Looks like coal. -my guess is the material is partially some kind of alcohol, some photopolimer and some minerals added for the stability. And those minerals combust like this coal pieces. Now I am thinking if we rise the temperature really slow alowing the material fully oxidize and combust this may leave small amount of residual ash. my greatest problem for the moment is not the ash, it is the surface quality I am having after the burnout.

You could try silicone molds instead of rubber as a cheaper option. No vulcanizer, just mix and pour. - I will still need wax injector correct? And the silicone one may not give the same quality. I do not have any experience with this technique so far.

By the way, about the divesting liquid, how well does it work and is it expensive? I was doing mechanical divesting of Formula and I cannot say that it can win any stars in the favourite activities competition.