Photographing a fire can be both challenging and satisfying. Challenging, because those flickering flames just won’t stay put for more than an instant at a time. Satisfying, because a crisp photo of a campfire or bonfire is a thing of golden beauty and lasting memories.
Photos of fires evoke special memories of star-spangled nights encircling a crackling fire with friends and loved ones. Looking at these photos years later helps us remember each campfire as a unique event, a particular kind of fire that feels different to the memory than any other.
But what exactly are we photographing when we frame up the flames in our viewfinders? Light, surely, but from where? What makes a fire burn? We all know it is the wood that is burning, but why do flames appear above and around the wood, not just on its surface? Conversely, why are there glowing coals inside the flames?
And why does putting water on a fire douse it? Why does putting a fire blanket on it smother it?
I’ve done some investigating into how wood burns, and have discovered that fires are both simple and complicated at the same time. And it is all to do with a chemical reaction that is self-sustaining once it gets going.
It may seem counter-intuitive to start with Secondary Combustion; no doubt you are thinking “but what about Primary Combustion?” I’m starting with secondary because I find it easier to think first in terms of the process by which a fire starts, rather than thinking in terms of fuel.
Wood is made up of molecules that are remarkably like sugar. Like the sugar we eat, these molecules are an easy source of energy. While there are many types of molecules that make up what we call a piece of wood, the main molecule in wood is cellulose. In a live fire, cellulose molecules (C6H10O5) break down into carbon dioxide and water.  This chemical reaction is the reverse of photosynthesis, in which plants assemble cellulose from carbon dioxide and water. Plants build wood from CO2 and H2O, and fire turns wood back into CO2 and H2O again. Sounds simple, doesn’t it? But it gets more interesting from here.
Firstly, it isn’t really the cellulose inside a piece of wood that catches fire and starts the whole chain reaction going. It is gaseous cellulose and other volatile molecules floating above the wood.
Cellulose molecules are linked tightly to each other through an oxygen atom. Fires catch alight only after wood is heated enough that some cellulose molecules break away from their oxygen links. Once in the heated air above the wood, the unstable molecule reacts with nearby oxygen molecules. In this reaction, the atoms in the cellulose and the O2 swiftly recombine into carbon dioxide (CO2) and water (H2O) molecules. CO2 and H2O molecules have a lower energy state than cellulose, and the extra energy is released in the form of light and heat.  Voilà, light and flames and photographs!
Ironically, it takes heat to start a fire, because without heat, the cellulose will just stay in the wood and not combust with the oxygen. However, once the initial heat is applied and the process starts, it is self-sustaining; the heat generated by the chemical reaction causes more cellulose to be liberated from the wood which then combines with O2 with the side effect of producing more heat which liberates more cellulose and so the cycle goes on. The fire will continue until the wood is exhausted or the fire is put out, whichever happens first.
However, what I’ve just described is only half the story. The flaming molecules of cellulose and oxygen are what cause the flickering flames in which we delight. But what about the glowing wood, the motes flying through the flames, and the hot coals that toast our marshmallows? That’s where primary combustion comes in.
Primary combustion is what creates the coals and embers of the fire. This is direct burning of the solid wood rather than the burning of gases given off by the wood as happens in secondary combustion.
As the volatile molecules are used up and the fire dies down, the temperature around the wood drops, and the combustion of molecules such as cellulose no longer takes place as rapid gas combustion above the wood, but as slower burning directly within the wood. Because it takes place at a lower temperature and with less exposure to atmospheric oxygen, the solid wood burns more slowly, and tends to retain its carbon to become charcoal.
If the fire burns hot enough for long enough, all of the charcoal will also be used up and there will be nothing left of the wood but ash, which is all the components of wood that cannot burn.
Of course primary combustion is usually taking place at the same time as secondary combustion, which is why the firewood glows even while the flames flicker. And as bits of wood pop and crackle, tiny pieces of wood undergoing primary combustion break away and fly up into the flames as motes of glowing coals.
Stopping the fire reaction
We all know that if we pour enough water over a fire, it will go out. But why? It is because water reduces the amount of energy available to the chemical reaction of fire. It takes heat to make cellulose molecules break away to combust in the air. When water is added to a fire, it soaks up all the heat near where it has fallen, and the cellulose molecules in that area aren’t hot enough to break away. Unless a new source of heat is applied, the fire reaction can’t recommence.
Water is also the reason that fresh twigs and logs are often difficult to use in a fire – the water inside unseasoned wood interferes with the heat buildup that commences the fire reaction. If the fire is hot enough, and there is enough moisture in the wood, water escapes from the wet wood as steam. As the wet wood dries, it becomes usable fuel for the fire.
Smothering a fire with dirt or a fire blanket targets a different aspect of combustion. The goal with smothering a fire is to remove the fire reaction’s access to oxygen. Without access to oxygen molecules, wood molecules such as cellulose cannot react and recombine into CO2 and H2O and so no new heat is produced and the fire reaction cannot sustain itself.
So when we take out our cameras to photograph a fire, there is so much more to know about what we are recording than just the beauty of the flames. A fire photograph shows how much primary and secondary combustion is taking place, the speed of combustion, the quantity of coals being created, how wet the wood is, and how hot the fire is. These are the characteristics that form the uniqueness of that particular fire, and even if we don’t consciously recognise them, they are the characteristics that create our memories of that moment of fire.
All photographs by Sabrina Caldwell; other than re-sizing for webuse, photos have not been altered in any way.
The cycle of Cellulose and Oxygen CO2 and H2O diagram is a composite of six photographs, manipulated arrow clipart and text. Many thanks to Harry Ward for his endless patience in throwing molecule models in the air for me to photograph!
 Curkeet, Rick. Wood Combustion Basics: http://www.epa.gov/burnwise/workshop2011/WoodCombustion-Curkeet.pdf
 More than you ever wanted to know about wood combustion: http://www.freepapers.ir/PDF/10.1016-B978-0-12-691240-1.50008-7.pdf?hash=TKG-N3UcGaAsu_WECcUg2w
Clipart arrows used in diagram from http://bit.ly/1GzWqk9