(1):

(n.) A scale of oxide of iron, formed in forging.

(2):

(n.) A kind of brick. See Dutch clinker, under Dutch.

(3):

(n.) Scoria or vitrified incombustible matter, formed in a grate or furnace where anthracite coal in used; vitrified or burnt matter ejected from a volcano; slag.

(4):

(n.) A mass composed of several bricks run together by the action of the fire in the kiln.