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. 30] 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.
Fig. 30.
After rounding, it is necessary 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 adhering lime and dirt. If the water is hard, it is best to add to it, before putting in the butts, a few pailfuls of clear lime-water, to precipitate the lime bicarbonate,[T] which would otherwise cause a deposit of chalk on the surface of the butts; this would not only make the grain harsh, but afterwards, 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.
[T] Lime softens water containing lime bicarbonate in solution by combining with half the carbonic acid, when the whole is precipitated as normal carbonate or chalk. CaO + CaCO3 . H2CO3 = 2CaCO3 + OH2. This is Clark's process. See also [p. 84].
This suspension in water is frequently 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, and this should never be omitted from sole-leather treatment where bright colour and clean buff are desired. Some tanners go so far as to bate best butts slightly with hen-dung, but with such treatment firmness and weight are lost. Washing in weak solution of sugar, or ammonic chloride or sulphate, or of sulphuric, or hydrochloric acid, may be adopted. It is essential to use acids nearly free from iron, as it may be precipitated on the butts and give a bluish colour in the liquors, and the acid must be of such a strength as neither to allow the iron to be precipitated, nor, on the other hand, perceptibly to plump the butts, which in this stage would endanger buff and colour. 100 cc. may neutralise 15-20 cc. of lime-water for this purpose. Hydrochloric acid and chlorides have a tendency to prevent plumping, and are therefore better adapted for dressing than for sole leather. Great care must also be taken to prevent putrefaction, or the use of putrid solutions, if firmness and plumpness are desired.