For the modern process of manufacture, good, even and well-flayed hides are selected, and unhaired either by sweating, or by a very short liming, which must be assisted by rockers or some mechanical mode of moving the hides, so as to get them unhaired in the shortest possible time and with the least injury to the fibre. Sodium sulphide (see [p. 147]) may be employed with great advantage. The fleshing and scudding are performed as usual, according to the mode of unhairing adopted. The hides are then very commonly rounded, and the bellies tanned in the usual manner; but sometimes the whole hide is made into crown leather.
As crown leather is naturally almost white, it is usual at this stage to colour the hide with bark or other liquors. As in this case simply colouring and not tanning of the grain is required, high-coloured liquors, made by steaming materials with much colour and little tannin, are preferable. For this purpose wood extracts, such as chestnut, quebracho, or oak-wood are said to be very suitable, and beech, pine or alder bark may also be used. In practice, chestnut and hemlock extracts, and occasionally cutch are employed; but the last named is not to be recommended. A chestnut liquor of 71/2° Tw. or 5° B. (34° Bark.), with constant handling or in a paddle-tumbler, will give a satisfactory colour and grain in 1-2 hours. This rapid colouring is preferable to the slower process, which occupies 24 hours in weaker liquors. If sweated, the hides are now plumped with sulphuric acid, but only to a very moderate extent. This process is best performed in a paddle-tumbler; about 31/2 oz. of sulphuric acid are required per hide, and a time of 6-12 hours according to the water employed. The liquor may be several times used, strengthened with the necessary quantity of acid. Limed hides do not require further swelling. The hides are washed through clean water, and hung up to dry somewhat.
The hides are next spread on a table, flesh-side uppermost, and covered with a layer of the tanning paste nearly 1/4 in. thick. The composition of this paste may be varied according to the relative prices of different materials, and the amount of hard fats must be regulated according to whether or not appliances are provided for heating the tumbler. A good mixture is 7 parts common wheat-flour, 7 of horse-grease, 1 of salt, and 1-2 of tallow. If too soft, more tallow may be employed. The salt is first added to the horse-grease, then the melted tallow, These fats are added little by little to the flour till a uniform paste is obtained. Another good mixture is 27 parts wheat-flour, 25 of bone-grease, 4 of tallow, and 4 of salt. Another recipe gives 28 lb. fine white flour made to a paste with 13-14 pints water and then worked up to a uniform mass with a tepid mixture of 28 lb. beef tallow and 28 lb. hard horse-fat (Pferdekammfett). These mixtures are all for use in warmed drums; a specimen of one used in a factory where the mixture was simply trodden in cold into the leather in open tubs is as follows:—7 parts flour, 9·4 of horse-fat, 2·8 of fish-oil, 7 of ox-brains and 0·7 of salt. The hides are next folded in bundles and placed in the drum; or in stocks, which are occasionally used for the purpose. If a drum be used, it must be of large diameter, 8-9 ft., provided with pegs inside, and should make about 25 revolutions per minute, so as to work the hides with considerable force. Much more care is needed in warming the drum, than is required in ordinary stuffing, and this is best accomplished by warmed damp air. This may be arranged by the use of an air-pump, which draws air through water warmed by exhaust steam, and forces it through the hollow axles of the drum (or drums); or a simple aspirator consisting of a cask filled with water may be connected to one axle, so that as the water runs out it will draw air through the drum from the opposite axle, which is connected with a cask half filled with hot water through which air is allowed to bubble. Probably the same effect could be reached in a still simpler and cheaper manner by the use of a steam-jet blower, such as Körting's. In any case the drum must be warmed to a temperature of 82°-104° F. (28°-40° C.). Warm dry air may also be used, but is not so suitable, as it dries the hides too much. The hides are tumbled 8-12 hours, hung up till half-dry, and the process is repeated. For very heavy hides, 4 tumblings may be required. In the later tumblings, a lower temperature, 95° F. (35° C.), may be employed, and the time extended to 15 hours.
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 the result of the affinity of the hide-fibres for alumina, a certain small proportion of free sulphuric acid being left in the liquor. The accumulation of this and of potash sulphate is the reason why such liquors cannot be used perpetually by mere strengthening with alum, but must be frequently renewed. The attraction of hide-fibre for alumina sulphate is so strong, that in presence of a sufficient excess of hide it may be completely removed even from dilute solutions. Alumina acetate or sulphate may be substituted for alum with equally good results in practice, the only advantage of the latter being its easier preparation. Ferric and chromic salts and iron or chrome alum, may be substituted for common alum, and are absorbed in a similar manner, and in presence of common salt give equally pliable leathers, of a buff and pale greenish tint respectively. Without salt, the leathers are hard and brittle. In all these cases, the tanning agent may be to a large extent removed by simple washing with water. The tannage may be rendered more durable by passing the leather before drying through a weak bath of sodic carbonate or even lime-water, which precipitates the alumina, iron, or chrome in a basic form on the hide-fibres. Soap baths may also be used, by which aluminic, ferric, or chromic stearates and oleates are formed, possessing considerable toughness and resistance to water. So far as the writer is aware, no mineral tannage has yet been produced which will not yield gelatin when treated, first with dilute acid and then with boiling water; but this is rather a gain than otherwise, as leather scraps might be utilised for glue. There seems no reason why good and durable leather, for boot-uppers and for many mechanical purposes, should not be fabricated with salts of iron and chromium in conjunction with salt. If eggs and flour were also used, products similar to calf-kid would be obtained. Iron-leathers may of course be blacked with infusions of galls or many tanning materials, or with logwood. Ferrous salts have no tanning properties.