The currying of crown leather is very simple. It is set out on flesh and grain, and boarded to raise the grain. Mossner, before currying, washes 2 hours in water and brushes with tepid soda solution (1 in 60). The yield of weight is small, only amounting to about 30-40 per cent. of the raw hide employed, and hence the price per lb. must be considerably higher than that of tanned leather to yield a profit. The above information is mostly drawn from articles by W. Eitner ('Der Gerber,' iv. 1 _et seq._) and Franz Kathreiner ('Gerber Zeitung,' 21st December, 1875). CHAPTER XIX. MINERAL-TANNED LEATHER. The invention of the earliest form of mineral tanning, that with alum and salt, dates from remote antiquity; but as it is in large measure the type of all that has been since done, it deserves examination in some detail, at least as regards principles. In practice it is used alone in curing skins with the hair on, and for making white leather for laces and other purposes; and, in combination with oil and albumen, which, as we have seen, are the tanning agents in the case of "crown leather," it forms the process for producing calf and glove kids, as will be described under those headings (pp. 223, 225). Careful researches by Reimer (Ding. Polyt. Jour., 205, p. 143 _et seq._) show (what has long been known in practice) that alum alone is not capable of making a pliable leather. The salt, nevertheless, does not enter into combination with the alum, or even with the hide. Its function is partially physical, increasing the diffusion of the solution, and partially chemical, as in the presence of acids (and salts of acid reaction) it precipitates the coriin, and prevents it from gluing the fibres into a horny mass as it dries. Prof. Knapp has shown that this is the first essential in producing leather, and that raw hide may be converted into a pliable material with all the properties of white leather by simply withdrawing the water with alcohol, in which coriin is not soluble, and by which it is therefore precipitated. This leather, containing when dried no added constituent, is of course at once reconverted into raw hide by soaking in water. Both the salt and a portion of the alumina is removed from tawed leather by soaking in water, and it then dries hard and horny, and by boiling in water will yield a considerable percentage of gelatin. The alum is not absorbed as a whole. It is a double salt (alumina and potash sulphate or alumina and ammonia sulphate), and only the alumina sulphate is absorbed, potash (or ammonia) sulphate accumulating in the liquor. The alumina salt retained by the hide, especially in presence of much salt, contains slightly more than its normal proportion of alumina to acid, or in chemical language is to some extent basic. This is caused partly by the lime remaining in the skin from the unhairing process, which neutralises a portion of sulphuric acid, but in part is