Present participle of culm; the process of producing or accumulating coal slack or dust.
From culm (coal dust), with -ing forming the present participle. Culm itself comes from Welsh 'colm' or similar Celtic roots referring to coal waste.
In mining regions, 'culming' became a distinctive word for a specific industrial process, and you can still find old culm heaps dotting the landscape in former coal-mining areas like Wales and Appalachia.
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