LIGHT LEATHER MANUFACTURE 97 



state, this may not be so effective, owing probably to the 

 secondary changes in tanning (Part I., Section III., p. 46), 

 but such leathers are usually " retanned " or prepared for 

 dyeing by sumaching (which process also incidentally 

 bleaches). The tannin mordant assists materially in the 

 fixation of the dyes. In the case of basic dyestuffs, lakes 

 also are formed, i.e. there is a mutual precipitation of 

 oppositely charged colloids (-j-dye, tannin). The dyeing 

 of leather is thus a case of colloid reactions even more com- 

 plicated than that of tanning. 



Another finishing operation typical of the light leathers 

 is " graining " or " boarding." In this the skins after dyeing 

 and drying are worked by a board which is covered by cork, 

 rubber, perforated tin or other material, and so grips or 

 " bites " the leather. The object of " graining " is to work 

 up the grain pattern by pushing or pulling a fold on the 

 skin with the board. The nature of the grain varies with 

 the thickness and the hardness of the skin, with the amount 

 of pressure applied, with the nature of the board, with the 

 direction of the boarding and with the total number of 

 directions boarded. There is thus infinite scope for variety 

 of finish, and hence arise bold grain, fine grain, hard 

 grain, straight grain, cross grain, long grain, etc. The 

 operation requires considerable skill and experience. In 

 the case of skins with little natural grain (such as sheep- 

 skin) embossing and printing machines impress the desired 

 pattern. 



In seasoning, a dressing is applied containing essentially 

 albumins and emulsified fats, e.g. egg albumin and milk. 

 Colouring matters are also often added to intensify or 

 modify the shade. After seasoning the goods are usually 

 " glazed " by a machine which rubs the seasoned grain with 

 considerable pressure, by a glass or hardwood tool, and so 

 produces a high gloss, for which the seasoning is very largely 

 a preparation. Light leathers are very lightly oiled with 

 linseed or mineral oil. 



