Tanning mineral

Mineral tanning was probably first practiced in ancient Mesopotamia and then spread to Egypt, the Middle East, and the Mediterranean Sea area (Levey 1958). Mineral-tanned leather is soft to handle, has a velvety texture, and is almost white, a color practically impossible to achieve by other tanning processes. It is, however, very sensitive to humidity and water under wet conditions the alum in the leather is hydrolyzed (decomposed by water), forming sulfuric acid, a very strong acid that attacks the leather and causes its rapid decay. Mineral-tanned leather that has been humid or wet for a more or less extended period of time loses some of its characteristic properties, such as softness, pliability, and strength, and becomes hard, horny, and brittle. 

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. 

Further investigations, funded by the European Commission CRAFT programme, have also been undertaken to develop a leather with both the resistance to acidic atmospheric attack exhibited by mineral tanned leathers and the physical characteristics of vegetable-tanned skins demanded by book binders and other leather workers. As a result of this work, leathers with these properties are now available commercially. 

A milestone in manufacturing leather was the discovery of the tanning properties of trivalent chrome salts like chrome sulfate at the end of the 19 century. All the tanning agents have been used so that vegetable, synthetic and mineral tanning can be distinguished. 

To determine the method used in mineral-tanned leathers, it is necessary to test the sample for certain inorganic compounds. The most common mineral tanning compounds are chromium salts sometimes aluminum salts are used, more rarely, iron salts. They may also be used in combination. Spot tests can be used for the detection of these inorganic tanning agents. The sample required is so small that it may be taken from manufactured products (shoes, small leather articles) without visible damage. 

Slabbert N P 1980 Metal complexes and mineral tanning. Ds Leder 31 79-82... 

Features Stable to mineral tanning agents, acids and bases under normal tannery conditions... 

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