Macdonald, and said to be capable of unhairing 800 aides a day, is shown in Fig. 905.
When the hair is very tho roughly loosened, as by sweating, or destroyed, as by sulphide of sodium, it is not uncommon to work it off by friction in the stocks ; but it is very doubtful whether the saving of labour is not more than compensated by the loss of weight, consequent upon submitting the hide, while its gelatine is in a partially dissolved condition, to such rough usage.
After unhairing, the loose flesh and fat are removed from the inner side of the hide by a sharp edged knife E (Fig. 903), partly by brushing or scraping, partly by paring. It is necessary not only to out off the visible adhering fat, but to work the hide well, so as to force out that contained in the loose ereolar tissue, which would not only impede tanning, but is liable to soak completely through the hide, producing most unsightly blotches. Several machines have been introduced to supersede hand-fleshing, but with only partial success. One of the best is Garric and Terson's machine (Fig. 904), which gives a very level flesh, free from galls, and without so much loss of weight, but scarcely so clean as desirable, while the saving in labour is not great. Molinier's machine (Fig. 906), and that of Jones and Rocke, are well adapted for skins, but hardly capable of fleshing an entire hide. All these machines are very similar in principle, the working parts consisting of drums with oblique or spiral knives.
When unhaired and fleshed, the hides intended for sole-leather, are, in England, almost invariably "rounded," or separated into (1) " butts," which are the best and thickest parts, and receive the most solid tannage, and (2) " offal," which is thinner, and for which a cheaper and more rapid tannage is sufficient. Fig. 907 shows the customary division. Frequently the butt is divided down the centre, and the halves are then called " bends." A piece called a "middle" is sometimes taken between the butt and the shoulder.
After rounding, it is neces sary to get rid of the lime, as completely as possible, before taking into the tan-house. For this purpose, the butts are usually suspended in fresh water for 12-24 hours, and frequently shaken up in it to remove ad= hering lime and dirt. If the water is hard, it is best to add to it, before putting in the butts, a few
pailsful of clear lime-water, to precipitate the acid carbonate of lime, which would otherwise cause a deposit of chalk on the surface of the butts ; this would not only make the grain harsh, but after wards, by combining with the tannin of the liquors, would cause bad colour. For the same reasons, it is important that limey hides should be as little exposed to the air as possible, as the latter always contains a small amount of carbonic acid, which renders the lime insoluble.
This suspension in water is generally considered sufficient for sole-leather, but it removes the lime very imperfectly. In olden days, it was customary not only to wash the hides much more thoroughly in water, but to "scud" them (i. e. work them over with a blunt knife), to remove lime, and the detritus of hair-roots and fat-glands. This is now frequently omitted from sole leather treatment, but no doubt leads to a more complete re moval of the lime.
Tanning Materials. — Before describing the management of the hides in the tan-house, it is necessary to say a few words about one or two of the prin cipal materials used, and the methods of preparing them.
Further details of their nature and origin will be given in the article on Tannin.
Oak-bark is one of the oldest of tanning materials, and the leather produced by its aid is still considered for many purposes the best. For sole-leather, its weakness in tannin (8-12 per cent.), the slowness of its action, and the light weight of the leather produced, render it unavailable alone except for the very finest class of work. It is, how ever, generally used in admixture with stronger and cheaper materials, such as valonia.
Valonia, the acorn-cup of an evergreen oak growiug in Greece and the Levant, is perhaps the most important of materials to the sole-leather tanner. It contains 25-35 per cent. of a tannin somewhat similar to oak-bark, and, like it, com municating a light-coloured bloom to the leather, but giving much greater firmness and weight, and a browner colour.