THE PREPARATION OF RUBBER 77 



merely of well- ventilated sheds, so arranged as to afford 

 a free circulation of air over the rubber. In others a 

 current of diy air, or of hot air at a temperature of 90° 

 to 100° F., is employed to facilitate the drying, and the 

 circulation is maintained by means of a fan. Vacuum 

 driers similar to those used in rubber factories are also 

 employed. 



Smoking of Rubber. — Recently large quantities of plan- 

 tation Para rubber, prepared in sheets, have been sub- 

 jected to a process of smoking after coagulation. There 

 is a general impression amongst planters that the smoking 

 improves the strength of the rubber, and it is a fact 

 that, as a rule, smoked sheet realises the highest prices 

 at the sales of plantation rubber. The smoking is usually 

 effected in the drying-chamber, either by passing in smoke 

 from a fire placed outside, or by having a fire, suitably 

 enclosed, on the floor of the chamber. The smoke is 

 obtained by burning hard wood, coconut husks, palm 

 husks, and other similar substances. 



This method of smoking sheet rubber after coagulation 

 is entirely different from the process employed in Brazil 

 for the preparation of fine hard Para, in which the latex 

 is exposed, in thin films, to the action of the smoke. 

 Machines have, however, been designed for use on 

 plantations which enable the Brazilian process to be 

 imitated, and interesting experiments with these are in 

 progress. 



Tackiness of Rubber. — The chief defect which is liable 

 to occur in prepared rubber is the development of the 

 soft, sticky condition known as " tackiness." It has been 

 already pointed out that rubber will become sticky if 

 exposed to sunlight or to too high a temperature during 

 its preparation or subsequently, and many cases of tacki- 

 ness are, no doubt, due to these causes. In other in- 

 stances, however, carefully prepared specimens of rubber, 

 which have not been over-heated or exposed to sunlight, 

 become tacky, either as a whole or only in parts, and the 

 causes of this change are very obscure. It has been 

 suggested that the effect is produced by bacteria, and in 

 support of this view it is pointed out that contact with 

 a piece of tacky rubber will induce tackiness in sound 

 rubber. The evidence which has been adduced to prove 

 that the change is due to bacterial action is, however, 



