What happens when you 'burn' ash? This popped into my my mind while watching a match burn (don't ask me what I was doing with a burning match...).

Now as I know it, 'ash' is what you call the residual, grey-black powdery material left over following the combustion of wood.

I'm under the impression that ash is largely just carbon, with little or no organic constituents present along with it (since I'm pretty sure that pure powdered carbon is black, not grey-black).

Now I heated what remained of the matchstick with, well...another matchstick and found out, unsurprisingly, that the ash underwent practically no visible change. So I scooped up some ash into a crevice in a concrete block and then blow-torched it for about 2 minutes. Same result. However, while I was heating it this time, it glowed orange, it subsided as soon as I turned off the torch.

Thinking about it, if I did heat it strong enough it should decompose completely to black colored elemental carbon. If that is the case, to what temperature should you heat it?

I don't think I've considered everything there is to consider in this situation, which is why I've adopted a tentative tone while typing out this question.

Could there be other, side-reactions/effects as well? What would they be?

Additionally if anyone happens to know; what is it that contributes to the grey color that ash normally assumes?
The principal component of wood ash appears to be calcium carbonate. (Wiki entry, "wood ash".) Other components include compounds of potassium and phosphorus. If you heat the calcium carbonate strongly enough, it will decompose into CO2 and calcium oxide. Apparently, strong heating is likely to result in less carbon instead of a larger percentage. The remaining calcium oxide is pretty stable as to temperature although addition of water will readily convert it to calcium hydroxide.

Calcium carbonate is white. White mixed with the black of any carbon present would result in a grey color.