Skins alum-processed

It has been suggested that vegetable tanning developed from a desire to colour oil or alum-processed skins. Interestingly the earliest surviving recipes for the preparation of leather, dating from Babylonian times about 3000 years ago, show that a combination of these three processes were employed ... 

Another tanning method is mineral tanning, which involves soaking a skin in a solution of alum and salt. This is called tawing. The products of this process are white and open-pored, and become stiff and hard when dried. Due to its open-pored texture, tawed leather is often treated with additives that fill in some of the irregularities and add to the skin s strength. Historically, these fillers have included flour, grease, egg yolks, and fat. 

Conversely, when the technique of chrome tanning was being developed in the late nineteenth century, it was found that the leather produced would dry out as a hard, stiff, inflexible material. It was only by the addition of fatty lubricating products in the form of an emulsion, in what became known as the fat liquoring process, and by mechanically working the skin that a useable product could be made. In a similar way, the production of alum-tawed skins involved the use of such fatty materials as egg yolk or olive oil and mechanical softening procedures. 

Widely-used alternatives to the alum-based tawing pastes were the various oxidisable marine oils, which were applied in the production of the chamois and buff leathers. The oils were trampled into the skins in a similar manner to that used in tawing. The skins were then hung in warm, airy stoves for the oxidation process to take place. After the oiling and stoving sequence had been repeated three or four times, the leather was washed off in alkaline liquors to remove excess oil, dried and worked mechanically to soften them. 

---