Why, when you continue to burn ash, do the ashes eventually change from black to white?

Submitted by genitalwaffles t3_xzosvv in askscience

Whenever you burn wood, plants, or other organic material, the residual carbon becomes black ash. However, if you continue to burn that material, eventually the ashes will turn white. What are the mechanisms involved in further combustion of the material that result in the ash changing from a compound which essentially absorbs all photons in the visible spectrum making it appear black, to a compound that largely scatters light in the visible spectrum appearing white? Thanks!

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taphead739 t1_irngv5v wrote

If you have something black during a combustion process that‘s not ash but the remaining carbon. The carbon reacts with oxygen in the air to form carbon dioxide, a gas. Ash is the product of oxygen reacting with everything not containing carbon: calcium, magnesium, and a few other metals. The oxides of those metals (reaction products with oxygen from the air) are usually white in color and do not become gaseous at usual combustion temperatures. That is why this is what‘s left at the end of burning something and why it is white.

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obax17 t1_irnhc9q wrote

A general answer, sure a chemist could give a more complete/detailed answer.

The black stuff isn't ash, it's soot, made up mostly of unburned carbon (giving the black colour). Ash is pale grey-white and consists of non-combustible compounds. When you get charred wood, for example, the black is carbon that hasn't burned yet, and if you let it continue to burn, the black carbon will eventually burn away and leave behind the pale ash. The black dust that you may be thinking of is just fine particles of soot, which are a component of smoke, drawn up by the heat and release of other gasses which make up the smoke.

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piccadillyst t1_irnrw3m wrote

Replying to taphead739 (#64,586)

Interestingly, most of these are basic oxides, meaning that they form very alcaline solutions in water. This was exploited in ancient times to prepare soap from fat.

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l3lindsite t1_irnuzmt wrote

Replying to piccadillyst (#65,010)

You're talking about deriving lye from ash in order to make soap. I was wondering about that. You said numerous metals end up getting oxidized in ash. So how does mixing ash with water give you lye specifically so you can combine with with fats and oils to make soap? How do you go from a random mix of oxidants to a specific alcaline chemical?

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piccadillyst t1_irnxpe0 wrote

Replying to l3lindsite (#65,130)

It's important not to mix up terms here. What you get after combustion are oxides: Na2O, CaO, K2O, MgO, etc (probably some superoxides as well, but I digress). These are not oxidants, because the metals are in very stable oxidation states. But they are very reactive towards water: the oxide anions "steal" protons from water, giving hydroxide ions as product. Specifically, in the case of K2O, the reaction would be something like: K2O + H2O ➡️2K+ + 2OH-. The hydroxide ions then react with triglycerides in the saponification reaction.

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tregosmasher t1_irny4iy wrote

Replying to l3lindsite (#65,130)

Lye is sometimes used to mean sodium hydroxide. If you bought lye from a chemical supply warehouse, that’s what you’d expect. Lye can also mean combining ash with water. You have conflated these two types of lye, which is incorrect. Wood ash lye is typically high in potassium hydroxide.

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I2ichmond t1_iro50qc wrote

Replying to taphead739 (#64,586)

Here to supplement with this: https://www.youtube.com/playlist?list=PLGnWLXjIDnpBVRqu5lz5JGaQxjPs7q3CJ

Watching these Primitive Technology videos on fire got me curious about all the different kinds of combustion we take advantage of. He has videos in that playlist where he makes a charcoal mound which is designed to burn wood only through the first stage so you end up with the leftover carbon, which can be burned all over again to produce much more intense heat. Charcoal is sort like refined wood (or refined organic tissue, in a broader and weirder-sounding way)

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wombat5003 t1_irocw1l wrote

That actually makes sense to me too, cause I had an old recipe for lye soap and you had to take a lot of ash and put weight on it in like a funnel (I’m going from vague memory here) and the lye would drip down and then you mixed that with fat, and herbs, then buried it for a few weeks country style soap…

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Lurker_IV t1_irqmf93 wrote

Replying to I2ichmond (#65,462)

Charcoal is pure carbon. The heat of fire without any available oxygen in the mound vaporizes any volatile compounds and burns them off leaving the carbon.

Charcoal fires can burn up to 100-degrees hotter than wood fires, iirc, which is why it is needed for metalworking.

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Nixeris t1_irqnp3n wrote

Replying to alucardou (#66,367)

When scientists discovered a metal that could be derrived from the roman alumin recipes, they called it Aluminium.

Found a metal in Magnesia? Call it Magnesium.

In California? Call it Californium.

The element is Blue-gray? Call it Blue-grayium. But make it fancy and use the Latin term for Blue-gray, 'Caesius'. Caesium or cesium.

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malastare- t1_irr13hd wrote

Replying to Lurker_IV (#68,853)

>Charcoal is pure carbon.

Not really. It still has some of those metals that end up as white oxides when you completely burn the charcoal, and it still has a decent amount of bound oxygen. The volatile compounds are mostly gone (with a bunch of the carbon) but all the trace metals are still there too. Some processes are able to remove some of them, with the goal being a result that retains as much carbon as possible, but burning even the best charcoal still results in non-carbon ash.

Coke (a slightly different process) has a notably higher carbon content, but even that isn't nearly close enough to "pure carbon" to be considered chemically pure carbon. It's better as a carbon source for steel and more expensive that charcoal as a heat source, but from what I can find, its still just shy of 90% carbon.

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l3lindsite t1_irti3sl wrote

Replying to tregosmasher (#65,246)

Okay so ignoring the ego tripping below what im gathering is pure lye is a particular chemical potassium hydroxide but wood ash has large amount of it in it so when you dillute it in water you are getting a mix of chemicals with a high concentration of lye within it, tho it isn't entirely pure lye. How could one refine this process.

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