A fragment of incombustible matter left after a wood or coal or charcoal fire WordNet 3.0 & Open English WordNet

Clinker is a generic name given to waste from industrial processes, particularly those that involve smelting metals, welding, burning fossil fuels and use of a blacksmith's forge, which commonly causes a large buildup of clinker around the tuyere. Wikipedia

Waste from industrial processes Wikipedia Disambiguation

Partially or mostly burnt material that results from incomplete combustion of coal or wood etc. Wiktionary

Partially or mostly burnt material. Wiktionary (translation)