Ferric iron, like alumina, forms an “alum,” a double sulphate of iron and potassium, Fe₂(SO₄)₃K₂SO₄, 24Aq, forming fine pale-violet crystals, but dissolving to a yellow-brown solution. (It must be distinctly understood that iron-alum and chrome-alum contain no alumina, but are simply called alums because of their similarity of constitution, iron or chrome taking the place of the aluminium. Iron-alum, in conjunction with salt, can be used for tanning, giving a pale buff-coloured leather very similar to an ordinary alum leather. Thus the presence of a small quantity of iron in an alum used for tawing is of no consequence, except as affecting the colour of the leather. In impure sulphate of alumina such as “alumino-ferric,” it, however, generally exists in the green ferrous state, and only acquires tanning properties on oxidation. Without common salt iron-salts are still less satisfactory tanning agents than those of alumina under the same conditions, as the acid is yet more loosely held, and though basic ferric salts are taken up in considerable quantities by hide, the leather produced is thin, and usually brittle. Professor Knapp devoted much study to the production of a commercial sole-leather by basic iron-salts; and took several patents, which did not prove practically successful, though the brittleness was to some extent overcome by the incorporation of compounds of iron with organic materials such as blood and urine, of iron-soaps, and of rosin and paraffin in the leather. Like most mineral tannages, the process was far more rapid than that with vegetable materials. Knapp’s basic tanning liquor was made by the oxidation of ferrous sulphate with a small quantity of nitric acid. Patents have also been taken for the oxidation of ferrous sulphate by peroxide of manganese in presence of sulphuric acid, which produces basic ferric sulphate in mixture with manganese sulphate, which has also some tanning properties. Attempts have also been made to tan by treatment of the hide with solutions of ferrous sulphate, and subsequent exposure to the air, in order to oxidise the iron on the fibre and convert it into a basic ferric salt, but have not proved of any commercial value. The principal use of iron at present in leather manufacture is in dyeing blacks (see p. 413), but in this case, its feeble hold upon acids in the ferric state, and its tendency to act as an oxidising agent, or oxygen carrier, renders the blacks somewhat unstable, and is frequently injurious to the leather. There is also little doubt that the presence of ferric salts in leather blacks has a great tendency to cause the resinification of the oil, known as “spueing,” by promoting its oxidation.