soot, smoke dried
煤 centers on the dark, powdery residue left by fire: soot and smoke-dried deposits.
煤 combines 火 (fire) with 某, which likely contributes the sound and may relate to the dark color of soot. The exact historical development of 某 in this context is uncertain.
Fire 火 on the left and 某 on the right: picture a fire leaving a dark, sooty residue on something nearby. The 某 shape suggests a dark, smudged surface.
For バイ, imagine buying a chimney sweep's brush to clean soot: buy -> バイ, and the brush scrubs away the dark soot from the fire.
soot
soot of burnt pine
to be sooty; to be stained
dust and soot; particulate matter
sweeping the soot from one's house (to welcome the kami of the New Year)
dirty smoke; soot and smoke
soot-colored bamboo (coloured)
color of soot (colour)
sooty albatross (Phoebetria fusca)
sooty owl (Tyto tenebricosa); greater sooty owl