Merriam-Webster offers me three meanings for charcoal as a noun (I paraphrase, but only slightly):
- a dark or black porous carbon prepared from vegetable or animal substances crisped in a kiln
- a piece or pencil of said carbon used in drawing, or the drawing prepared therewith
- a dark grey (a lovely dark grey, not black at all, really, which you wouldn’t necessarily expect after the lead-off definition although they do cover themselves with that “or”)
To these I now add a fourth:
- a completely unsuitable additive for toothpaste
This needs just a smidgen of backstory.