34 ANIMAL PROTEINS 



that it is not a good plumping material. It makes in all 

 respects an excellent blend with myrabolans. I^ike all 

 catechol tans, the resulting leather darkens on exposure to 

 sunlight. 



Oak bark, from Quercus robur, is the ancient tanning 

 material of Britain, and is still used to a limited extent. 

 It contains about 13 per cent, of tannin and is mainly a 

 catechol tan, but also contains a pyrogallol derivative. 

 It yields catechin, and gives a red colour with the sodium 

 arsenate test, but also will yield some bloom and gallic acid, 

 and gives a blue-black with ferric salts. The tannin itself 

 is exceedingly similar to that of mimosa bark, but the 

 material contains about 2,\ per cent, of sugar, which makes 

 it possible to employ oak bark alone for making sole leather. 

 It is noted for yielding a sound, durable leather of good 

 typical tan colour. Its tannin combines well with hide 

 and penetrates quickly. The fatal disadvantage of oak 

 bark is its weakness in tannin strength compared with 

 other materials. This results in heavy freight and heavy 

 cost per unit tannin, bulky storage, expensive handling in 

 the factory, comparatively large bulk of spent tan, after 

 relatively greater trouble in extracting, and the impossi- 

 bility of making the strong liquors so necessary in these 

 days to produce good weight in a short time. No satis- 

 factory extract has yet been made from it. 



Pine bark, from Pinas abies, is one of the staple materials 

 of the Continent. It contains up to 14 per cent, of a catechol 

 tan, and, unlike most of this group, contains a high propor- 

 tion of sugar and will give good results alone. Hemlock 

 bark has been the staple tanning material of North America. 

 It is obtained from the hemlock, or Pinus canadensis. It 

 contains up to n per cent, of tan and much phlobaphene, 

 and yields a characteristic red leather of good quality, but 

 which rapidly darkens with sunlight. It contains some 

 sugar, but is usually employed in conjunction with sulphuric 

 acid or with sugary materials. Mallet bark yields another 

 catechol tan similar to that of mimosa, but somewhat less 

 astringent and more yellow in colour. Quebracho wood 



