The Principles of Leather Manufacture

by H. R. Procter

Puering is a very similar process, applied to the finer and lighter skins, such as glove- and glacé-kids and moroccos, in which dog-dung is substituted for that of birds, and, as the mixture is used warm and the skins are thin, the process is generally complete in a few hours at most. Neither bating nor puering are very effective in removing lime, and seem to act principally by some direct effect of the bacterial products on the swelling of the pelt.

Drenching is occasionally used (e.g. on calf-kid) as a substitute for bating or puering, but more frequently follows the latter, and serves to cleanse and slightly plump the skins before tanning, and complete the removal of lime. The drench-liquor is an infusion of bran made with hot water, and allowed to ferment under the influence of special bacteria, which are always present in vats used for the purpose, and which develop lactic and acetic acids.

It will be noted that all these methods are fermentative, and their effect is not simply the chemical one of removing the lime, but the bacterial action leads also to solution of the cementing substance of the hide-fibres, and produces a marked softening effect on the leather, together with considerable loss of hide-substance. In the manufacture of the softer leathers this effect is generally desired, and no process would be satisfactory which did not produce it; but in other cases, such as harness- and strap-butts, firmer and heavier weighing leathers would be preferred, if it were known how to make them. The putrefactive processes would be gladly relinquished, if satisfactory substitutes could be found, not only on account of their offensive character, but because of their uncertainty and danger to the goods; and even if lime only were removed, the necessary softness could often be obtained by appropriate liming and tanning.

It will be best, therefore, to deal first with the purely chemical methods which aim only at removal of lime, before considering those involving bacterial action. Unfortunately, the chemical problem is not so simple as it might at first sight appear. The alkaline lime clings obstinately to the hide-fibre, and can only be removed very slowly, if at all, by mere washing. On the other hand, the use of any excess of strong acid is absolutely precluded, because of its powerful swelling effect on the pelt, in the tanning of which it would prove even more injurious than the lime, making dark-coloured and brittle, or tender, leather. This effect is not to be avoided by the use of even very dilute solutions of strong acids, since the affinity of hide-fibre for them is so strong that it will abstract practically all the acid from even a decinormal solution, leaving it quite neutral. What is required is an acid of extremely weak affinities, forming soluble lime salts, and obtainable at a low cost; or, on the other hand, a salt of some weak base which could be displaced by lime, and which would not act injuriously on the pelt. With certain precautions, and in special cases, however, the stronger acids may be used successfully.

In the cases of sole- and belting-leather no softening is desired, and formerly tanners usually contented themselves with a very perfunctory washing in water, trusting to the acids present in the liquors to complete the removal of the lime. Even pure distilled water effects this removal very slowly and imperfectly, owing to the strong attraction of the lime for the fibre; and if “temporary hard” water is used, the lime present in the hide combines with that present in the water and is precipitated as chalk in the surface of the hide. This may be prevented by previously adding a small quantity of lime or lime-liquor to the water before use to soften it (see [p. 95]); but unless this is very carefully done, the free lime present in the water prevents it from removing any from the hide. The safest way is not to add lime direct to the water, but to change the latter gradually, so as to allow the lime already present to soften the new portion of water.

A much more efficient method is to suspend the butts in water to which small portions of diluted acid are successively added till the lime is nearly, but not quite, neutralised. If carefully used, sulphuric acid[86] is perhaps as good as any, but, of course, any excess will spoil the colour or “buff” of the leather.

[86] The use of sulphuric acid for this purpose was patented by H. Belcher of Wantage (No. 14,943), but was used some years previously in several tanneries known by the author.

Acetic, formic, and lactic acids are safer than sulphuric, but are somewhat costly, and must not be used in appreciable excess. Crude pyroligneous acid may be used, and it has a considerable antiseptic effect owing to the phenols, etc., which it contains. Hydrochloric acid is not suitable for sole-leather, on account of the bad effect of chlorides on plumping. Sulphurous acid[87] is perhaps the best, and its acid properties are so weak that slight excess does little harm, but the neutral calcium sulphite is insoluble, and to actually dissolve the lime the hydric sulphite must be formed, which can only occur in presence of excess of the acid. Unless such excess is used, the colour of the pelt in the early liquors is apt to be somewhat greyish. Probably a very good method would be to suspend the butts in a solution of sulphurous or some other acid of about ^N⁄₂₀ strength, sufficiently long to remove all lime from the surface and slightly to plump it but not to penetrate to the centre of the hide, which should then be suspended in water until any excess of acid had been taken up by the unneutralised lime still present in the middle of the butt, which at the end of the operation should be rather alkaline than acid. The course of this, or any other bating operation can be followed by cutting the hide, and moistening the cut surface with alcoholic solution of phenolphthalein, which is turned red, or pink, by the least trace of free lime.

[87] Manufacture of sulphurous acid, see [p. 24]; testing, see L.I.L.B., p. 37.

In using mineral acids it is of great importance that they should be perfectly free from iron, and that the vat employed should contain no iron which could become dissolved, since, if present in the bating liquid, it is sure to be fixed by the hide, especially if the quantity of acid used is insufficient to neutralise the whole of the lime.