Why, when you continue to burn ash, do the ashes eventually change from black to white?

Whenever you burn wood, plants, or other organic material, the residual carbon becomes black ash. However, if you continue to burn that material, eventually the ashes will turn white. What are the mechanisms involved in further combustion of the material that result in the ash changing from a compound which essentially absorbs all photons in the visible spectrum making it appear black, to a compound that largely scatters light in the visible spectrum appearing white? Thanks! via /r/askscience https://ift.tt/XPpxlnM