What exactly are ashes?

Submitted by krFrillaKrilla t3_107165l in askscience

Ashes cannot be burnt and therefore seemingly can't be disposed of unless thrown into a black hole. They can't be what the burnt object was because (according to what I've been told) all of the subjects "information" has been lost when it was burnt, and there is none left in the ashes and it can't be recovered. If this is the case and ashes aren't any part of the original object, then what are they and why are they always the same?

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Jeff-Root t1_j3pnife wrote

Ashes cannot be burnt because they are the solid remains of something which was burnt.

Burning something means breaking chemical bonds in a material and forming new bonds with an oxydizer, which most often is oxygen. Some materials burn entirely to gasses and vapors, leaving no ashes behind. For example, hydrogen gas combines with oxygen to form water vapor. Methane and other hydrocarbons, which are composed only of hydrogen and carbon, combine with oxygen to form water vapor and carbon dioxide. Wood, paper, and many other materials contain elements which form solid residues when burned. These elements include silicon, calcium, and iron. Ashes are the portion of a burned material which does not turn into gas or vapor. Ashes typically include oxides of silicon, calcium, and iron.

Most ashes can easily be turned into something else by being eaten. After a forest fire, microbes consume the ashes and turn them into dirt, in which plants grow. Plants absorb the waste products of the microbes in the dirt through their roots, and turn it into new wood, or leaves, or fruit.

-- Jeff, in Minneapolis

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wm_berry t1_j3q058x wrote

Ashes from one oxidant (e.g. oxygen based) may be able to be burnt again with stronger oxidants (e.g. fluorine based).

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PopeBrendicus t1_j3pk5i7 wrote

I think you're confusing multiple different concepts here. I'll let someone more versed in information theory weigh in on the black hole discussion, but ashes themselves are just the non-burnable parts of whatever you set on fire.

In a log, for example, the vast majority of the mass is water (which can evaporate into the air) and organic compounds that will be turned into carbon dioxide and carbon monoxide (this is the chemical reaction you are doing when you see fire). After you completely, 100% turn the organic compounds into gasses, you are left with ashes, which are largely the micronutrients that were stored in the plant and used to keep it healthy. Calcium, potassium, magnesium, iron.

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Jeff-Root t1_j3poyzp wrote

If you look carefully at the ashes from different burned materials, you will see that they are not the same. Ashes of a burned log are quite different from ashes of a burned bone. And even in cases where the ashes look similar, the material that was burned to make them can usually be determined by chemical analysis. Different materials have different proportions of ash-producing elements, so determining the proportions of those elements in the ash can show what material was burned.

-- Jeff, in Minneapolis

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