44 ANIMAL PROTEINS 



deposited in great concentration at the surface of the fibre, 

 but for good weight it is necessary to use also the less 

 adsorbable and more diffusible tans, which penetrate the 

 fibre itself. Hence it is necessary for good weight to use 

 a blend of materials, and so supply many grades of 

 liability to adsorption. It is particularly advantageous 

 to blend judiciously the two main types of material, the 

 pyrogallol and catechol tans. It is also necessary for good 

 weight to present to the pelt the more diffusible and less 

 adsorbable tannins first, in order to secure the maximum 

 diffusion into the interior of the fibre before the exterior 

 of the fibre is heavily coated with the heavily adsorbable 

 and astringent tans. The least adsorbable materials are 

 therefore used in the early stages of tanning, and the most 

 adsorbable materials at the end of the tanning process. 

 Thus gambier is added to the early liquors (suspenders), 

 solubilized quebracho to the later liquors (handlers), and 

 mimosa bark extract to the final liquors (layers). There 

 is also another excellent way of ensuring this progressive 

 astringency of the liquors; this consists in leaching the 

 required blend of materials together (or mixing them in 

 the case of extracts) and presenting the mixed infusion to 

 the nearly tanned goods, which adsorb chiefly the more 

 astringent tannins. The liquor is then used for goods at a 

 less advanced stage of tanning, which again take the most 

 adsorbable constituents. This is repeated until the stage 

 is reached when the fresh pelt is inserted into the nearly 

 exhausted liquor, which naturally contains only the least 

 adsorbable substances. This system is almost universal, 

 and in practice is known as " working the liquors down the 

 yard." It has the additional advantage of being a syste- 

 matic method of economically exhausting (" spending ") the 

 tan liquors. When free acid is present in the tan liquors, 

 it tends to distend the fibres composing the pelt by a strong 

 and rapid adsorption. Thus distended or plumped the 

 fibres present a still greater surface for adsorptive operation, 

 but the distension naturally leaves less space between the 

 fibres for the diffusion of the sol. Hence acid or " sour " 



