THE LATEX AND HOW IT IS COAGULATED 133 



will probably be found that rubber from old trees yields the 

 superior finished product. 



Black, discoloured latex is sometimes yielded by certain 

 trees, especially during very dry weather. This is often very 

 puzzling to planters, who are at a loss to account for the phe- 

 nomenon, and often very wild theories are suggested. This dis- 

 colouring of latex is accounted for to some extent by an excess 

 of tannic acid in the bark of certain trees, owing, perhaps, to 

 some peculiar local condition of the soil. The latex is not really 

 black, but has a dark, discoloured, streaky appearance. As 

 a general rule most of the discoloration disappears gradually 

 from the rubber sheets as they dry. The sheets may at first 

 show dark streaks, but these in most cases, at all events, seem 

 to fade away as the sheets become dry. 



It is most important that, before latex is poured into the 

 Shanghai jars or other receptacles for the purpose of coagula- 

 tion, that the latex be well strained through wire sieves. If too 

 thick to pass readily through, water should be added. This is 

 seldom necessary, and the less water added to latex at any time 

 the better. The sieving intercepts many impurities which 

 would otherwise lower the quality of the rubber and, in some 

 cases, might do damage in the washing machines. 



If first-grade rubber is to be of best quality and colour 

 the lump rubber should not be put into the Shanghai jars or 

 rubbed through the strainer. It should be kept apart and put 

 through the mills by itself. 



To make certain of obtaining clean rubber, the usual 

 practice on some of the best estates is to stir the latex well 

 round with a stick after it has been strained through into the 

 Shanghai jars. After it has been well stirred round, the surface 

 is skimmed with a ladle, and it is then stirred a second time and 

 again skimmed. The careful attention thus given removes 

 many impurities which have passed through the sieves and 

 greatly improves the quality and the colour of the rubber 

 subsequently made. The skimmings should not be thrown 

 away but added to some of the lowest-grade rubbers. The 

 removal of all scum is essential, as otherwise fermentation is 

 apt to set in, and the sheets of rubber in such a case are often 

 discoloured. 



Latex should never be exposed to sunlight, as that stimu- 



