Hard, fused residues left over from burning coal or other fuel; or plural of clinger/clinker (something that clings or a metallic sound maker).
From Dutch 'klinker' (brick, or hard object that makes a clink sound), related to 'klinken' (to ring/clink). The term shifted to mean coal residue in industrial English.
Clinkers were the nightmare of coal miners and furnace workers—these rocklike chunks would build up and block fires, making 'clinker problems' an entire category of industrial headaches that engineers had to engineer away.
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