Drawing medium of natural, coloured earth or its synthetic or fabricated equivalent. Natural chalks, available until the late 18th century and early 19th, were made from a relatively few types of coloured earth. The restricted number of suitable sources reflects the fact that to be a good drawing material, the chalk had to be dense and consistent in colour and value. Moreover, as Vasari wrote in 1550, it must be ‘soft enough to be easily sawn and reduced to a fine point suitable for marking on leaves of paper’. Until the introduction of manufactured chalks in the late 18th century, the colours of chalk were restricted to black, red and white.