Each of these classes of tanning materials has characteristic effects which render them easily distinguishable. The use of combinations of vegetable and mineral tannins has lately increased, and it is possible that the blending of the two classes of materials may produce an ideal tannage for certain classes of leather. In fact, this result is already claimed for a chemically combined tanning material which, according to the American patent, is prepared by the following method: 125 lb. of solid quebracho extract is dissolved in the same weight of hot water and allowed to cool; 16 lb. of commercial caustic soda dissolved in two or three times its weight of water is added, and the mixture agitated about half an hour; 150 lb. of chromium sulphate is then added. In this way an insoluble tannate of chrome is produced, but, on boiling and agitating, it changes to a greenish brown colour and forms a sulphotannate of chrome. The combination of the alum tannage and gambier (a vegetable extract) has been used successfully for years past. Another combination which has given good practical results is the tannage with alum and chrome salts in the manufacture of glove leather. The vegetable materials containing tannin should be arranged botanically, but the following classification is simpler for practical purposes. NATURAL TANNING MATERIALS 1. _Barks._--Oak, Hemlock, Pine, Fir, Alder, Khaki, Willow, Cork, Mimosa or Wattle, Babool, Larch, Mangrove, Spruce, Elm, Birch, Pomegranate, Cebil. 2. _Leaves, Twigs, etc._--Sumach, Mangrove, Mango, Eucalyptus, Pistacia, Lentiscus. 3. _Roots._--Canaigre, Palmetto. 4. _Fruits._--Myrobalans, Valonia, Divi-Divi, Cascalote, Mangosteen, Pomegranate, Celavinia, Bablah, Algarobilla. 5. _Excrescences._--Gall Nuts, Chinese Galls, Pistacia Galls, Tamarisk Galls. TANNING EXTRACTS 1. _Woods._--Oak, Quebracho, Hemlock, Chestnut, Mimosa, Mangrove, Spruce. 2. _Barks._--Oak, Wattle or Mimosa, Larch. 3. _Shrubs, Leaves, etc._--Gambier, Cutch, Catechu, Kino, Sumach. 4. _Fruits._--Myrobalans, Valonia. 5. _Roots._--Palmetto. Of these materials, only about twenty are of much importance commercially, the principal being oak, chestnut, quebracho, hemlock, valonia, gambier, myrobalans, mimosa or wattle, sumach, mangrove, divi-divi, spruce, larch, and babool.