Bones do burn. At approximately 1292 degrees Fahrenheit, calcium phosphates begin to fuse together and change in color and shape. While not entirely reduced to ash, modern cremation does an excellent job of vaporizing and deteriorating bone.

Biology

Human bones are comprised of a collagen-rich matrix pregnant with calcium phosphate and other minerals, according to Dr. Elaine N. Marieb in "Human Anatomy and Physiology." Calcium phosphates are inorganic mineral salts present in the form of tiny crystals, and make up roughly 65 percent of our bone mass.

Chemistry

Most tissue composing the human body begins combustion at 572 F. In cremation, the deceased is exposed to several hours of intense heat ranging between 1400 F and 1800 F, vaporizing all organic matter.

Result

Some portions of the inorganic bone do survive this process, only as chips and re-fused fragments of calcium phosphate. In modern crematories this remaining material is sifted through a specialized processor and ground into a uniform substance similar to powder. This powder constitutes the remains of a body and is primarily cremated bone.