VEGETABLE TANNAGE 39 



of the phlobaphenes, and a great variety of quebracho 

 extracts are now available which deal with this problem 

 in different ways. In some the more soluble reds are simply 

 left in the extract under the idea that they are really tannins 

 and may be of some use in some part of the tanning process ; 

 in others they have been removed by settling and filtration at 

 appropriate temperatures and concentrations ; in most, how- 

 ever, they have been solubilized by treatment with alkalies, in 

 the presence of reducing agents, notably by heating with 

 sodium bisulphite in closed vats. The base combines with 

 the phlobaphenes, which are made completely soluble and 

 available for tanning. Sulphurous acid is evolved, and its 

 reducing powers assist materially in retaining and promoting 

 a good colour in the product. Such " sulphited extracts " 

 are now extensively manufactured in this country from the 

 imported " crude " extract, and sold as liquid extracts con- 

 taining 30, 35 or 40 per cent, of tan according to the require- 

 ments of the buyer ; " mixed extracts," which are solubilized 

 quebracho blended with about 15 per cent, of myrabolans, 

 are also used. 



By solubilizing quebracho with excess of bisulphite an 

 extract is obtained which possesses considerable bleaching 

 powers, and such extracts are also extensively manufactured 

 he " vat ting " or bleaching of heavy leather after tannage. 

 The excess of sulphurous acid not only bleaches the leather, 

 but also swells it up and thus permits a further absorption of 



iig tan liquor, which is conducive to good weight. These 

 bleaching extracts are usually of 36-38 per cent, strength in tan. 



Gambier is an extract of the leaves and twigs of the 

 eastern shrub Nauclea gambir. It is a catechol tan of 

 peculiarly mellow quality and great practical value. It 

 ont.iiii : . < ' i liin, but little j.lilobapliene. and yields 



a beautifully soft leather, but without weight. It is an 

 edingly suitable material for the early stages of tanning, 

 and is much liked for tanning leathers that have to 

 be curried, and is widely used in the manufacture of 



r leather. It is, however, an exceedingly expen 

 tannin, and the extract is made in a very crude way by 



