VEGETABLE TANNAGE 33 



on the side of the pits, being thus known as " whites." 

 The phlobaphenes or " reds " are also typical of catechol 

 tans from which grow catechins ; they can be formed by 

 boiling with dilute mineral acids. They are considered 

 to be anhydrides of the catechol tans. They are difficultly 

 soluble in cold water, but freely in hot, and in cold alcohol 

 and dilute alkalies. They are true tannins and alone are 

 capable of making a red leather, but in practice are often 

 found as mud in the tan liquors owing to their limited 

 solubility. They naturally influence the colour of leather 

 made with catechol tans, which is usually distinctly redder 

 than the leather made from pyrogallol tans. Infusions of 

 catechol (cp. catechin) give a green-black colour with iron 

 alum. The sodium arsenate test gives a red colour due to 

 catechin. The chromate and iodine tests mentioned for 

 pyrogallol tans give negative results with the catechol tans, 

 but bromine water gives a precipitate, and sulphuric acid 

 a crimson colour. 



Mimosa bark is one of the most important catechol 



tans. It is usually obtained in this country from Natal 



(" Natal bark ") ; but the tree (Sydney green wattle, Acacia 



mollissima) is a native of Australia. It is being cultivated 



now extensively in South Africa, and forms a most valuable 



portion of the Empire's stock of tanning material. Its 



more extensive use has been long recommended by the 



author, 1 but its gradually increasing employment in British 



tanneries has been greatly accelerated by the war, which 



has prevented its delivery in Germany and has cut off 



Turkish valonia from Britain. It yields about 30 per cent. 



of a stable and excellent tannin, and will produce a firm, 



ible leather, with a colour much less red than that 



obtained from many other catechol tans. It is an astringent 



and if carelessly used yields a harsh or even " drawn " 



grain. Most of the tannin is easily extracted, yielding a 



>n which jxmetrates fairly quickly and gives 



good \\vi-lit. It contains less than I per cent, of sugar, 



which unfortunately rapidly ferments to carbonic acid, so 



* y.s.c./.. 1908, 1193- 



3 



