548 MARINE PRODUCTS OF COMMERCE 



Chemical Reactions Involved in Tanning 



In the study of leather manufacture, like every other manufacturing process, 

 the chemical reactions involved form the basis upon which a complete under- 

 standing depends. Let us, therefore, consider the reactions taking place in the 

 order of their occurrence. 



Liming. In removing the hair or plumping the stock a solution of milk of lime 

 is generally used. Lime is obtained by "burning" limestone or calcium carbonate, 

 whereby the gas (carbon dioxide) is driven off and calcium oxide remains. This 

 calcium oxide, when treated with water, is converted into calcium hydroxide 

 which, when mixed with water, forms a suspension known as milk of lime. Cal- 

 cium hydroxide acting upon the hide substance forms soluble compounds with 

 the albuminous materials, thus causing partial hydrolysis with a resulting swelling 

 and opening of the fiber bundles. To a certain limited extent, also, the lime tends 

 to saponify the fatty matter, thus making its removal less difficult in the subse- 

 quent operations. 



Bating. The action of the bate through the ammonium chloride present is to 

 remove the excess of lime, and a depleting effect upon the hide substance is 

 brought about by the enzymes which it contains. The removal of the lime and 

 the depleting action produce the desired softness and flaccidity required before 

 actual tanning. 



Vegetable Tanning Materials. Tannic acid is the primary substance contained 

 in tanning solutions. It combines with the gelatinous substance of the hide, form- 

 ing an insoluble, nonputrescible product. Tannic acid alone, however, will not 

 make a satisfactory leather as it requires the presence of other substances, called 

 "non-tans," to secure the desired results. Sources of vegetable-tanning agents 

 are: the barks of numerous trees, such as oak, chestnut, and hemlock; the wood 

 of some trees, such as quebracho; and the leaves of certain shrubs, such as sumac. 

 The real tanning substances are obtained by extraction with water from all these 

 materials. 



Chrome Tannage. In the production of leather by the chrome process the 

 active substance is basic sulfate of chromium. This compound may be produced 

 in various ways, but in practically all cases it results from the reduction of sodium 

 dichromate in the presence of sulfurous acid. The tannage is due to a deposition 

 of chromium hydroxide and lower basic substances about the fiber bundles. 

 Chrome tannage has the advantage over vegetable tannage in that the time re- 

 quired is much less and the resulting leather has characteristics which are very 

 desirable for use in the manufacture of shoes. 



Dyeing and Finishing. In dyeing leather natural colors as well as coal-tar dye- 

 stuffs are employed. For bright shades the basic dyes are most commonly used, 

 but for certain purposes both acid and direct colors are applied. Within the past 

 few years pigment finishes have met with considerable favor and are now being 

 used to a very large extent. To give leather the proper strength, fullness, and 

 desired appearance it is necessary to introduce certain fats and oils at various 

 stages in the process of manufacture, and to apply special finishes for producing 

 a bright or dull appearance as the demand may indicate. The most common oils 

 and fats are cod oil, neatsfoot oil, and degras, while the finishing materials consist 

 of gum tragacanth, Irish moss, egg albumen, shellac, gelatin, and various alginates. 



