Shellac is natural, but it needs to be mixed with denatured alcohol to become a wood finish. I don't feel like going down to my shop, but I would imagine commercial shellac in the can, like Zinsser's, could potentially have other chemicals mixed in to help its shelf life or speed up its drying process. But even then, it still uses denatured alcohol, and alcohol and fire aren't a good combo.

You could make your own, which would just be shellac flakes and denatured alcohol, but it'd still be alcohol based. I'd burn first, then apply your finish.