I'm not sure the large bones would be ash. Cremation uses temperatures of just under 1,000 degrees C. After cremation, the bones remain and are run through a crusher to turn them into fragments. My understanding is that the bones are rather dry and brittle, but very recognizable before the crusher.
Glass making takes place at temperatures in excess of 1,500 degrees C. Although considerably hotter, I don't think it would completely destroy bones. I think larger bones such as the femur would survive relatively intact, although very, very dry and brittle. (Any moisture in the bone would be removed.) DNA would be gone, and because the fragments would have been blown over a wide area, I'm not sure anyone other than a specialist would recognize anything.
But other than very broadly speaking, no one has any experience with this, so you can fake it until you make it. Normally in a bomb explosion, people near the explosion end up in body parts, which may or may not be recognizable.
You might look up some of the descriptions of Hiroshima and Nagasaki. An atomic explosion generates heat in the temperature range you're talking about or higher.
Best of luck,
Jim Clark-Dawe