The Principles of Leather Manufacture

by H. R. Procter

In place of applying the caustic soda first, and the calcium chloride subsequently, hides may be first treated with calcium chloride solution, and then with caustic soda, or the caustic soda may be applied to the flesh side of the hide by painting. These modifications are covered by Messrs. Pullman’s patent, but they are willing to grant licences for experiments at a nominal fee.

Alkaline carbonates are much milder in their action on hide than the corresponding hydrates, and although they will unhair hides, in absence of lime, their action is somewhat uncertain and slow. “Polysulphin” (Polysulphin Co., Keynsham) owes its unhairing power principally to the sodium carbonate, and not to the small traces of sulphur compounds which it contains.

Sodium carbonate occurs in commerce in three forms: “soda ash,” a more or less pure dry sodium carbonate; “soda crystals,” or washing soda, Na₂CO₃·10Aq, containing 62·95 per cent. of water of crystallisation, and efflorescing in the air; and Gaskell and Deacon’s “crystal soda,” Na₂CO₃·1Aq, containing only 14·5 per cent. of water of crystallisation. It must be remembered that where carbonate is used in conjunction with lime it becomes causticised and converted into NaOH.

Sulphides.—The practice of using realgar, or red sulphide of arsenic (Ger. Rusma) as an addition to limes for fine leathers is one of considerable antiquity. It has the property of loosening the hair and epidermis structures with less solution of cement-substance than lime alone, and hence produces a leather of fuller and closer texture. It will, however, be convenient to defer the consideration of this agent till after that of some of the more modern and simpler substitutes, such as the sulphides of sodium and calcium. Sulphides of the alkalies and alkaline earths, if used in strong solution, say 5 per cent. or upwards, have the effect of very rapidly reducing the harder keratin-structures, such as hair and wool, to a pulp, attacking first the interior cells, so that the hair crumples up like a string of sausages, and in a few hours, or even, with very strong solution, in a few minutes, the whole mass becomes so completely disintegrated that it can be swept off the hide with a broom, or washed off in a tumbler. At the same time, the action on the substance of the hide, and especially on the cementing substance, is very slight, though the grain is swollen and temporarily rendered somewhat tender. On the other hand, when used in weak solutions, say ¹⁄₄ per cent. and under, in conjunction with lime, the hair is but little injured, while the hair-roots and dirt are rapidly loosened, and results are obtained very similar to those with arsenic.

Sodium Sulphide (Na₂S·9OH₂).[80]—For the methods of valuation and determination of sodium sulphide, see L.I.L.B., p. 28.

[80] In the Laboratory Book the water of crystallisation is given as 10 Aq. Later researches show that pure crystals of the commercial sulphide only contain 9 Aq., or 67·5 per cent. of water.

Hides suspended in solutions of sulphide of sodium of 2 to 3 per cent. strength unhair rapidly.

For the commoner classes of sole-leather, hair is frequently removed by painting on the hair side with a 15°-28° Tw. (30-40 per cent.) solution of (crystallised) sulphide of sodium thickened with lime, applied with a fibre-brush, and folding the hide in cushions in a damp place, or packing in a tub. The hair is reduced to paste in a few hours. The same effect is produced by drawing the hides through a similar solution without lime, of which sufficient is retained by the hair to destroy it. The workmen must be provided with indiarubber gloves to prevent the caustic effect of the solution on the skin and nails. Skins and lighter hides are conveniently unhaired by painting the mixture on the flesh side, when it will loosen the hair or wool in a few hours without destroying it.

For dressing-leathers and the finer sorts of sole it is best employed as an addition to ordinary limes to the extent of ¹⁄₄-¹⁄₂ per cent. on the weight of the hides or skins, when the hair is loosened more rapidly than with lime alone, and with less loss of hide substance.

Good samples of sulphide of sodium consist of pale-brown, almost colourless crystals, containing 28 to 32 per cent. of dry sodium sulphide, which readily deliquesce on exposure to air. Fused sodium sulphide can now be obtained, which contains nearly twice as much actual sulphide as the crystalline form. The dark green colour possessed by many samples of sodium sulphide is due to the presence of iron sulphide. If carefully used no serious harm can accrue from its presence. If allowed to stand a short time in solution the iron sulphide will settle out.