firewood Residue - Cottonwood?

Question

I've recently been burning some salvaged cottonwood in my woodstove and have noticed a hard, vitreous residue as well as ash. This residue coats and sometimes bonds with the firebrick. What is it and is it toxic? ( I usually dump the ashes in my garden and lawn.)

Answer 1

http://en.wikipedia.org/wiki/Creosote#Wood_creosote