226 RUBBER AND 



after masticating and mixing, is pressed into a solid 

 block. The block is then frozen hard, an operation 

 which takes a long time owing to the low conductivity 

 of rubber for heat. After freezing, the block is cut into 

 sheets by one of two methods. In the older method 

 the block of rubber was sliced horizontally, being raised, 

 after the removal of each sheet, by an amount equal to 

 the thickness of the sheet. In the second method a 

 cylindrical block is made to rotate against the knife 

 blade, which thus slices off a continuous sheet. The 

 knife is kept wet during the process, and special 

 methods of sharpening and setting have to be 

 adopted. 



Raised sheet is prepared by spreading rubber in 

 solution over cloth, in a very thin film. A series of 

 such films are spread over the cloth, the solvent being 

 allowed to evaporate between each spreading. Finally 

 the cloth is detached from the sheet of rubber thus 

 produced. A similar process is employed in water- 

 proofing, in which a thin layer of rubber is permanently 

 attached to the surface of the cloth. Sometimes two 

 layers of cloth are united by a thin layer of rubber 

 placed between them. It was for waterproofing that 

 rubber in solution was first employed. Sheet rubber 

 prepared on cloth in this way carries the grain of the 

 cloth upon its surface. Sheet having a perfectly 

 smooth surface can also be prepared from solution by 

 spreading the dissolved rubber over the surface of a 

 plate of glass. 



