A large proportion of rubber articles are manufactured 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, 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.