Oak bark is not as high in tannin content as many other materials, and its use, which has extended over many centuries, has been due more to its ready availability than to any other factor. A high quality oak bark will contain 12 to 14% tannin, while old oak heartwood will contain 6 to 9%.
Despite its relatively low tannin content, oak bark was at one time used extensively in the manufacture of some very fine leathers, especially in England. In fact, the English leathers of the past, which were known throughout the world for their high quality, were produced largely by means of high quality stock and a long, slow, oak tannage. Slowness of penetration, however, along with declining availability, are important reasons why its use is not nearly so extensive as in the past.
Oak tannin is a combination of the pyrogallol and condensed tannins, in the ratio of one to two, but the real nature of the tannin is still somewhat obscure. The tanning has a medium pH and moderate salts and acid content. See also: VEGETABLE TANNINS . (175 , 306 )