It would seem probable that the real solution of the problem would be found by a different process altogether. In this connection it is interesting to note that a Continental proposal to unhair by enzyme action only has been found most practicable with goods for chrome, and, in the author's opinion, some development on these lines, in which a lipolytic enzyme is used in addition to a proteolytic, might solve the difficulty, and give a rapid depilation which dispenses with liming, plumping and deliming with the consequent loss of valuable hide substance.
In the usual short, mellow, sulphide liming it is clear that there is not much advantage in a "round" or "set" of pits. Hence the one-pit system is popular on account of the less labour involved.
The above remarks are less applicable in the case of chrome sole leather. In this case weight is a great consideration and plumping is necessary. In such leather the chrome tannage is supplemented by the use of waxes, which fill up the spaces between the fibres and give solidity and waterproofness to the finished article. With this leather an ordinary sole leather liming in sharp liquors is not unsuitable, handling the goods from "mellow to fresh," but there is, on the whole, a tendency to shorten the process to about a week by using more sulphide.
Processes for deliming pelt for chrome leather should also be chosen by our guiding principle of hide substance conservation. Here again short processes involving little change in swelling should be preferred. Now, the ordinary bating and puering processes give (1) neutralization of lime by organic acids combined with weak bases; (2) the solation of some hide substance; and (3) a "pulling down" effect on the swollen pelt. Now, neutralization is quite superfluous, as the acids of the chrome liquor (one-bath or two-bath) can quite well accomplish this; the solvent effect is undesirable altogether; and the "pulling down" effect is also unnecessary if the goods are not plumped up. With any method of liming, however, some plumping is obtained, and this creates a problem of practical importance. In the huge quantities of pelt which go for chrome upper leathers, a primary consideration is the soft, "kind," or mellow feel of the grain in the finished leather. This is obtained only by tanning the pelt when the grain at least is in a thoroughly deplumped and inelastic condition. It is essential to delime not only so that the alkaline plumping effect is completely removed, but also so that no acid plumping effect succeeds it. The practical problem is to decide whether, in any particular instance, dung puers and bates are necessary to obtain this result. Bating is clearly not very desirable, on account of the length of the process, during which hide substance would be lost unnecessarily, and also because there will usually be a slight alkaline swelling. Puering with dog-dung infusions is preferable; it is not such a long process, the liquor is just acid to phenolphthalein, and the action is more intense, and by puering for a short time only the loss of hide may be confined to the grain and flesh only, whilst the desired inelasticity of grain-pelt is soon obtained. Many large firms have admittedly found themselves unable to dispense with puering, but others have succeeded in substituting for it the use of non-swelling deliming and lyotrope agents like ammonium chloride and boric acid. In all cases it is futile to delime or puer the grain and then allow the goods to stand until the centre lime has diffused outwards. The goods must pass into the chrome liquors when in the correct condition. For heavy chrome leather a surface deliming with boric acid is all that is necessary. Even that is superfluous when the goods are to be pickled before tanning.
Types of Two-bath Chrome-Tannage.—Although the original process of the Schultz patent is quite a practicable one, many modifications have been introduced. These modifications have been made to suit the class of goods under treatment, to suit the particular mode of application which is available or suitable, and to effect economies of chrome and other material, and of time, and also to combine with the tannage a pickling effect or a partial alum tannage. Other modifications arise from the precise acid, neutral, or alkaline condition of the pelt, being for example obviously necessary when pickled stock replace neutral pelts. The many two-bath processes which have been found useful have been classified previously by the author[6] into three types: (1) The "Schultz type," in which such quantities of dichromate and acid are used that there is no excess of free acid (other than chromic), but an excess of unaltered dichromate; (2) the "Acid type," in which the chromic acid is completely free and the liquor contains also some excess of mineral acid also; and (3) the "Neutral type," in which neither of these main constituents is in excess, just sufficient mineral acid having been used to liberate all the chromic acid from the dichromate.
[6] "Types of Two-bath Chrome Tannage," Leather, 1909, 227-259.
Now:—
K2Cr2O7 + 2 HCl = 2 KCl + 2 CrO3 + H2O
204 73
Taking the commercial hydrochloric acid as a 30 per cent. solution, 73 parts will be obtained in about 250 parts of commercial acid. Hence 294 parts dichromate need 250 parts commercial hydrochloric acid for the above reaction;[7] in other words, 5 per cent. dichromate needs 4¼ per cent. commercial acid. Similarly 6 per cent. and 4 per cent. of dichromate need 5.1 per cent. and 3.4 per cent. respectively of commercial acid. If therefore such quantities be used we have the so-called "Neutral type" of chroming bath. If less quantities of acid be used we have the "Schultz type," and if greater quantities of acid be used we have the "Acid type." The original Schultz patent used 5 per cent. dichromate and 2½ per cent. hydrochloric acid, and well exemplifies its type, for there is much undecomposed dichromate. The composition of some chroming baths in common use on a practical scale are given below under the heading of their type:—
| Type. | Dichromate. | Hydrochloric Acid. | Salt. | Aluminum Sulphate. |
| Schultz | 5 | 2½ | — | — |
| 5 | 2½ | — | 3 | |
| 5 | 2½ | 5 | — | |
| 5 | 2½ | 10 | — | |
| 6 | 3 | — | — | |
| Acid | 4 | 4 | — | — |
| 4 | 4 | 5 | — | |
| 5 | 5 | 5 | 3 | |
| 5 | 5 | 10 | — | |
| 6 | 6 | 15 | — | |
| 3 | 3 | 15 | 4 | |
| 2 | 4 | 10 | — | |
| 4 | 15 | 24 | — | |
| Neutral | 5 | 4¼ | 5 | — |
| 5 | 4 | — | 2½ | |
| Chromic acid | ||||
| 5 | — | 5 | — | |
| 6 | — | 8 | — | |
| 4 | — | 10 | — |
[7] Commercial acids of course vary in strength, and the amount needed varies accordingly.