Description

Chalk used in school classrooms comes in slender sticks approximately .*5 of an inch (nine millimeters) in diameter and 3.*5 inches (*0 millimeters) long. Lessons are often presented to entire classes onchalkboards (or blackboards, as they were originally called) using sticks of chalk because this method has proven cheap and easy.As found in nature, chalk has been used for drawing since prehistoric times, when, ccordingrchaeologists, it helped to create some of the earliest cave drawings. Later, artists of different countries and styles used chalk mainly for sketches, and some such drawings, protected with shellac or a similar substance, have survived. Chalk was first formed into sticks for the convenience of artists. The method was to grind atural chalk to a fine powder, then add water, clay as a binder, and various dry colors. The resultant putty was then rolled into cylinders and dried. Although impurities produce natural chalk in many colors, when rtists made their own chalk they usually added pigments to render these colors more vivid. Carbon, for xample, was used to enhance black, and ferric oxide (Fe 2 O 3 ) created a more vivid red.