[123] Leather Trades Review. Later investigations have shown that the temperature of the water is unimportant if alkali be added, but chrome-alum dissociates to some extent in hot water, and comparative experiments have shown that solutions of the normal salt made with the aid of heat act on skin as if more acid than those made in the cold.
Eitner[124] has pointed out the important effect that differences of basicity have on the tanning properties of chrome solutions. Normal chrome sulphate or chrome-alum colours the leather quickly and equally throughout, and swells the pelt on account of its practically acid character, but gives a thin and lightly tanned leather, from which much of the chrome washes out, unless it is at once “neutralised” in alkaline solutions. As the chrome solution is made more basic, the tannage penetrates more slowly, but is heavier and more thorough, the colour is darker and bluer, and much less of the chromic salt is removed by washing with water. When the basicity becomes excessive, the solution becomes unstable, and decomposes on dilution with water into a very basic salt which is precipitated, and a more acid solution than that given by a moderately basic salt. The effect of such solutions on the leather is very unsatisfactory, producing the bad effects both of too acid and too basic salts. The pelt is apt to be swollen and lightly coloured by the more acid salt, but at the same time the actual tannage proceeds very slowly, and in extreme cases it is difficult to tan through, while the surface becomes over-tanned, and the grain often tender and even brittle from the incrustation of precipitated basic salt. Eitner likens the effect of the more acid liquors to the quickly penetrating and lightly tanning vegetable tans, such as gambier, and that of the more basic to the heavier tannages, such as valonia; and within limits, advantage may be taken of these facts in adjusting the liquors to the character of the leather it is desired to produce. In sulphate liquors, he considers the salt CrOH.SO4 as most suited to general use, and in the case of chrome-alum, this is produced by the use of 286 parts of soda-crystals, or 106 parts of dry sodium carbonate (1 molecule) to 998 (or practically 1000) parts by weight (1 mol.) of chrome-alum. (In using washing soda, care must be taken to employ clear crystals of the salt, and not those which have become white by loss of water.) In place of soda, Eitner makes a similar basic liquor by boiling 1000 parts of chrome-alum with 248 parts (1 mol.) of sodium hyposulphite until the whole of the liberated sulphurous acid is driven off, and the sulphur deposited. In comparative experiments by the Author, no difference could be detected between the tanning effects of the two solutions, and that with soda is both cheaper and more easily made. If the solution with hyposulphite is not boiled, a more acid liquor results, in which part of the chromium is combined with sulphurous acid, forming an unstable compound which may prove useful in certain cases.
[124] Gerber, 1901, pp. 3 et seq.
Eitner states that he has made chrome-solutions of various types, containing organic compounds in combination with the chrome-salt, which combine with the leather, producing a fuller and softer tannage, but he gives no details as to their preparation, as they are made commercially by the “Erste Oesterreichische Soda-Fabrik” at Hruschau. The writer has found that in some cases by the addition of say 3 parts of sugar, or still better of glucose, to 10 parts of the chrome-alum in making up the basic liquor, a much fuller and plumper leather is produced, which dries perfectly soft, even without staking or fat-liquoring; and it is probable that many other organic compounds may be found which produce similar effects. The addition of very small quantities of even neutral tartrates or lactates, and probably of many other organic salts or acids, have a remarkable effect in lowering the apparent basicity of the solution, and it is possible that these may also be usefully employed in combination with very basic liquors. It is highly probable that the unsatisfactory tanning liquors produced by direct reduction with some samples of glucose are due to the presence of small quantities of some organic acid produced during the oxidation. It has been found that these solutions may be made to tan by the liberal addition of soda. It is probable that more satisfactory results in chrome-tanning will be attained by the direct addition of known organic substances to basic liquors of definite constitution, than by the somewhat uncertain products of organic oxidations.
