Ash mainly based of carbonates of the mineral elements. Mostly sodium- and potassium carbonate. Last one also called pot ash. Melting can be looked up.
I'd doubt that carbonates are the final form of trace refractories in burnt organic material, probably stops at the various oxides.
To the O.P.: you should look up the composition yourself, and see what is possible, and what's not. You can start with the definition of 'refractory'.