RUBBER PLANTING 225 



contains either litharge or some form of carbon black. 

 Finally, there are the filling materials which are intro- 

 duced in order to reduce the specific gravity of the 

 rubber or to lower the cost. Rubber is sold by weight, 

 and the materials used for toughening have mostly high 

 specific gravities. Substances of low specific weight 

 are therefore introduced in order to counteract the effect 

 of the heavy bodies. For this purpose whiting and 

 French chalk are largely employed. For cheapening, 

 a large variety of substances is used, foremost among 

 them being old vulcanised rubber in a more or less 

 reclaimed condition. Fatty oils undergo a kind of 

 vulcanisation with sulphur, and are used as cheapeners 

 in combination with reclaimed rubber. With low grades 

 of rubber such substances as leather waste and sawdust 

 may even be incorporated. 



The further treatment of the mixed rubber varies 

 according to the kind of article which is to be manu- 

 factured. Very commonly it is treated in one of three 

 ways. It is either pressed into blocks, or rolled into 

 sheets, or moulded in what is known as a forcing 

 machine. 



Preparation of Sheet Rubber. 



A large proportion of rubber articles are manu- 

 factured from sheet rubber. The highest class of sheet 

 rubber is cut from block, but this method is less used 

 than formerly, on account of the increased perfection of 

 other methods. For making "cut sheet," the rubber, 



