234 RUBBER AND 



bisulphide is exceedingly poisonous and inflammable, 

 but no other solvent has been found to give equally 

 good results. The strength of the solution used is 

 generally about 2^ per cent. The sheet rubber, which 

 must first be very thoroughly dried, is immersed in 

 the solution for three minutes or less, according to its 

 thickness. 



Whatever the method of vulcanisation employed, 

 there is always a certain amount of after effect, owing 

 to the prolonged slow action of the residual rubber. 

 Articles are therefore almost always rather less perfectly 

 vulcanised than their final condition requires. The 

 goods afterwards improve to some extent by keeping. 

 In the case of articles in which a high degree of 

 permanent elasticity is required, the excess of sulphur 

 must be removed by boiling in caustic soda, followed 

 by a thorough washing in water in order to remove the 

 alkali. This process is necessary, for example, in the 

 manufacture of elastic thread, which is cut from vulcan- 

 ised spread sheet and afterwards freed from sulphur in 

 the manner described. 



Reclaimed Rubber. 



During recent years the scarcity and high price of 

 fresh rubber has led to the use of enormous quantities 

 of old rubber, reclaimed in various ways, either in 

 combination with fresh rubber or alone. Old rubber 

 may be simply ground to powder and employed as 



