This is what I was told in another forum:
I don't use burlap for several reasons:
1) some burlap is now reinforced with non-natural fibers, polyester, plastic... these make a toxic smoke
2) some burlap, basically anything that brought a product in from a foreign country, will have been fumigated. Burning would release some nasty stuff.
I have used clean corrugated cardboard, fuel pellets, cotton dish rages with a little mineral oil on them, but mostly now, I use wood shavings (hamster bedding).
Reading the old posts on this subject has given me great ideas. This fall I will collect all the old sumac blooms I can find. In the meantime, I will try several of the other member's methods.