The quantity of salt to be added depends on the qualities desired in the leather, and upon whether chloride or sulphate liquors are employed; salt in chloride-liquors increasing the softness of the leather, but in excess tending to flatness, while in sulphate-liquors it practically diminishes their basicity by converting the chromium sulphate into the equivalent chloride, which, as Eitner points out, behaves as a less basic salt, and hence but little advantage is to be gained from its use. It is best to begin with a very weak liquor, to avoid drawn grain, and for the same purpose a preparatory tannage with alumina salts, or an addition of alum or sulphate of alumina and salt may be made to the first liquor, as the attraction of the chrome salt for the fibre is sufficient to produce a chrome tannage, even in presence of excess of alumina salts. 10 lb. of chrome alum will tan about 100 lb. of wet pelt, but more must be used for the first parcel; as to avoid loss of time, the skins may be tanned out in a pretty strong liquor. The bath has a tendency to become acid by use, and before strengthening, it may be necessary to add some more soda solution. Very little additional salt is required, as it is only absorbed by the skins to a small extent, probably as chromic chloride. As the liquors gradually become charged with sulphates, it is best to work them out like bark liquors, and not to go on strengthening the same liquor indefinitely. If old liquors are used for green goods, it is not necessary to neutralise them with soda before use, as Eitner has shown that less basic liquors colour more evenly and with less tendency to produce drawn grain.
Basic chrome liquors, such as have been described, may also be used in chrome combination tannage. It is generally best to let the light vegetable tannage precede the chrome, and lightly tanned skins, such as “Persians” and East India kips, acquire many of the qualities of chrome-tanned leather by the treatment. The effect is still further increased by a previous detannisation of the leather with alkaline solutions (see [p. 241]). Several firms beside Dennis now supply basic chrome liquors ready prepared for use.
The time of tannage will of course vary with the thickness of the goods, and for calf-skins will usually extend over some days, though it can be much quickened by drumming. The tannage is generally best accomplished in the paddle, but can be carried out by frequent handling in pits or tubs, or, where very smooth grain is important, by suspension. When the goods come out of the final liquor, they may be allowed to lie in pile for twenty-four hours, or even for some days, with advantage, as the surplus chrome liquor is pressed out, and the tannage becomes more complete. They are then washed with plenty of warm water, till it ceases to be coloured with chrome. They may be kept for an almost unlimited time in a wet condition, as they do not bleed, and have little tendency to heat even in pile. They have now reached the stage at which we left the “two-bath” leather, and the subsequent treatment may be the same in both cases.
Although by both processes, the chrome-salt fixed in the fibre is of a decidedly basic character, it still contains enough acid to act injuriously on the leather in course of time, and to lead to serious inconveniences in its subsequent treatment. Before proceeding further, this access of acid must be removed or neutralised, and it is not too much to say that most of the troubles experienced in the fat-liquoring arise from neglect or mistake in the washing and neutralisation. The difficulty in the process arises from the fact that while the acid should be reduced to a mere trace, it must not be entirely removed,[125] as chromic oxide itself does not seem capable of tanning, and at any rate the effect of excess of strong alkalies is at once to render the leather hard and pelty. Borax is one of the safest neutralising materials, about 3 per cent. on the wet weight of the pelt being required, in not more than 1⁄2 per cent. solution. Eitner recommends the use of silicate of soda, which, sold as a solution of S.G. 1·5, is somewhat stronger and much cheaper than borax. Hyposulphite of soda and whitening together neutralise more rapidly and completely than either alone. Other salts of weak acids may also be used, the acids exercising a regulating influence which prevents neutralisation going too far. Sodium carbonate or bicarbonate, or ammonia may also be used, but with these it is difficult to get even “neutralisation,” or to avoid the risk of carrying the process too far. Even a thorough drumming with a milk of “whitening” (calcium carbonate) is effective. With the latter there is no danger of overdoing the process, but in some cases the adhering whitening and precipitated calcium sulphate are troublesome in later operations. In any case the neutralising should only be carried so far that the skins show no acid reaction to litmus paper.
[125] Procter and Griffith, Journ. Soc. Chem. Ind., 1900, p. 223.
It is probable that one of the great causes of difference between “one-bath” and “two-bath” leathers is the presence of free sulphur in the latter. This may also be introduced into “one-bath” leather, by treating it in the wet chromed state, without washing out the chrome liquor, with excess of a solution of hyposulphite, or of an alkaline polysulphide, which at the same time will neutralise the skin. The more acid the chrome liquor, the greater the quantity of sulphur which will be introduced. The simplest means of distinguishing “two-bath” from “one-bath” tannages is to test for the presence of sulphur, by wrapping up a silver coin, with a piece of the leather in paper, and leaving the parcel for an hour in the water-oven, or some other warm place, when the presence of sulphur will be shown by the blackening of the coin. Of course a sulphurised “one-bath” leather will give the same reaction.