OIL TANNAGE
As tanning agent unsaturated glycerides are used. The favored ones are found in cod-liver oil. These fatty acids may have one up to six double bonds in the aliphatic chain, but 15 % should have at least four to give the necessary reaction products from oxidation and polymerization to give characteristic ‘chamois’ leathering effect under normal conditions of tanning. Oil tanned leathers are light, soft air-permeable and resistant to washing.
In oil tannage for chamois leather, the flesh splits of sheepskins after usual beamhouse processes are brought to the iso-electric point(ie. pH=5). This makes it easier to bring them to a moisture content of 50% by pressing, samming, or squeezing, to expell all the interfiber water, leaving only damp, hydrated fiber structure. This is important in making the skins porous to air and oil. The skins are then drummed with 40%(of their weight) of the cod-liver oil, which should be spread over the surfaces of the fibers and be almost completely absorbed by interfacial tension forces. The skins may then be hung up or drummed in warm air. Oxidation reactions of the oil will now occur with exothermic liberation of heat, increase of proxide value of the absorbed oil, acrid fume liberation, yellowing of color and leathering or tanning of the skins. Surplus oil is removed by warm damp pressing and then washing with warm solutions of sodium carbonate in water at pH 8-9 to saponify the greases. This surplus oil has no longer tanning properties. Of the 40% oil offered only 5-7 % remains fixed to the fibers. Modern systems remove the surplus oil by solvent degreasing system hpwever the resultant leather is waterproof unless treated with alkaline surfactants. Resultant chamois leather is very soft, very stretchy, rapidly absorbs 600% water, which can be readily wrung out and makes it suitable for window cleaning.
Chamois leather when adjusted to suitable pHs shows poor affinity for anionic dyes but good affinity for basic, cationic dyes and strong affinity for reactive triazinyl dyes, which may fix also on oxidized oil residues.
