=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. [Footnote 6: "Types of Two-bath Chrome Tannage," _Leather_, 1909, 227-259.] Now:-- K{2}Cr{2}O{7} + 2HCl = 2KCl + 2CrO{3} + H{2}O 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 4-1/4 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 2-1/2 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:--