<P> However, raw rubber was perishable . In 1839 Charles Goodyear discovered the process of vulcanization, a method that would cure rubber, making it durable . Rubber erasers became common with the advent of vulcanization . </P> <P> On March 30, 1858, Hymen Lipman of Philadelphia, USA, received the first patent for attaching an eraser to the end of a pencil . It was later invalidated because it was determined to be simply a composite of two devices rather than an entirely new product . </P> <P> Erasers may be free - standing blocks (block and wedge eraser), or conical caps that can slip onto the end of a pencil (cap eraser). A barrel or click eraser is a device shaped like a pencil, but instead of being filled with pencil lead, its barrel contains a retractable cylinder of eraser material (most commonly soft vinyl). Many, but not all, wooden pencils are made with attached erasers . Novelty erasers made in shapes intended to be amusing are often made of hard vinyl, which tends to smear heavy markings when used as an eraser . </P> <P> Originally made from natural rubber, but now usually from cheaper SBR, this type contains mineral fillers and an abrasive such as pumice with a plasticizer such as vegetable oil . They are relatively hard (in order to remain attached to the pencil) and frequently coloured pink . </P>

What is the shape of an eraser called
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