It does not fall within the scope of this book to describe in detail the working methods for the production of the different kinds of chrome leather, but a few precautions common to all forms of the process may be named. It is not absolutely important in all cases that goods should be completely freed from lime before chrome-tannage, but in this case a sufficiency of acid must be allowed in the first bath to neutralise the lime introduced. Pretty thorough liming is generally advisable, to plump and separate the fibres, but as a rule the bating or puering of goods for chroming should not be excessive,[118] but should be planned not to remove more than is absolutely necessary of the hide-substance, as the chrome tannage is in its nature soft and light, and does not lend itself to artificial fillings, such as the flour and egg-yolk of the calf-kid process. Skins are sometimes freed from lime by “pickling” ([p. 89]), and pickled skins may be chromed without depickling, which will be done by the dichromate, but in this case the acid contained in the skins must be considered in the composition of the chroming bath. Skins, indeed, which are pickled with a sufficiency of acid may be chromed in a neutral dichromate bath, and this is sometimes a convenient mode of procedure. To prevent drawing of the grain during tanning, skins not unfrequently receive a preliminary tannage with alum, or sulphate of alumina, and these materials, together with salt, may be introduced into the chroming bath, in which case they will liberate a portion of the chromic acid, as has been mentioned in connection with the Heinzerling process. Alum, chrome-alum, and acid salts, such as sodium bisulphate, may be substituted for the acid in the chrome bath, but organic acids must not be used, as they would reduce the chromic acid. The quantity of free chromic acid in the chrome bath is of the most vital importance to success, as it, and not the dichromate (which may be present in considerable excess), regulates the amount of chrome taken up by the skin, and the subsequent degree of tannage. It is very possible to injure leather by overchroming, rendering it rough, harsh and even tender. If a bath containing excess of bichromate is to be re-strengthened, it may be assumed as a rule that all the free chromic acid has been absorbed by the skins, and while it is merely necessary to restore the strength of the dichromate to its original amount, the full quantity of acid must be used which would be required in preparing a new bath. Where, as in Eitner’s acid chrome bath, the whole of the chromic acid is liberated, the bath may be exhausted by a second pack of skins. Many tanners, in order to avoid the complications of remaking a bath, run away their chrome liquors after once using, containing all the excess of dichromate which has been used. With proper chemical control, this should not be necessary, and is objectionable, not only from its wastefulness, but on account of the very poisonous character of the unreduced bichromate. Even weak dichromate solutions, especially if warm, are liable to cause painful and obstinate eruptions on the hands, but this rarely occurs to tanners, as the poisonous action of the solution is removed on reduction. It is well, however, to arrange that men who handle skins in the chrome bath, should subsequently also work in the reducing bath. Methods of analysis of used chrome liquors are given, L.I.L.B., pp. 142 et seq. Those for the determination of acidity are not however easily applicable in the presence of alum and salts of chromic oxide.
[118] Goat-skins for glacé kid need thorough puering to produce a smooth grain.
The skins, on coming from the chroming bath may be allowed to lie for some time without serious injury, but should be carefully protected from the action of light, which reduces the chrome at the expense of the skin, and renders the subsequent tannage irregular. It is found that skins, if brought into a weak or neutral reducing bath, are apt to “bleed” or lose chromic acid, which is reduced wastefully in the bath. On the other hand a strong “hypo” bath is apt to draw the grain and contract the skins, owing to the tannage taking place too suddenly. A somewhat strong “hypo” bath is therefore often employed as a preparatory “dip,” the skins being simply drawn through it to fix the chrome on the surface, piled on a “horse” and subsequently reduced in a bath of ordinary strength. The tendency to bleed is lessened, but at the expense of the pelt, by the reduction which takes place if the skins are allowed to lie overnight in the chromed state. Eitner states that skins chromed in an acid bath (i.e. where the whole of the chromic acid is in a free state) show little tendency to bleed. After reduction, the skins are well washed with warm water, and their subsequent treatment is the same as that of skins tanned by the one-bath process, which is subsequently described (see [p. 211]).
Naturally in practical work, the reduction cannot be made to proceed rigidly in the definite steps described by Eitner on [p. 206], but all go on in different proportions together, though by supplying the acid in proper quantities, and at proper intervals, they may be made in the main to follow in the given order. Both on this account, and because neither the exact amount of chromic acid in the skins, nor the sulphurous acid lost by escape into the air can be exactly determined, the reduction cannot be conducted on theoretical principles, but the best conditions must be empirically determined. Eitner states that 12 parts of thiosulphate dissolved in 400 parts of water, and 6 parts of (40 per cent.) hydrochloric acid are sufficient for 4 parts of bichromate per 100 of wet pelt employed in the chrome bath, of which not more than one-half to two-thirds is absorbed; and that if equal parts of bichromate and acid are employed in chroming, the acid used in reducing may be lessened to 5 parts. In this case it must not be forgotten, that if the partially exhausted chrome-bath is used for a second pack of skins, which are afterwards finished in a bath of full strength, nearly the whole quantity of bichromate used in making up one bath will be absorbed by the skins. The amount of acid consumed in reduction will be greater, the more rapidly it is added, owing to increased escape of sulphurous acid. It is better to add the acid, previously diluted with water, in 8 or 10 successive portions, more rapidly at first, and more slowly during the latter half of the operation, each portion of acid being added as soon as no further change of colour appears to be caused by that already given. These changes are the more rapid the lighter the goods. The colour darkens at first to olive-brown, then gradually becomes green, and finally blue, and when this colour is uniform throughout the thickness of the goods, no further acid need be added. For goods which have been chromed in an acid bath, Eitner states that no acid will be needed for the first twenty to thirty minutes. It is important to have a sufficient excess of thiosulphate in the bath when reduction is complete, in which case the goods may be left for some hours or overnight in the bath, to complete “neutralisation,” but Eitner prefers to use a fresh bath of 11⁄2 parts of thiosulphate in 400 parts of water for this purpose, the bath being used, after settling, for making up the reduction bath for the next lot of goods, for which 11⁄2 parts less thiosulphate is used. The goods must be kept in motion during reduction, either in a drum or a covered paddle.
