Vegetable Tanning Processes

The dried rough leather was sold to a currier whose first operation was to dampen the skin and soften it. This was followed by a scouring operation in which the leather surfaces were scrubbed clean. The skins were then pared down to the required thickness after which they were worked on a bench to remove loose tanning materials, flatten and stretch them. They were then partially dried and impregnated with a warm mixture of tallow and fish oils. If a soft, fine product was required, further mechanical operations were undertaken. These had the effect of separating the tanned fibres and allowing 

By the last decade of the century, the effects of these changes had become only too apparent, and efforts were made to determine the causes of the problems and eliminate them. Conservators are, however, still working today on 

Retannage with condensed tannins (wattle or quebracho) was much better than with the hydrolyzable tannins (myrobalans) or chestnut tannins. Wattle tannins reacted more slowly than quebracho tannins. Use of a wattle retannage with neutralization gave the most stable leathers. The wattle tannin were also superior because of the lower level of non-tans as they also reacted with the chromium and increased the amounts of chromium stripped off in the vegetable retannage (13). 

The most recent major advance in vegetable tanning processes is the development of a combination vegetable-aluminum tannage (85, 225, 233). Leathers produced with this type of tannage have heat stability even greater than commercial 

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Liritan A leather tanning process, introduced in 1960 and now widely used worldwide. The leather is first pickled in a solution of sodium hexametaphosphate and then soaked in baths of various vegetable products. 

Iron, titanium, and zirconium salts can be substituted for the chromium ones.146 Vegetable tannins, such as 4.20 can also be used, but they slow down the tanning process. Similar phenols can be found in the residue from tea leaves left after the manufacture of instant tea. Perhaps, they could be used in making leather to eliminate or reduce the amount of another waste product. Getting reagents to penetrate the hide is a problem. Newer methods, such as ul-trasonication and supercritical fluid extraction, may help reduce the time required to make leather, so that these alternative-tanning agents can be used instead of the chromium. 

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 ... 

The predominant tanning technique in the world is chrome tannage (about 85%). As a multi-purpose material chrome tanned leather can be transferred to leather with different properties (to different leather types) by further processing. However, a significant quantity of vegetable tanned leather is manufactured, too. Part-tanned chrome-free - often termed wet white - leather is also produced, in small but growing quantity. 

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