Why Doesn’t Wood Melt

Jesse asks: Why doesn’t wood melt?

FireJesse, I love this question. It seems like everyone knows things are solid at certain temperatures. If you heat them up, they will become a liquid. Heating it further will turn the substance into a gas. This elementary truism of science is every grade school students’ claim-to-fame at their science fair. At some point, however, every person growing up looks at a burning fire and says “Hey, what gives?”

The Melvin in the cheap seats yells, “The easiest answer is that wood’s combustion temperature is lower than its melting temperature, duh!” While the politically incorrect stereotypical jock answers, “Lol, whut?”

The problem with melting wood revolves around what combustion is, and what temperature the combustion of wood happens at.  Combustion, also known as burning, is simply a chemical reaction that takes place where the combustible material (in this case wood) in the presence of an oxidizer (usually the air around the fire) changes its chemical composition and decomposes the material into other chemicals. The process is one that’s exothermic. As such, light and heat can be released.

Wood is mostly made up of things like cellulose, lignin, and water. As wood combusts, it’s broken down into products like charcoal, water, methanol, and carbon dioxide. Unlike water turning back into ice, if you cooled down the resulting products of burning wood, it obviously does not change back to its original composition. Thus, all of the ash left in your fireplace after that romantic evening with your special someone.

All materials that combust will have a natural temperature at which the process will begin taking place. The higher the temperature, the quicker the process becomes (usually). If that temperature is lower than the temperature at which the material will melt, that material will never (naturally) melt because it just turns into other chemicals.

As for wood, it will begin a process known as pyrolysis at temperatures around 500-600 degrees Fahrenheit. Pyrolysis is also an exothermic reaction that tends to be self sustaining. At these temperatures, wood will begin giving off up to 100 chemicals, including methane and methanol (the same stuff they put as additives in gasoline), that will begin to burn. Once those chemicals begin burning, they will increase the temperature and the remaining char (the burned black bits present after the fire goes out) left behind will begin to further decompose, things like calcium, potassium and magnesium.

At this point Melvin might again pop up and say, “methanol at room temperature is liquid, and other chemicals like magnesium and calcium can be liquid, so wood can melt!” I say, be quiet. Once those things are present, the substance is no longer wood, as such, not really melted wood.

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Bonus Facts:

  • It’s almost impossible to get complete combustion of wood. Don’t fret. It’s this incomplete combustion that allows us to have fire. It’s the byproducts of the incomplete combustion that actually burn and allow pyrolysis to continue giving us a fantastic energy source to cook our hot dogs. If you were to get absolute complete combustion of wood, the only byproducts would be carbon dioxide and water.
  • Wood will combust at temperatures lower than the point at which pyrolysis will allow it to give us a continuously visible flame. Wood will slowly char between 248-304 degrees Fahrenheit. Decaying wood with low water content will also ignite at 304 degrees, while the various types of wood will usually ignite between 374-500 degrees, depending on the chemical composition of the wood.
  • In case you were wondering what might make wood burn, here are some common things used to start fires and their average temperature their heat gives off, in Fahrenheit.
    • Ventilated cigarettes- 752-1,436
    • Unventilated cigarettes- 550
    • Matches- 1,112-2,552
    • Candle flames- 1,112-2,552
    • Electrical spark- 2,400
    • Electric arc- Up to 6782
    • Lightning- 54,000
  • Most wood is made of about 50% carbon, 44% oxygen, 6% hydrogen. Softer woods will tend to have more carbon and less oxygen than harder woods. Up to 85% of the mass of wood and 60% of its heating potential is contained within the gases that are produced when the wood is burning from pyrolysis.
  • According to the National Fire Protection Agency, there were 370,000 house fires in the United States in 2011. They accounted for 13,910 injuries and 2,520 deaths. The direct cost of their damage was estimated at 6.9 billion dollars. Cooking is the leading cause of house fires with 42% of them starting in the kitchen. While 7% of the house fires started in the bedroom, 25% of the deaths were a result of those fires. 24% of the deaths were caused by fires that started in the living room, family room or den, combined while they only accounted for 4% of the fires.
  • 60% of home fire fatalities occurred in homes without working smoke detectors. Get them people! Many fire departments have programs that give away free smoke detectors year round. Call them and ask. If they do, they will usually install them for you as well! There, you now have no excuse not to get one.
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  • Will wood or other combustible material melt if you leave it in a vacuum and turn up the heat? The idea is to remove the oxygen that allows combustion to take place. Or will other chemical reactions start taking place between the different chemical components in the wood?

  • Off of previous comment–When placing a log in a heated vacuum would it then melt? Wouldn’t this take away the ability of the wood to be combustible.

    If the convection is needed for it to melt, then why not place the log in a chamber of a noble gas such as radon?

    • Very good question. If you heat wood in an inert environment, rather than melt, it will sublimate, or go directly from solid to a gas. Well, mostly. Wood isn’t just one compound, it’s a combination of lignin and cellulose (mainly). So you will get methane, charcoal, and a few other compounds, in addition.

  • Thank you for this clear and straightforward explanation.