What does "clinker" mean?

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.

Additional senses

  1. 2.A scale of oxide of iron, formed in forging.
  2. 3.A kind of brick. See Dutch klinker, under Dutch.

Sources