SECTION III. OIL TANNAGES 



THERE are very obvious analogies between the fat tannages 

 discussed in Section II. and the oil tannages now to be dealt 

 with, but there is nevertheless a distinct departure in principle 

 involved. In the oil tannages the mechanical treatment is 

 generally more vigorous, and the " drying " process is 

 conducted at a much higher temperature, with the result 

 that there is a vigorous oxidation of the oil. This results 

 in the formation of insoluble oxidation products .which coat 

 the fibre and play an essential part in the production of a 

 permanent leather. Pungent vapours are evolved in the 

 drying operations, amongst which is acrolein and probably 

 also other aldehydes, and it is thought by Procter that these 

 aldehydes also are essential tanning agents and typical of 

 the process (cf. Section IV.). Fahrion considers that the 

 tanning action is due solely to unsaturated fatty acids with 

 more than one double linkage. Garelli and Apostolo, 

 however, believe that the tannage is due to a coating of 

 fatty acid whether saturated or not. These observers made 

 leather with stearic and palmatic acids in colloidal aqueous 

 solution. 



The manufacture of chamois leather from the flesh 

 splits of sheepskins comprises the largest and most typical 

 branch of the oil tannages. The sheep pelts are split in 

 the limed state, and the fleshes are given another sharp 

 liming which may last up to a fortnight. They are next 

 " frized," i.e. scraped over the beam with a sharp two- 

 handled knife, to remove roughness and loose fat. The 

 goods are next thoroughly washed in running water and 

 drenched. A paddle drench is often preferred, and if not 

 used the handling should be frequent. Paddling drenching 

 reduces the time required from about 16 hours to about 



