Boric acid is well known as an inorganic compound that complexes to carbohydrates and keeps them from burning, they just char. When you treat
wood with it it looks much the same...the wood chars with a torch and doesn't burn. It will glow red and then cool "instantly" when you turn off the torch. May be worth blending some boric acid into the mix or else perhaps use boric acid treated wood? Boric acid treated wood is commercially available as a preserved wood since it resists insects and
fungi.
Alternatively it might be interesting to add a silane or colloidal silica so as to create a glass protectant during the charring phase. That could also lead to a more resistant surface as well.
Do you
think the PVA glue is used to better capture the CO2 produced so that it blows the PVA polymer into a foam? I would think if the corn starch was fully dissolved that it could do the same so I'm guessing the PVA is there more just for making it putty like.
Fun stuff! Thanks for sharing!