98 FOREST UTILIZATION 



IV. Sulphate process. 



It is adopted in mills originally arranged for caustic soda 

 process. The chemical used is sodium sulphate, the 

 price of which is only one-third that of caustic soda. It 

 is reclaimed out of its watery solution by evaporating 

 and melting. This process gives the old soda mills a new 

 lease of life which were about to be forced to the wall 

 by the superiority of the sulphite process. 



x 



V. Electric process. 



The electric current is used to obtain from an 8% solu- 

 tion of common salt (Na Ci) its composing parts, 

 viz., caustic soda and hydrochloric acid. 



These substances, alternatingly acting upon the wood pre- 

 pared in the manner described under II, a, b, and c, dis- 

 solve the lignin and destroy the incrustations of the 

 fibre, so that pure cellulose remains in the digesters. 



Two digestors are used, connected with the positive and 

 the negative electrode of the current respectively. 



The process is said to be faster and cheaper than the 

 sulphite process. No bleaching required. 



XXXII. TANNING MATERIALS AND TANNERIES. 



A. Tanning materials. 



Tanning materials used in the United States were in 1900: 



Hemlock bark, 1,170,000 cords. 



Oak, 445,000 cords. 



Gambier, 128,000 bales. 



Hemlock bark extract, 13,000 barrels. 



Oak bark extract, 54,000 barrels. 



Quebracho bark extract, 20,000 barrels. 



Sumac bark extract, 8,500 barrels. 



Chemicals, $2,225,000 worth. 

 In the sole leather, belt leather and harness leather industries, 



vegetable tanning material is still preferred. Mineral or chem- 

 ical tannage, however, has been developed during the last ten 



years to a degree threatening to entirely supplant the old 



methods. 

 Since 1900, extracts obtained from chestnut wood have gained 



both favor and importance. 



B. Tanbark in particular. 



I. Notes on tanbark. 



(a) The corky layers of bark do not contain any tari- 



nin and are usually shaved off. In Europe, 

 young oak bark not having any cork is prefer- 

 ably used. . 



(b) Fresh bark contains on an average 45% water 



and shrinks heavily during the drying process. 



