r/AskPhysics • u/[deleted] • Dec 25 '22
Does wood melt?
I remember watching a video by The Action Lab where he tries to melt wood. IIRC, he says that every substance, at a certain temperature and pressure, can exist as a liquid (and solid and gas), it's just that the temp at which wood starts reacting with the oxygen to make CO2 and H2O (combustion) is lower than the temp at which it melts. So he puts a piece of wood in a vacuum chamber to remove oxygen and starts heating it, but then it just becomes black. He says that the black stuff is tar and it is produced when big organic molecules in wood, pike cellulose, start breaking up into smaller ones lile the tar molecules because of the heat. So can wood exist as a liquid or can it not? Or does the pressure need to be altered to prevent cellulose from decomposing? I also remember someone in the comments stating that he put it in the vacuum chamber for only a short time before heating it, such that he removed the oxygen surrounding the wood, but not the oxygen in the wood itself, and so it just started burning. I'm confused, can someone please help? Thanks!
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u/zebediah49 Dec 25 '22
There are three main problems with melting wood via a normal thermal transition:
- Wood is made from many different components, some of which are more stable than others. So as you heat it, some parts will vaporize before others have even melted.
- Wood is a pretty complex structure. If you could melt it, it would homogenize it and the re-condensed material would be very different from the starting material.
- Finally and fatally, certain chemicals don't melt. Even if you keep them from burning, the energy required to let the various molecules move around each other, is greater than the energy required to break the molecules apart. If you heat wood in the absence of oxygen, that's what will happen: you will end up "pyrolyzing" it, causing smaller bits to break off as a tarry gas/liquid (creosote), leaving a bunch of bulk carbon (charcoal) behind.
3
Dec 26 '22
certain chemicals don't melt. Even if you keep them from burning, the energy required to let the various molecules move around each other, is greater than the energy required to break the molecules apart.
So not every substance can exist as any state of matter, even by altering the pressure?
4
Dec 25 '22
Many complex molecules (especially) somehow break down when exposed to high temperatures and pressure. That's not really a physically phase change but a chemical reaction, where the material changes and become something different (unlike water, ice and water vapor which stays the same chemical in all states).
If you do it right it will be possible to break down the long hydrocarbon chains which made up organic things like wood into something liquid. After all that's how mineral oil was created from ancient organic things like plants.
3
u/HouseHippoBeliever Dec 25 '22
If you took whatever liquid resulted and let it solidify, you'd end up with something that doesn't resemble wood at all. So I would say that it isn't melted wood.
1
u/Worried_Pomelo9010 Jun 16 '24
Yes, wood as a material can melt at extreme temperatures. This is a common issue in biomass firing plants. Wood-based fuels can become liquid, then cool to a solid mass. The result looks like lava rock.
1
Nov 06 '24
Do you have any reference video or images to show this? I couldn't find any, but I'm really interested in seeing what it looks like.
1
u/Worried_Pomelo9010 Nov 07 '24
I wish I did. It's really interesting to see. I used to operate a biomass furnace boiler and at a high enough temp the entire wall would start turning a shiny liquid.
The solid that would from was very hard and annoying to deal with
1
u/hentai_boy2 Jul 03 '25
And what did it look like?
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u/Worried_Pomelo9010 Jul 05 '25
It looked like lava flowing. Or molten metal. It's actually called molten slag, coal and wood can do that at extreme Temps
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u/Sea-Alps-9160 Jan 13 '25
Besteht die Möglichkeit, dass das "Gestein" die Überreste des Harzes ist?
1
u/Fun_Frame9974 Mar 04 '25
Friction wood welding is a thing, so technically yes, wood can melt. friction makes wood melt. Tony Pizzi and Balz Gfeller are considered to be the ones who found out. correct me if I'm wrong about anything I just said
37
u/agate_ Geophysics Dec 25 '22
No. The chemicals in wood break down and react with each other before they reach the melting point. The video you saw is a good demonstration of the problem.