Tans and Dyes 



355 



exploited principally are Rhizophora mangle, Rhizophora mucrunata, and Bruguiera gymnorhiza, 



yielding barks containing from forty to fifty per cent: of tannin. The bark is merely stripped 



from the stems and branches, broken up into small pieces, and dried in the sun, preferably 



under cover. When dry it is packed into bales weighing about one cwt. The manufacture 



of mangrove tanning extract, and " cutch " is carried on on a considerable scale in Borneo 



and some other East Indian islands. 



- 



Mallet Bark 



This tanning material is derived from Eucalyptus occidentalis, and has been exported during 

 the last few years on a large scale from South Australia. The bark, which is of medium thick- 

 ness, is very hard and shows a cinnamon-brown colour ; it contains as a rule from thirty- 

 five to forty-five and occasionally up to fifty per cent, of a readily soluble, yellow-brown 

 tannin, which yields a firm, tough, light-brown leather. 



DYE-STUFFS 



Dye-stuffs of vegetable origin have become almost a negligible quantity in the world's 

 commerce since the introduction of the so-called " aniline dyes," and what was at one time an 

 industry of great importance to many agricultural countries has now sunk to comparatively 

 small proportions. 



The vegetable dye-stuffs owe their characteristic colouring powers to the presence of 

 small quantities of highly coloured substances (dyes) secreted by the plants. In general 

 the " dyes " are not readily retained by fabrics unless the latter are first treated with a 

 mordant. This consists in steeping the fabric in a solution of a weak salt of one of the 

 metals iron, chromium, aluminium, or tin. The steeped fabric is then dried and treated 

 with a current of steam whereby a fine layer of metallic oxide is formed all over it, i.e., the 

 fabric is " mordanted." If it is now placed in an infusion of the dye-stuff, the " dye " forms 

 a coloured insoluble compound with the " mordant," which is resistant to light and cannot be 

 washed out. The coloured compound formed with each metallic oxide used as a mordant 

 is different, and by this means a considerable range of tints may be obtained from each dye-stuff. 



Indigo 



This blue dye-stuff is obtained from a number of indigo-bearing plants (Indigofera tinctoria, 

 I. arrecta, etc.) cultivated in India, Java, 

 and Natal. It does not occur naturally in 

 the plants but is formed by a process of 

 fermentation. Fresh plants, collected at 

 the flowering period (late summer in India 

 and Java), are thrown into large vats con- 

 taining water, and are thoroughly broken 

 up by means of sticks wielded by natives. 

 By this means the soluble matter contained 

 in the plant is dissolved out. The watery 

 extract is drawn off into a second vat, 

 where it is thoroughly churned up so as to 

 expose it as much as possible to the air. 

 This " churning " is accomplished in India 

 usually by natives, who wade about in the 

 vat and beat the liquid with sticks, though 

 the more enterprising planters have adopted 

 the Javanese method of agitation with a 

 current of steam or compressed air. The 



INDIGO 



