Drawings in chalk are executed in black, white or colored chalks. They are usually executed on tinted paper and chalk is combined with ruddle or pastels. White chalk is also used for a form of highlighting known as heightening. A distinction should be drawn between natural chalk or whiting (mined calcium carbonate, ranging in color from white to greyish black) and precipitated chalk, an artificial calcium carbonate which may contain colored pigments. Drawings were done in natural chalk primarily from the late 15th century (Italy) until on into the 18th century. Since the 19th century, however, precipitated chalk has mainly been used for this purpose.