Black CHALK, a name given by painters to a species of earth with which they draw on blue paper. It is found in pieces of from two to ten feet in length, and from four to twenty inches in breadth, generally flat, but somewhat rising in the middle, and thinner towards the edges, commonly found in large quantities together. While in the earth it is moist and flaky; but being dried, it becomes considerably hard and very light, but always breaks in some particular direction; and, if attentively examined when first broken, it appears of a striated texture. To the touch it is soft and smooth, stains very freely, and by virtue of its smoothness makes very neat marks. It is easily reduced into an impalpable powder, without any diminution of its blackness. In this state it mixes easily with oil into a smooth paste; and being diffused through water, it slowly settles in a black slimy or muddy form; properties which make its use very convenient to painters, both in oil and water colours.