VEGETABLE TANNAGE 37 



the liquor passes in succession through all the vats over 

 wood less and less spent. The temperature is highest in the 

 vat containing the fresh water and nearly spent wood. In 

 open vats of wood or copper the temperature is near boiling- 

 point, whilst in the closed autoclaves (copper or bronze) 

 tile pressure reaches about two atmospheres and the tem- 

 perature about 130 C. (266 F.). The series may contain 

 5, 7, 9 or even 12 vats, and the liquor obtained has a strength 

 of 3 to 4j Beaume (22 ro 33 Bkr.). 



After extraction the liquor is allowed to stand, and 

 much insoluble matter settles out resins, wood, fibre, etc. 

 The clarified and settled liquor is then passed through a 

 cooler up to about 55 C., and then run into the decolorizing 

 plant, a deep vat fitted with a copper steam coil and 

 mechanical stirrer attached to power. The best decolorizer 

 is bullock's blood, which is run into the vat and well mixed. 

 The temperature is next raised to about 70 C., causing the 

 blood albumin to coagulate. It carries down with it a 

 little tannin, but much colouring matter. After standing 

 a few hours the settled liquor is run off direct to the 

 evaporator. A multiple-effect evaporator is usually em- 

 ployed, and the concentrated liquor, which has a strength 

 of about 25 Beaume, is run into suitable oak casks. The 

 extracts contain 27-32 per cent, tannin. An extract made 

 with open vats has about 7 per cent, soluble non-tanning 

 matters, whilst a " pressure extract " may contain up to 



er cent, of these " non-tans." Pressure extracts obtain 

 also a better yield of tannin, which more than compensates 

 for the slightly lower price. Open extraction yields, how- 

 ever, the purer product and an extract with better pene- 

 trating powers, and is consequently the more suitable for 

 drum tannages. Chestnut extract is extensively used by 



heavy leather tanners. 



Oakwood Extract is manufactured from the wood of 

 the con n IK n oak (Quercus robur). The centre of the indu 

 has been the oak forest of Slavonia. The wood contains 

 2-4 per cent, of a tannin very similar to that of chestnut 

 wood, but somewhat more astringent. 



