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 employed, and the concentrated liquor, which has a strength of about 25° Beaumé, 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 12 per 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, however, the purer product and an extract with better penetrating powers, and is consequently the more suitable for drum tannages. Chestnut extract is extensively used by the heavy leather tanners. =Oakwood Extract= is manufactured from the wood of the common oak (_Quercus robur_). The centre of the industry 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. The manufacture is also similar to that of chestnut extract, but decolorization is often omitted, and greater care has to be taken and in other ways to keep the colour within limits. One of these is to strip the wood more completely of bark. Another is to operate at as low a temperature as possible, about 110° C. The extraction is made in large circular vats about 14 feet high and holding about two tons material. A battery is composed of about eight vats or extractors. Open extraction is used, and the liquor is passed forward after 2-3 hours' boiling, so that the material is spent in about 24 hours. A liquor of about 5° Bé (36 Bkr.) is obtained, and the strength of the material reduced from 4 to 1/4 per cent. of tannin. Getting rid of insoluble matter is a difficulty, and is attained by settling, by rapidly cooling, and then passing through a filter press of wood. For evaporation a double-effect vacuum pan is preferred, which operates first at about 113° F., and afterwards at 140° F. with a higher vacuum. The liquor is concentrated from 5° to 25° Beaumé (s.g. 1.036 and 1.210 respectively). The extract has a much higher colour than chestnut, and is not used now as much as some years ago. As the principal supply was German, it has been unavailable.