerly the principal source of alum, is a bituminous clay containing much iron sulphide, and which when calcined yields aluminium sulphate. As aluminium sulphate does not crystallise readily, and was difficult to free from iron, potassium sulphate was added to the liquor obtained by leaching the calcined shale, from which, after concentration by boiling, potash-alum, a double sulphate of potassium and aluminium, Al₂(SO₄)₃,K₂SO₄,24Aq, was easily crystallised out. Alum is now usually made by decomposing clay or bauxite with sulphuric acid, and ammonium sulphate is generally substituted for the potassium salt, yielding ammonia-alum, a double sulphate of aluminium and ammonium of similar constitution to potash-alum. Ammonium alum is easily distinguished from the potassium salt, by the strong smell of ammonia which it evolves on the addition of caustic soda or lime. So far as is known, there is no practical difference in tanning effect between the two salts, and ammonium alum is cheaper, and slightly stronger, its molecular weight being 906, as against 948 for the potassium salt. Either alum dissolves readily in cold water to the extent of about nine parts in 100 of water, and more easily, and to a much larger extent in hot water, from which the excess crystallises on cooling. It is said that for purposes of leather manufacture, alum solutions should not be boiled, and, though it is improbable that this produces any change, it must be remembered that chrome alum on boiling really does undergo decomposition to free acid and a more basic salt, indicated by change of colour from violet to green, from which it slowly returns to the violet form on cooling.

Alums are only valuable in leather manufacture in proportion to the aluminium sulphate which they contain, the potassium or ammonium sulphate taking no part in the reaction, and since improved methods have rendered possible the production of aluminium sulphate practically free from iron, it has largely taken the place of alum, than which it is both cheaper and stronger. Crystallised aluminium sulphate, Al₂(SO₄)₃, 18Aq, has a molecular weight of 666, which is of equal value to 906 of ammonia-alum, and 948 of potash-alum. Iron is the most objectionable impurity in both alums and aluminium sulphate, and may be detected by the addition of potassium thiocyanate, which will produce a red colour, or potassium ferrocyanide (yellow prussiate of potash), which will produce a blue. As the iron may be present in the ferrous condition, it is safer first to boil the alum solution with a few drops of nitric acid or bromine water. For more accurate determination of iron see L.I.L.B., pp. 20, 136.

No satisfactory leather can be produced with a solution of alum or aluminium sulphate alone, the skin drying horny, and incapable of softening by stretching. In practice, salt is always used in addition, the proportion being very variable, but averaging about half the weight of alum, or two-thirds the weight of sulphate of alumina employed. The mode of action of the salt has long puzzled chemists, and it has been supposed that its use was to convert the aluminium sulphate into chloride, a reaction which takes place to some extent, but which fails to explain the production of a soft leather, since aluminium chloride, though freely taken up by the skin, produces no more satisfactory leather than aluminium sulphate. The real explanation is found in