All the figures are percentages of the weight of pelt. As K{2}Cr{2}O7 has a molecular weight of 294, and Na{2}Cr{2}O{7}·2H{2}O 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, 2CrO{3}, 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, Al{2}(SO{4}){3}·18H{2}O (mol. wt. 666), may be replaced by ordinary potash alum, K{2}SO{4}·Al{2}(SO{4}){3}·24H{2}O (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.