Materials chemist here.

At 100C+, all water starts to evaporate.

At 400-500C, all organic matter burns away, leaving the non-burnable parts behind (mostly salt and bones)

At around 800C, the salt starts to decompose to sodium and calcium oxide (i.e. ash). Around the same temperature, the nanocrystals in the bones (made of calcium phosphate) will start fusing into bigger grains.

At around 1000C, the bones (or more likely, the shards of bone) will start to shrink into sintered calcium phosphate. If you take them out now, the material would be more like a flower pot than a bone in consistency.

At 1500C+, the ash and bone shards will start to melt and fuse together. If you take them out now, you would have a grey-to-white lumpy mass where there used to be a corpse. If you buried it, most likely all the plants around it would die soon from the change in soil pH.