Have you ever wondered “why does fire pop and crackle?” What is it within the combustion process that causes the fire to pop or make an incessant crackling noise? Sure, it’s comforting, especially after a long day on the trail, but it’s still an interesting question, worth pondering as you relax in the warm glow.

The answer is pretty scientific. It boils down to moisture, combustion, oxygen, heat, and wood. Even the driest wood retains a bit of moisture and as the heat expands through the wood fibers, the moisture and air (held tight in small pockets) expand until it bursts, releasing the air and moisture with a loud pop.

The more your fire crackles and pops, the higher the level of combustion inefficiency. A combustion process that is ongoing but slow and lazy allows for the creation of more pockets of air and moisture. As the fire continues to slowly spread through the cellulose fibers of the wood, it bursts open more of these pockets.

How does fire burn wood?

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Understanding why a fire pops means understanding why a fire burns or, at least why it burns wood. It may seem pretty simple, after all, you light your kindling, and the fire burns. What could be so complicated about that?

The fact is, it’s a lot more complicated than you’d think and the first time you get slapped in the face with hot coal from a spitting fire might make you rethink your gloss-over analyses of combustion.

Here’s how the entire process works:

  • Fire requires three components: heat, oxygen, and fuel
  • Starting the fire creates heat
  • Oxygen fuels the fire, increasing the level of heat
  • The heat “decomposes” the wood at a cellular level
  • The decomposing wood ignites as the fire continues to consume the fuel
  • For ignition, the wood must reach 500ºF to truly burn
We've also written an article covering how hot a campfire gets if you're interested in the temperatures.

That’s the gist of it, although it can certainly be broken down further if we chose to do a full-blown, scientific breakdown of the process of combustion.

What’s important to know, however, is that the process occurs in this way and the popping that comes from releasing oxygen and steam, is caused by it.

Water’s reaction to fire

Water doesn’t burn and it’s impossible to ignite it. The only thing that water can do is steam and that’s why you will often hear hissing sounds after the initial pop.

Oftentimes when the fire sufficiently degrades the perimeter of a pocket of air and moisture, the air explodes out while the steam is a little slower. instead of all coming out at once, it will hiss like a train whistle.

This is especially true if actual water is boiling inside the pocket. It’s more common to see this when you’re burning “green” wood or wood that has recently been cut down and still retains a vast amount of moisture.

Trees consume water through their roots, after all, and that water spreads up the trunk, into the limbs, and the leaves. When you cut down a tree, all of that moisture is trapped inside. “Seasoned” wood is firewood that was stacked away and allowed to shed its moisture over a year or two.

Safety issues with a popping fire

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There is no such thing as a piece of firewood that doesn’t have moisture inside of it, even when it is considered to be seasoned wood. So, no matter what you do, there is going to be some degree of crackling and popping going on when you have a campfire going.

These are the things you should consider:

How you build your fire, in terms of the firewood that you’re burning and how you lay it out, is as important as where it’s located.

Get the right type of firewood

You don’t want moisture as it reduces the fire’s efficiency and creates a lot of popping that isn’t always safe, especially if the fire is located close to the woods, dry grass, or your camping equipment.

If you can haul firewood for your camping trip, you want to get well-seasoned wood that is dry and will reduce the amount of popping. There are several ways to check this as most people don’t walk around with a moisture meter in their back pocket.

  • Look for cracks in the ends of the wood as dry wood will crack
  • Knock a few pieces together and if you get a hollow sound, you’re good
  • Dry wood always weighs less than greenwood

If you season your own wood, the best way to store it is to keep it outside, covered, and elevated from the ground. Let it dry throughout an entire summer and only cut it late in the winter season or during the spring time.

Depending on where you live, you won’t need to purchase any kind of fire starting logs or equipment because of what is called lighter wood. Lighter wood is primarily found in the south and comes from the heartwood of a pine tree.

After a pine tree falls over in a storm, the heart of the tree hardens and dries on the outside, harboring the majority of the highly flammable sap within. You can shave pieces of lighter wood off for kindling and it is capable of getting a roaring fire going in minutes, even when it’s wet.

You don’t want softwoods and heavy sap but the lone exception is lighter wood. Besides, you’re not using lighter wood as the heart and soul of your fire. If you do, you won’t stand anywhere near the fire all night and you’ll burn down half the trees around you.

There’s little doubt about the efficacy of lighter wood and if you want to enjoy the spectacular display by throwing a large knot of lighter wood in the fire, be sure that it’s an open fire and well away from trees.

Sparks can travel a good distance

This is why you want seasoned wood only. The less the fire pops, the lower the odds of a stray piece of screaming hot coal flying past your face and landing in a pile of pine straw, which would really be the worst luck but it happens.

You minimize the sparks by sticking with seasoned hardwoods, such as oak. Softwoods are your pines or trees with a lot of moisture and you should stick with pine only when it comes to lighter wood for kindling.

Hardwoods burn long and slow but hot and they create an excellent bed of long-term coals that you can shore up and use in the morning to cook breakfast before you break camp.

Choose a good location for your campfire

Straight dirt and/or gravel are the best foundations for a fire. The objective is to stay away from any moisture that could potentially make its way up into the wood and your fire.

It sounds counterintuitive as most people would think that a damp spot would be best, however, picking a spot like that is practically begging for some extra popping in your campfire.

If you have large rocks laying around, don’t be afraid to scoop them up and surround your fire with them. Rocks don’t burn unless, of course, you’re talking about a volcano and you’re not building a volcano, just a fire.

Build yourself a nice pyramid/teepee shape because it accomplishes two things. First, it allows more oxygen to penetrate, which means more heat, more fire, and more combustion efficiency. Second, it keeps your wood off of the ground and away from any moisture lingering in the dirt.

Keep your tent and gear away from the campfire

Nothing will ruin a camping trip quicker than a hot coal that lands in your tent while you’re rooting around in your pack, blissfully unaware. It may seem like a good idea to place your tent close to the fire, for warmth and all, but not really.

Besides, if you’re building a fire to stay warm, your tent is going to sit off in the distance while you stay close to the fire, wrapped up in your mummy bag.

You want to keep anything that’s even remotely flammable well away from the fire, especially if you’re forced to use green wood or wood that is somewhere along the halfway point from green to seasoned.

Propane or butane devices and cannisters are a big no-no as well, because you just don’t want to tempt fate. Fate doesn’t care if your propane tank goes off next to your food and water supplies.

Final thoughts

Air and moisture make your fires go pop and its often fun to watch and enjoy, so long as the popping isn’t turning your nice coals into flying shrapnel. If it gets too bad, tone it down on the green wood you’re feeding into the fire.

Whether you’re camping out or heating up the fireplace at home, always stick with what is safe by burning hardwood for some good solid coals and using the sap-heavy lighter wood to get the party started.