SECTION II. FAT TANNAGES 



FOR the manufacture of a permanent leather the essential 

 requirements are that the fibres of the hide or skins gel 

 should be dried in a separate condition, and that they should 

 be coated by some waterproof or insoluble material. Many 

 substances fulfil the first but not the second of these con- 

 ditions. For example, the dehydration only may be accom- 

 plished more or less by salt (as in curing hides), still better 

 by salt if a little mineral acid be used (as in pickling), and 

 by other salts such as potassium carbonate and ammonium 

 sulphate, and dehydrating agents such as alcohol. Such 

 " temporary leathers," however, are not water-resisting, as 

 the second requirement has not been fulfilled, viz. the 

 coating of the fibres with some more or less waterproof 

 material. Thus if pelts dehydrated with alcohol be treated 

 with an alcoholic solution of stearic acid, the second con- 

 dition is fulfilled and a permanent leather is obtained. 



Now, many tanning agents accomplish these two require- 

 ments only imperfectly. As we have noted in the preceding 

 section, the alum-tanned leathers are not very water resist- 

 ing, and much of the tannage will wash out. Leathers 

 made by the vegetable tannages usually contain some excess 

 of vegetable tanning matters which are soluble in and 

 removed by water, though much tannin can no longer be 

 thus removed, owing to the mutual precipitation of the 

 oppositely charged tannin sol and hide gel. The necessity 

 for fulfilling the second requirement mentioned is one reason 

 for the practice of following these tannages by applications 

 of oil, fat or of both. In this way the isolated fibres are 

 not only dried separately, but are coated with a typical 

 water- resisting material. 



In the fat tannages an attempt is made to fulfil this 

 second requirement without the use of any specific " tan- 

 ning agent " for producing the first requirements ; i.e. an 



