What is 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 called?

clinker — 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. (Webster's 1913.)

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