RUBBER PLANTING 163 



air. In practice the use of hot air is often adopted, the 

 method being based on the processes customary in tea- 

 withering lofts and in cacao-curing houses. Prolonged 

 exposure to a high temperature is not however to be 

 generally recommended, owing to the softening effect 

 upon the rubber, and the danger that tackiness may 

 arise. The use of cool air artificially dried has been sug- 

 gested, such drying being effected either by mechanical 

 cooling or by the use of such a substance as calcium 

 chloride. However, the technical difficulties in the way 

 of adopting such a process have not been entirely 

 overcome, and the method is scarcely used in practice. 

 The most rapid and convenient way of removing the 

 moisture is by the use of vacuum driers. In these 

 machines the thinly creped rubber is spread on trays in 

 a square chamber or oven provided with a door which 

 can be shut quite air tight. The chamber is heated by 

 steam to a temperature of about 90 degrees F. It is 

 then exhausted by means of a powerful air pump, and 

 the pressure is kept low in this way for one or two 

 hours. A large machine is capable of dealing with 200 

 to 300 Ibs. of rubber in two hours. The rubber is taken 

 from the drier in a soft and woolly condition, and is 

 generally re-creped whilst still warm. It should be 

 pointed out that certain manufacturers are of opinion 

 that rubber dried in this way is inferior in nerve to 

 samples slowly dried in the open. Other experts how- 

 ever consider vacuum-dried rubber to be as good as any 

 other. 



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