I actually tested this out by accident and was surprised that I could burn bones to ashes. I found a dead possum and had a very large wood burn going on at the time and so I threw the possum into the hot coals and sure enough after about 6 hours of being embedded in the hottest coals part of the blaze there was basically nothing left. I was surprised that this happened as I expected there to be charred bones left but the possum disintegrated. The possum bones were much smaller than human bones of course and the fire I had going was absolutely huge. I burned a very large dead tree. The logs were 2 feet diameter logs about 4 foot long logs. The tree was about 60 feet tall and so my burn pile was absolutely huge and so I had a very hot fire going. My burn pile was about 10-12 feet diameter and about 5 feet high of ashes. Anyway this surprised me and made me realize that if someone did have a large enough fire I do think its possible to basically get rid of a body. Even though there would be microscopic fragments left the problem is the burn pile of ashes was so large and things get so scattered that even the best forensic pathologist would most likely not be able to isolate anything of significance. Especially if someone was constantly tending to the fire and stirring the contents and making sure things got dismembered and separated and scattered all through the burn pile of ashes. And we are talking "white hot" here folks. The large fire I had going was so hot you could not hardly get within 10 feet of it for just a second and have to turn away. I could hardly throw any more wood on the burn pile because I could hardly get close enough to toss another log. Super super white hot. JMO of course based on my experience.