Aside from all the CCA treated bits that have been mentioned, not something
I want burn in my home, I use all of the ash from my stove around my place,
but allow the white small fine stuff to wash away in the driveway with the
rain as it helps it to set like concrete. The larger black bits are great on
the veggie garden or anywhere and allow more air and water to enter the
soil.

Hope this helps,
Peter