Skins alum-tawed

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. 

Oil-tanned chamois leather Alum-tawed skins Formaldehyde-tanned leather Aluminium-tanned skins Vegetable-tanned leather (hydrolysable) Vegetable-tanned leather (condensed) Chrome-tanned leather... 

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. 

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. 

The fur treated in this manner, on the flesh side has the appearance of a fresh skin if it isdesired to impart to it the appearance of tawed leather, this can be accomplished in a very simple manner by spreading over it a cold solution of alum or aluminium sulphate, leaving it to dry and then dabbing it over with a sponge dipped in ammonia. In this way the aluminium hydroxide is separated from the flesh side of the skin, which thereby has the appearance of tawed leather. 

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