158 RUBBER AND 



rubber. Acetic acid has also less destructive effect on 

 the finished rubber than most of the mineral acids. It 

 is generally recognised however that an excess even of 

 acetic acid is harmful to the manufactured product. It 

 is therefore important that only just sufficient acid 

 should be added to ensure complete coagulation. The 

 exact amount required appears to vary considerably 

 under different circumstances. Parkin found that com- 

 plete coagulation was produced by the addition of 

 09 per cent, of pure acetic acid, or one part in about 

 iioo of pure latex. Working with latex from the old 

 Henaratgoda trees, we have found that this amount is 

 often insufficient, whereas on many estates a smaller 

 proportion is generally found to be effective. It is to be 

 recommended that in large factories the bulked latex 

 should frequently be tested on a small scale in glass 

 vessels, in order to ascertain the smallest amount 

 necessary. If the latex has been kept for some time, 

 its alkalinity may be reduced by the action of putre- 

 factive bacteria, and a smaller amount of acid will then 

 be required. 



The correct amount of acid, well diluted with water, 

 is added to the latex, which is then thoroughly stirred 

 and allowed to stand. When crepe is being manu- 

 factured the coagulation usually takes place in enamelled 

 buckets. From these the spongy rubber is removed after 

 an interval of half an hour or less and transferred at 

 once to the washing machines. For the manufacture of 

 sheet rubber the latex must be set in shallow pans. In 



