39 



Tanning Substances. 



accumulates below the false bottom, and as this fills the liquor is forced by the 

 pressure of the water, which is continually supplied to the first extractor, over into 

 the second extractor, and so on throughout the series, until from the last a com- 

 paratively strong liquid is obtained. This is then run into a vacuum-evaporator 

 of the Yarian or similar type, in which it is exposed in thin layers for a short time 

 to a temperature of about 80 c C, whereby it is converted into a thick liquid of 

 specific gravity 12. Thence it is run into an ordinary vacuum-pan, and evaporated 

 in vacuo to a satisfactory consistence. The process thus briefly outlined yields 

 a satisfactory extract (cutch) for dyeing purposes, but one of the greatest 

 objections to the use of mangrove bark or extract for tanning purposes is, that 

 it produces a leather having an objectionable red colour, and consequently it is 

 necessary in manufacturing extracts for tanning purposes to decolorise the liquor 

 at one stage in the process. This is necessary not only because of the colouring 

 matter naturally present, but also because in the process of manufacture the 

 application of heat to the tannin and extractive matters leads to the production 

 of dark-coloured substances by their partial decomposition. The decolorising 

 process is usually carried out with the liquor as it comes from the extractors, and 

 consists in the addition of small quantities of albumen, alum (or aluminium sulphate) 

 or similar material, which forms an insoluble compound with a small portion 

 of the tannin, and as this settles out it carries with it a great part of the objection- 

 able colouring matters. Bleaching agents such as sulphur dioxide or salts yielding 

 this have also been used for the decolorisation of mangrove and other extracts 

 to be employed for tanning pui'poses, but none of these methods have so far 

 succeeded in completely and permanently eliminating the red colour. This is 

 the most serious difficulty in the way of an extension of the use of mangrove bark 

 as a tanning agent, and recently the German Colonial Society has offered a prize of 

 3,000 marks for a satisfactory and permanent method of decolorisation. The 

 conditions under which this prize is offered are given in Der Tropenpflanzer, 1905 

 pp. 475—6. 



The total cost of a small plant arranged on modern lines for the manufacture 

 ot mangrove extract, and with a sufficiently large capacity to be remunerative in 

 working, would probably be from £450 to £500. 



QUALITY OF LEATHER PRODUCED BY MANGROVE BARK. 



Reference has already been made to the dark-red colour of the leather 

 tanned by mangrove bark or extract, and the fact that this colour depreciates its 

 value. Apart from this defect, mangrove-tanned leather when properly made 

 seems to be of fair quality and suitable for shoe leather and similar purposes. On 

 this point Professor Proctor, of the Leather Industries Laboratories of the Leeds 

 University, says that " mangrove bark is now largely used in conjunction with pine, 

 oak and mimosa barks, and that extract prepared from Cerio'ps Candolleana 

 makes a good but dark-red leather ": and Professor Korner, of the Freiberg Tannin 

 School, as the residt of his experiments on the value of mangrove extract as a 

 tanning agent, states that i( it yields a soft and pliant, workable leather, which, 

 however, possesses an objectionable red colour, which can only be modified suffi- 

 ciently for commercial purposes by using it in conjunction with myrobalans, 

 valonia, sumach or similar light-coloured tanning materials." 



EXTENT OP TRADE IN MANGROVE BARK. 



It is impossible to give any exact figures showing the total amounts of 

 mangrove bark and extracts imported into the principal European countries, since 

 these are not separately shown in the returns, but the following statements and 

 statistics seem to show that the demand for mangrove bark and cutch in Europe 

 has in recent; years reached a considerable total. 



