162 TANNING MATERIALS AND TANNERIES 



PARAGRAPH XXXIII. 

 TANNING MATERIALS AND TANNERIES. 



(A) OBJECT OF TANNING. Tannage tends to render the skin permanently supple and durable by 

 impregnation with tannin. 



(B) THE METHODS OF TANNAGE EMPLOYED nowadays are:- 



1. "Tawing" by means of alum salts. 



2. "Shamoying" by means of certain oils or acids of oils. 



3. "Tawing" by salts of chromium. 



4. "Vegetable tanning." 



Chromium tannage has many advantages; it is superseding vegetable tannage in many instances. In 

 the sole leather, belt leather, and harness leather industries, vegetable tanning material is still preferred. 

 Forest utilization is concerned in vegetable tannage alone. 



(C) VEGETABLE TANNING MATERIALS:— 



I. Tanning materials used in the United States, by over 600 tanneries, are:- 



Hemlock bark ... 800,000 cords, worth -v 900 per cord 



Oak bark 300,000 cords, worth .5 10-90 per cord 



Vegetable (notably chestnut) extracts, 200,000 tons, worth •y28'00 (in chestnut) per ton. 



Chestnut wood averages 8 per cent and chestnut oak bark 12 per cent of tannic acid, approximately. 

 Sumach leaves and mangrove bark, rich in tannic acid, are used on a small scale. 



II. Important are the importations into the United States of foreign tanning materials, particularly rich 

 in tannic acid, notably of:- 



Quebracho wood (containing 24 per cent tannin, from Argentina, Chile, Peru). 



Quebracho extract. 



Divi-divi, bean pod of Caesalpinia coriaria (containing 45 per cent tannin). 



Catechu (cutch, terra japonica), a cake obtained from boiled leaves, fruit, and wood of Acacia 



catechu. 

 Gambler, obtained like catechu from Uncarica gambir (exported from Singapore). 

 Myrobalans, the dried fruits of various species of the genus Terminalia (from East India). 

 Kino, a gum bled from many tropical trees, e. g., from Pterocarpus (Malabar-kino), from Butea 



(Bengal-kino), from Eucalyptus (Australian kino). 

 Valonea, the cup of the acorn of Quercus aegilops (from Mediterranean countries). 



III. Tannin percentages of dressed bark are, after experiments given in Sargent's 10th Census report: — 



(D) TANBARK IN PARTICULAR:— 



I. Notes on tanbark obtained from chestnut oak, and from hemlock. 



(a) The corky outer layers of bark do not contain any tannin. The inner layers (phloem and 

 bark cambium) are rich in tannin. 



(b) Fresh bark contains on an average 45 per cent water and shrinks heavily during the drying process. 



