Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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chrome tanning

A method of tannage stemming back to the discovery, in 1858, that leather could be produced by treating skins with basic chromium sulfate (Cr(OH)SO 4 ). The two basic methods employed today are the one bath and two bath methods, the former being most often used. The most widely used chemical in chrome tanning is sodium dichromate (sodium bichromate) (Na 2 Cr 7 O 7 . H 2 O), from which chromium sulfate is produced.

As in vegetable tanned leather, the degree of control exercised in the tanning process has great influence on the nature of the leather produced. If, for example, the final pH of a chrome-tanned leather is too low, the leather will be flat, hard, and wet, and may show grease spots on the surface; if it is too high, the leather will probably be plump, loose, dry, and may have a drawn grain or be too soft in the BLUE SORT . It is, therefore, imperative in chrome tanning to obtain the optimum pH, i.e., 3.4 to 3.5 in the one bath method, or 3.2 to 3.4 in the combination single and double bath method, and to maintain it.

The two bath method has almost been completely superseded by the one bath tannage, except in certain cases where the older two bath process is thought to give a particularly uniform tannage and a deposit of colloidal sulfur in the leather.

The major characteristics of chrome-tanned leather are its blue-green color and absence of filling power, i.e., an empty tannage. Chrome-tanned leather tends to be softer and stretchier than vegetable-tanned leather, and is very stable in water. Unlike vegetable-tanned or alum-tawed skins, chrome-tanned leather can withstand boiling water and has a shrinkage temperature higher at times than 100° C.; however. it does not resist perspiration or organic acids well and is difficult to emboss. In addition, it does not take gold tooling well and is difficult to fabricate in such operations as turning-in, etc. It is, on the other hand, a very durable leather. (101 , 164 , 248 , 306 , 363 )




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