Burn Baby Burn: The Science of a Perfect Yule-time Fire

The holidays are about ambiance. If you can’t surround yourself with the right festive atmosphere, then it will take much more of an effort to get into the spirit of the season. Obviously, decorations and a Christmas tree (or menorah or Festivus pole, depending on your leanings) go a long way to setting the mood; but nothing quite matches a crackling fire for creating warm and cozy feelings in the cockles of your heart.

The problem with fire is that it is difficult to manage and generally a pain in the butt. If you don’t keep it under control, it will burn up your presents and send you to the hospital. If you don’t constantly stock it, your fire will burn itself out and leave you shivering in the cold. That is probably why as human civilization has developed, we’ve outsourced our fire-making to power plants. But even if we are a lazy bunch of spoiled brats, we’ve never lost our love for just being in the presence of flames. Why else would they have an endless loop of a burning log on Netflix during December?

Streaming may be more convenient, but starting your own fire brings a sense of accomplishment and the effect is much more authentic. With that in mind, let’s dive into what makes a good fire.

First, most of us know that fire needs three elements: heat, fuel, and oxygen.

The interesting thing about fire is that it is not a thing by itself; rather, it’s more of an event. It is the outcome of taking a piece of matter containing compounds that react with oxygen and then heating them up enough so that they can combust. These compounds generally include hydrogen and carbon. Flames themselves are just super-heated carbon atoms that give off light as they rise away from the base of the blaze.

For our purposes, a fire starts when you heat up a piece of wood to a temperature where cellulose (the stuff wood is made of) begins to break down. As it does so, it releases gases containing reactive compounds. As heated molecules in cellulose are exposed to the oxygen in the surrounding air, they break apart and create new bonds, generally producing water and carbon dioxide. As atoms recombine, they rise and give off heat. The reason flames taper off into a point is that as the gases in the fire rise, they gradually spread out and begin to cool down. The tip of a flame is the last point at which they are close enough together for some atoms to still glow (incandesce).

Okay, so now we know what fire is, but how to we get one started? First, you have to pick the kind of wood you want to burn, as not all woods are created equal. Hardwoods like ash, oak, beech, and apple are dense, so they burn hot and last a long time; however, they are tough to heat to ignition temperature and they lack the satisfying crackle that a good Christmas fire needs. The reason they are dense is because the trees from which these woods come grow slowly with few air or water pockets between the wood fibers. By contrast, softwoods like pine, light easier and crackle like a cereal elf, but they burn quickly and the liquid sap they contain can create a flammable build-up in your chimney. The best choice for Christmas, then, is fir. Woods like Douglas fir are moderately dense, they crackle, and they have less sap.

The key to getting your fire going is in the shape of the wood, and the key factor in shape is surface area. To heat up enough cellulose to start the self-perpetuating chemical reaction we’re going for, we’ll want to start with pieces of wood with a larger surface area. This means that as much of their mass as possible is exposed to the oxygen with which we are trying to get it to react. To help you visualize what I’m talking about, imagine a piece of paper so large that it weighs 10 lbs (4.5 kg). It has a large surface area and will burn much faster and light much easier than a block of wood that weighs 10 lbs.

Once you get your fire going and it is generating enough heat on its own, you can throw in pieces with decreasing surface areas until eventually you can burn a big yule log. A big, slow burning piece of dense wood will keep you warm and entertained for hours as you drink your eggnog and revel in holiday cheer.

Now, get to roasting those chestnuts.