Not Medical Advice: For the purpose of cremation, incineration of an adult human body requires ultra-high heat (1400°F to 1800°F) to reduce the body to its basic elements and dry bone fragments.
The intense heat dries the body, burns the skin and hair, chars and contracts the muscles and vaporizes soft tissues.
Depending on the size of body, all that is left is the glow of the ashes, no flames, after 90 minutes. A person with a lot of fat will be burned up in up to three hours, whereas a small, frail person may take 80-90 minutes.
All that is left are the ashes from the body but there may also be some bone fragments which tend to be the larger bones (hip and the shin bones).
Bones technically do not melt, instead, are calcified so that they eventually crumble. Remains are called 'bone ash', and it is mainly composed of tricalcium phosphate which liquifies under high pressure at 1381°C (2517.8°F). This is much higher than the temperature used in cremation, the reason why bones remain 'intact' after burning.
Here are 5 things you need to know about direct cremation shared by Everplans.com.