All the figures are percentages of the weight of pelt. As K2Cr2O7 has a molecular weight of 294, and Na2Cr2O7 · 2H2O a molecular weight of 298, in practice they may be considered as interchangeable, weight for weight. The sodium salt is cheaper and more often used. The corresponding amount of chromic acid, 2CrO3, has an equivalent weight of 200, hence any weight of dichromate may in practice be substituted by two-thirds the weight of commercial chromic acid. Equivalent weights of commercial sulphuric acid are sometimes used in place of hydrochloric. The quantity depends upon the strength of the sulphuric acid used. Aluminium sulphate, Al2(SO4)3 · 18H2O (mol. wt. 666), may be replaced by ordinary potash alum, K2SO4 · Al2(SO4)3 · 24H2O (mol. wt. 948). In practice 7 parts of the former and 10 parts of the latter may be considered equivalent. It should be remembered that both these salts are hydrolyzed in solution, and therefore increase slightly the amount of free acid present. Their presence decreases the amount of chrome taken up, and as little or no alumina is found in the leather, there is usually small advantage in their employment. The use of salt is common but often unnecessary. It is considered desirable in baths of the acid type to prevent swelling by the excess of acid, and in baths made up from commercial chromic acid to replace correspondingly that normally formed from the reaction of dichromate and acid. It is used also in all baths which are intended to treat pickled goods. Like all electrolytes its presence decreases the adsorption of chromic acid.
All these conceivable modifications will make good leather, and the choice of a process often depends largely upon market prices. On the whole the tendency is to prefer the neutral or acid type, on account of the greater ease and completeness with which the bath may be exhausted. Pickled stock may be depickled before tanning, by a bath of salt, mixed with borax, whitening, or basic alum solutions. It may also be placed direct in the chroming bath, but the amount of acid thus added with the goods must be determined and allowed for when making up the bath. No allowance is usually necessary, however, if the "pickle" consist only of alum and salt.
The chroming operation is carried out usually in drums or paddles. Drums are preferable because more concentrated baths may be used; these solutions penetrate quicker and are easier to exhaust economically. They are also preferable for hides and heavy skins. Paddles are preferable where grain is important, and for light skins in which little time is needed. Small variations in the ratio of chrome to pelt, or in concentration of liquor, have little influence upon the resulting leather.
The analytical investigation and control of chroming baths is usually simple. A suitable volume of liquor is titrated with N/10 thiosulphate after acidifying with hydrochloric acid and adding potassium iodide. The operation should be conducted in a stoppered bottle, and the liquor allowed to stand for 10-15 minutes after adding the iodide and before titrating. A little fresh starch infusion should be added towards the end of the reaction. Each c.c. N/10 thiosulphate corresponds to 0.0033 gram CrO3 or 0.0049 gram K2Cr2O7. The same volume of liquor should also be titrated with N/10 caustic soda and phenolphthalein. Potassium chromate is neutral to this indicator, i.e. chromic acid acts as a dibasic acid. Any excess of hydrochloric acid is also titrated. More indicator should be added towards the end of the titration, as it is often oxidized. Each c.c. N/10 soda corresponds to 0.005 gram CrO3, 0.01 gram "half-bound" CrO3 (i.e. present as dichromate), 0.0147 gram K2Cr2O7, or 0.00365 gram HCl. If a c.c. N/10 thiosulphate and b c.c. N/10 soda be needed the type of chroming bath may be seen at a glance—
| If | The type is | The bath contains |
| b is greater than ⅓ a but is less than ⅔ a | Schultz | potassium dichromate and chromic acid |
| b is greater than ⅔ a | Acid | chromic acid and free hydrochloric acid |
| b equals ⅓ a | Neutral | chromic acid only |
If 10 c.c. chrome liquor require a and b c.c. of thiosulphate and soda respectively—
| I. | 10 c.c. of a Schultz bath contain (b-⅓×a) × 0.01 gram CrO3and { (a×0.0033)-[(b-⅓×a)×0.01] } × 1.47 grams K2Cr2O7 |
| II. | 10 c.c. of an acid bath contain (a×0.0033) grams CrO3 and {(b-⅔×a)×0.00365} grams HCl |
| III. 10 c.c. of a neutral bath (a×0.0033) grams | } | CrO3 |
| or (b×0.005) grams |
The second bath of the two-bath chrome tannage consists of a solution of sodium thiosulphate acidified with hydrochloric acid. The reactions in this bath are somewhat complicated, several occurring simultaneously. Broadly speaking, the final result is due to (1) the reduction of the chromic acid to a chromic salt by the sulphurous acid; (2) the formation of a basic chromic salt owing to the excess of thiosulphate; (3) the reaction of the added acid and thiosulphate to give free sulphur, which is deposited in and on the leather. The relative intensity of these effects is variable, according to the conditions of operation, e.g. the amounts of chemicals used, their concentration, the nature and condition of the goods, the time of application, the manner of application, etc. In practice the most favourable conditions are usually discovered empirically, but, broadly speaking, the goods are usually added soon after the thiosulphate and acid are well mixed. There is some evidence that the reduction is in steps, intermediate products such as sodium tetrathionate and chromium dioxide are known to be formed. The goods change from yellow to dark brown, then to green, and finally to the familiar blue. The sulphur makes the final colour a lighter blue than in the case of a one-bath tannage, hence the two-bath process is often preferred for "colours."
On account of the empirical character of this "hypo bath," it is impossible to fix any exact relation between the quantities of material used in the chroming bath, and the quantities of "hypo" and acid used in the reducing bath. The following rules, therefore, must be understood as rough approximations for practical use, and though they have been empirically discovered their theoretical significance is often fairly obvious.