I just made a quick 'model' on my triblet. Then I pushed some busted smalls , from .02 to .05 ct, that I had into it, and 'folded' the pmc over like they say in the instructions. The six 'o' clock diamond is upside down
Then I fired it in my dental oven.
After 900C for two hours and ten minutes the piece gets taken out. The diamonds are still there.
A close up through my microscope. I was not careful at all to finish to sample off.
This is a picture of one of the diamonds that I set upside down. I thought that the point of the culet being thin, would be exposed to a higher heat and would be the first to cook away. It did not, but seems to had cracked off. The outer tip does not have the glassy look it should, and that my first attempt to evaporate diamonds had. See http://www.meevis.com/jewelry-making-class-cooking-broken-diamond.htm
At two 'o' clock there is a section where a 'scale' looking piece has pulled off what seems to be 'burnt diamond' It's like frosted glass.. Apologies for the bad quality of the picture. I take it through the lens of my microscope. If I would buy another one I would DEFINITELY buy one with internal camera abilities.
My conclusion: It is doubtful that diamonds evaporate in 2 hours when fired in precious metal clay.