Black CHALK, a name given by painters to a species of earth with which they draw on blue paper, &c. It is found in the earth in pieces from two to ten feet long, and from four inches to 20 in breadth, generally flat, but somewhat rising in the middle, and thinner towards the edges, commonly lying 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 fresh broken, 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 soft 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 the painters both in oil and water colours. It appears to be an earth quite different from common chalk, and rather of the flaty bituminous kind. In the fire it becomes white with a reddish cast, and very friable, retaining its flaky structure, and looking much like the white flaky masses which some sorts of pit-coal leave in burning. Neither the chalk nor these ashes are at all affected by acids.
The colour-shops are supplied with this earth from Italy or Germany; though some parts of England afford substances nearly, if not entirely, of the same quality, and which are found to be equally serviceable both for marking and as black paints. Such particularly is the black earth called killow, said by Dr Merret
10 P
in
Chalk
Challenge. in his Pinax Rerum Britannicarum, to be found in Lan-
cashire; and by Mr Da Costa, in his history of fossils,
to be plentiful on the side near the top of Cay-Avon,
an high hill in Merioneth-shire.