1) Usually said to mean soot or coal-dust, and ‘smithy coom’ referred to the hard granular soot that forms over a smith’s fire.
1587 ‘The coame aboue the Smithes forge’. In the localities named Cowmes and Combs there were formerly smithy places and collieries. However, ‘culm’ in the sense of slack or small coal was used in burning lime and had a market value: 1521-2 to have for his beilding at Saynte Patricke’s, lyme, as it commeth from the kilne, and if he beilde at Dalymore to have colme of my cost and somme help with cariage, Halifax and Ireland. The two spellings are dealt with separately in the OED but they are the same word.