SOOT

Chimney dirt (4)

Solution has appeared in 2 crosswords (1 across and 1 down):

• Quick crossword No 16,123 — 2022-01-10 00:00:34
  19A: Carbon black (4)
• Quick crossword No 15,690 — 2020-08-19 23:00:05
  19D: Chimney dirt (4)