Rubber erasers are used extensively in drawing to remove pencil and ink marks. They are made of rubber combined with sufficient sulphur to give the proper hardness. Other materials are added in varying proportions to give different degrees of softness and suppleness. It is by these qualities that the different grades of erasers are distinguished.
The process of obtaining the rubber used for erasers, and for many other purposes as well, is a most interesting one. Rubber is obtained from the sap of certain tropical trees. A series of slanting cuts made in the bark allows the sap to run out (Fig. 98). A cup is hung at the bottom of the tree and gradually the milky sap runs into it. The contents of a number of these cups are then poured into a large vessel. A wooden paddle is dipped into the sap and when withdrawn is held over a fire made from palm nuts. The heat from the thick smoke hardens the sap. This process is repeated many times until a ball, called a biscuit (Fig. 99), is formed. The paddle is then withdrawn from the biscuit and the biscuit is ready for market. After coming to the market as balls or biscuits, the rubber is purified and made into sheets. Because of its softness and sticky nature, this crude rubber is useless for erasing and consequently must be subjected to a hardening process called vulcanization. This process consists in subjecting the rubber to supreme heat. After being vulcanized the rubber is suitable for erasers and other commercial products.
Fig. 98. - Tapping Rubber Trees.