The Physics and Chemistry of Fire

For me, Bonfire Night has never been about the rockets or the smoke. It is about the fire itself. That is where we burn the guy, thaw frozen fingers, and, if we are lucky (and you stick around long enough), bake potatoes or toast marshmallows (a habit I blame on American influence) in the embers. Beneath the tradition lies something beautiful: a bonfire is biology, chemistry, and physics all meeting together. I have been building fires since I was probably too young to hold a match. The secret? Listen to what the log has to say.

What a Log Really Is

Look at a piece of firewood. To most, it is just fuel. To a biologist, it is a tower of complex sugars. The main ingredients are long organic chains called cellulose, hemicellulose, and lignin [1]. Trees evolved these polymers to grow tall and haul water metres into the air. When we burn them, we liberate ancient sunlight. But wood is not pure carbon and hydrogen. Hidden inside are trace metals, especially calcium and potassium, that will later paint the flames with colour [2].

When I think of logs you are releasing energy from decades to a few hundred years. In contrast, the formation of coal is an incredibly slow process, taking place over millions of years. The most significant and high-quality coal deposits began forming around 360 to 290 million years ago during the Carboniferous period. This vast timeline means the coal we use today is the result of geological processes that occurred long before the age of dinosaurs.

How Wood Falls Apart

Strike a match, and you supply the activation energy. That is the little push needed to start breaking chemical bonds. A 2011 paper showed that woody biomass lights more readily than coal precisely because its activation energy is lower [3].

Heat the wood to somewhere between 280 and 420 °C, and those large molecules begin to fragment [4]. In a perfect world, combustion would produce nothing but carbon dioxide and water. But because lignin has those stubborn ring-shaped structures, the reality is messier. The fire spits out soot, carbon monoxide, and a range of organic fragments. Studies of pyrolysis reveal that these volatile compounds are exactly what give wood smoke its distinctive, mouth-watering aroma, the same one that flavours smoked food [5]. The calcium and potassium? They refuse to burn. They simply remain behind as the pale grey ash that coats your shoes [6].

Why Flames Glow Yellow

You have a spark. The chemistry is running. So why do the flames shine yellow? That is physics at work. Inside the fire, carbon atoms get incredibly hot. Their electrons absorb that thermal energy and leap to a higher energy level, what scientists call an excited state. When those electrons fall back down, they must shed the extra energy. They do so by releasing photons of yellow light [7].

Watch the base of the fire carefully. You may catch a hint of lilac or orange. That is the classic flame test from school chemistry. Potassium gives that pale purple tinge, while calcium produces a brick-orange colour [8].

Building the Perfect Bonfire

Now that we understand the science, let us put it into practice. Do not simply throw logs together and hope for the best.

1. Begin with the smallest pieces: You want a high surface area relative to mass. Twigs and kindling heat up fast because there is less material to warm, and they have plenty of contact with air. They will hit the ignition point, around 300°C, far sooner than a damp log will [9].
2. Respect the need for air: Combustion is an oxidation reaction. Without a good draught at the base, your fire will suffocate. Feed it oxygen from below [10].
3. Light low down: Hot gases rise. That is convection. As they climb, they warm the wood stacked above, driving out moisture and eventually providing enough energy for those upper logs to catch fire spontaneously [9]. Note, resorting to accelerants should be considered a failure to understand the science.
4. Remember the wildlife: If your bonfire has been assembled for days, it has become a luxury hotel for hedgehogs, mice, and insects. Give it a final check before you light the match.

From the breakdown of lignin polymers to the dance of excited electrons, every bonfire tells a story of science. Enjoy the show.

References

1. Rowell RM, editor. Handbook of wood chemistry and wood composites. 2nd ed. Boca Raton (FL): CRC Press; 2012. Chapter 3, Cellulose, hemicellulose, and lignin in wood structure. p. 35-60.
2. Vassilev SV, et al. An overview of the chemical composition of biomass. Fuel. 2010;89(5):913-933.
3. Park Y, et al. A study on combustion characteristics of wood biomass for cogeneration plant. Applied Chemistry for Engineering. 2011;22(3):296-300.
4. Misyura S, Morozov I. Thermogravimetric analysis of combustion and gasification of lignocellulosic biomass and its components. In: AIChE Annual Meeting Proceedings; 2010 Nov 7-12; Salt Lake City, UT. New York: AIChE; 2010. Paper 570j.
5. Dobele G, et al. Pyrolysis and smoke formation of grey alder wood depending on the storage time and the content of extractives. Journal of Analytical and Applied Pyrolysis. 2009;85(1-2):163-170.
6. Haykiri-Acma H, et al. Co-firing of biomass with coals: Part 1. Thermogravimetric kinetic analysis of combustion of fir (Abies bornmulleriana) wood. Journal of Thermal Analysis and Calorimetry. 2011;103(3):925-933.
7. Gaydon AG, Wolfhard HG. Flames: their structure, radiation and temperature. 4th ed. London: Chapman and Hall; 1979. Chapter 7, The emission spectra of flames. p. 180-210.