Oh! I am quite certain it was coal ashes. He heated with coal, in Pennsylvania. He got a massive pile delivered every summer, and we three kids would play "King of the Mountain," which left us covered in coal dust from head to toe...much to the dismay of our mother.
.
He was our
paternal grandfather - Mom felt he was turning us into wild heathens.
(There's probably some truth in that, but children
need to be heathens from time to time.) He was a colorful old man; taught me my first curse words...which I shouted at the top of my 4 year old lungs! (But that's another story...
.)
That said, I've done some reading, and the old ways were not always the best ways.
@BigC, I would use the wood ash, some googling tells me there are heavy metals left in the coal ash, which may not be safe in your edibles.