Clinkers are hard glassy chunks that accumulate in the furnace, they're basically the remnants left over from the burning coal. You could call them slag, too. Periodically you have to dig them out of the furnace and dispose of them; when I was a kid we had a long handled claw thing, you'd reach in the furnace, grab the clinkers, put them in a bucket and then haul them out of the basement.
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