In a paper on “Die Natur und Wesen der Gerberei” published by Professor Knapp, in 1858, he describes clearly a chrome tanning process with basic chromic chloride, formed by the addition of sodium carbonate to a solution of the normal salt, but he expressly states that the product was not more resistant to water than the ordinary alum tannages. How he fell into this error is hard to explain, for leathers produced according to his directions, resist not merely washing in cold but boiling water. As soon as the Schultz process proved successful, many attempts were made to evade the patent by the use of other reducing agents than the “hypo,” and other salts of sulphurous acid which it covered. Among these, the use of hydrogen sulphide, and acidified solutions of alkaline sulphides, and especially of polysulphides,[119] proved capable of practical use, though less convenient than thiosulphate, but were soon acquired by a combination, the Patent Tanning Company, together with Schultz’s original patents.
[119] “Liver of sulphur” or solutions, made by boiling sodium sulphide or soda with excess of sulphur.
Under these circumstances, Martin Dennis, either by fresh discovery, or otherwise, revived the original process of Knapp, which he patented[120] almost word for word, and offered a basic chrome tanning liquor for sale, without further restrictions on its use. This liquor was made by dissolving precipitated and washed chromic hydrate (easily prepared by precipitating chrome-alum solution with excess of alkali) in hydrochloric acid to saturation, and adding washing soda until the solution was rendered sufficiently basic. Such a solution may be used on skins prepared in the ordinary way, by diluting with water, and strengthening as the tannage proceeds, like a vegetable tan-liquor. It is doubtful if the patent is a valid one, as it was known that the use of such a solution was not new, and it was only granted in America on the representation, which has since been found to be mistaken, that chlorides alone were applicable for tanning, while Knapp had not restricted his statement to these salts. In reality chlorides and sulphates seem equally suitable, but to produce similar results the former must be made more basic than the latter. In any case the patent cannot cover the general principle of basic tanning, but only the particular liquor and mode of preparation specified. It was soon afterwards shown by the writer,[121] that a good chrome tanning liquor might be prepared by direct reduction of dichromate with sugar in presence of such a limited quantity of hydrochloric acid as to produce a basic salt. Suitable proportions are 5 mol. HCl to 1 mol. potassium dichromate, which produces a salt approximately Cr2Cl3(OH)3. The solution is easily made by dissolving three parts of dichromate in a convenient quantity of water, adding six parts by weight of concentrated hydrochloric acid, and then cane-sugar gradually till a green solution is obtained, when the whole may be made up to one hundred parts, and will be approximately of the same strength as a 10 per cent. solution of chrome-alum. A little heat may be needed to start the reaction, but too much should be avoided, as considerable heat is evolved by the oxidation; and as much carbonic anhydride is produced, which causes the solution to effervesce briskly, the vessel used should be of ample size. In place of cane-sugar, a good quality of glucose may be used, but some samples contain some impurity which produces a violet solution which will not tan satisfactorily. This liquor is in regular use in many tanneries, producing a good quality of chrome calf, but is somewhat variable in its effects according to the temperature employed in its preparation, and it appears to have no real advantage over a simple solution of chrome-alum, rendered basic by soda and with some addition of salt. A somewhat similar preparation is Eberle’s “chromalin,”[122] in which some organic substance, probably crude glycerine, is used to reduce the bichromate. The organic matters, and especially the organic acids which result from the oxidation of the sugar or glycerine, are not without influence on the tanning properties of the liquor. Of course these solutions may be rendered still more basic by the addition of sodium carbonate. A good stock-liquor, of approximately the same strength as that above described, is made by dissolving 10 parts of chrome-alum in 80 parts of tepid, but not hot water,[123] and adding with constant stirring a solution of 21⁄2 to 31⁄2 parts of washing soda in 10 parts of water. The chrome alum dissolves somewhat slowly without the aid of heat, and the solution is best made either in a small drum driven by power, or by suspending the crystals in a basket near the surface of the liquor, so that the saturated solution can descend.
[120] Martin Dennis, U.S.A. Pat. 495028, 1893; and 511411, 1893, 7732, 1893. E. Pat. Gallagher.
[121] Leather Trades Review, Jan. 12, 1897.
[122] Compare Eberle’s German patents, 119042, 1898, and 130678, 1899. The last of these appears to be anticipated, at least as regards the use of glucose, sugar and starch, by the writer’s publication in 1897 above cited.