Curious Kids: why does wood crackle in a fire?

Why does wood crackle in a fire? – Rocco, age 6 (nearly 7!)

Hi Rocco, that’s a great question. I love sitting in front of a fire, listening to it crackle and pop.

These noises are caused by pockets of trapped steam suddenly escaping, making a mini explosion!

To know why this happens, we need to understand what happens when you place a wooden log on a fire. First, the wood starts getting hotter. Inside the wood are pockets of trapped water and tree sap, which is the sticky stuff you sometimes see on trees.

In the same way water in a kettle heats up and turns into steam, so does the water trapped inside the log. So as the fire gets hotter, the water and sap inside start to boil and turn into gas. As the fire gets even hotter, these gases start to take up more space and expand (get bigger).

How do the gases burst out?

While the water and sap turn into steam, something also happens to the wood. Wood contains something called cellulose, which is the stuff that plants are mostly made out of.

When cellulose is heated, it starts to break down, or “decompose”. If you’ve ever forgotten an apple in your lunchbox over the weekend, and it turns brown and yucky, that means it has decomposed. When something in nature (like a piece of fruit) decomposes, it changes.

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When wood in a fire gets hot enough, the cellulose inside starts to turn into gas. This is when we see smoke coming out of the wood, sometimes even before that piece of wood has burst into flames.

The flames happen when the gas escaping from the wood starts to mix with the oxygen in the air. Oxygen is like food for fires – it makes them burn really bright.

As wood burns, the mix of expanding gases and cellulose breaking down makes the pockets of trapped steam burst open from the wood, one by one. This is why you hear the crackling and popping noises.

So the more water and sap there is inside the wood, the noisier the fire will be. If you’ve ever put damp wood on a fire, you may have noticed it makes a lot more noise than really dry wood.

How does the wood get water inside it?

But how does water and sap get inside wood in the first place?

Well, wood isn’t quite as solid as it looks. It has many tiny holes, too small for our eyes to see, and these holes have water and sap inside them.

We know wood comes from trees. And when trees are alive, they stay healthy by carrying water up their trunk through these tiny holes, which are called xylem vessels. When the tree is chopped down to make firewood, there is still water trapped inside these xylem vessels.

There are other ways water can get inside wood. If firewood is left out in the rain, it can soak up water that way. Or sometimes insects make small holes in the wood, which let water in.

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Sitting in front of a fire watching the flames and listening to the wood crackle and pop can be fun. Most of the time the mini explosions of the steam escaping are small.

But sometimes they can be big, and might even cause small chunks of burning wood to fly out of the fire! This is why it’s important always to keep a safe distance from a fire, or to use a fireguard.

The author thanks her nephews Aldous Nolan (6) and Fergus Nolan (5) for helping to improve this answer.

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