What happens to you when you die?
Specifically, what happens to you when you die and are cremated? Well, cremation uses furnaces that reach temperatures of around 760–1150 °C. Fire this hot vapourises almost all the tissue in the body, and what remains is mostly former bones.
The mineral in bone is hydroxylapatite, a type of calcium phosphate with the formula Ca5(PO4)3(OH). But in the heat of cremation, it’s at least partly transformed into tricalcium phosphate, or Ca3(PO4)2.
It also occurs naturally in cow milk and is used as food additive E341(iii) in table salt, sugar and baking powder, to stop it clumping together. But don’t worry: the food additive isn’t normally made from bones. That would be gross.
We covered this story in response to a listener question:
In many cemeteries there is much controversy about the removal after a designated period of metal plaques commemorating the location of cremated remains of the dearly departed. Needless to say, this is a source of anxiety to many families. It could be unjustified from a scientific viewpoint however.
To a chemist, the ashes are mostly calcium phosphate residue with trace metals (that are usually collected for recycling by the way).
What happens to the calcium phosphate in the ground? Is it like limestone and dissolves? If so, how long would it take? Does it need an acidic agent to do it (for example, carbonic acid?).
Cremation is the reduction by heat of non living remains until only chemical components are left. Theoretically, however it depends on the efficiency of the process. Commercial cremators are designed to produce temperatures of 870–980 °C, though depending on the individual case, much higher temperatures are possible, especially where there is excessive body fat.
Time taken depends primarily on the size of the body. Larger bodies take longer. So what is left at the end? Ashes. We call them ashes, though actually they are the non-burnable elements of the body. For the most part, this is bones and teeth.
Other things like dental and surgical prostheses may also remain, and are usually removed from the remains, along with larger bone fragments before they are delivered to the families of the deceased. The chemical makeup of what is left has been measured, how do we do this?
X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique that measures the elemental composition that exist within a material.
XPS spectra are obtained by zapping stuff with X-rays while at the same time measuring what bounces off it. The kinetic energy and number of electrons that escape from the top 1 to 10 nm of the material are measured and calibrated, which allows us to see what the material is made of, different elements reflect different amounts and wavelengths of energy.
XPS requires ultra high vacuum (UHV) conditions, because the atmosphere itself contains a large number of gaseous elements and molecules which would interfere with the detection process.
XPS detects all elements with an atomic number of 3 (lithium) and above. It cannot detect hydrogen or helium because the diameter of these atoms is so tiny, the probability of detecting them is almost zero.
So, as our listener suggested, the primary component of the cremated remains is Calcium Phosphate. There are other elements present, usually metals, such as Aluminium, Potassium and Zinc. Other elements may persist, such as various chemicals used in medical imaging techniques or embalming.
Unlike most other compounds calcium phosphate is increasingly insoluble at higher temperatures. But there is no reason to think it would not dissolve under normal rainfall conditions, even if buried, assuming the container allowed water to enter and come in contact with the material.
Primary reference: The analysis of burned human remains By Christopher W. Schmidt, Steve A. Symes
Lost in Science 
Fascinating. The Powerhouse Museum did a great triptych of blog posts about cremains. The converted coffee machine for pulverising what comes out of the furnace is truly ingenious and bizarre.
Great writeup. Just one correction—larger bone fragments are not removed. The cremated remains are placed in a pulverizing device, much like a blender with stout blunt blades. The fine powder with smaller fragments is what results after ~ 2 minutes of this process. (We actually go to a fair amount of trouble to make sure that as much of the fine “ash” as possible is swept out of the cremation chamber and then the processor—any crematory operator I know or work with would be horrified at the thought of discarding pieces of the cremated remains intentionally or even accidentally, if for no other reason than that could lose us our licenses!!)