Why Does Firewood Crackle?

We can’t deny it, there’s something rather alluring about the sight and sound of a crackling fire.  It just exudes a sense of warmth and cosiness. Never one to miss a trick, Netflix has recently cashed in on this phenomenon offering its paying viewers hours of footage featuring cracking log fires; presumably an innovative life hack for those yearning for the hygge lifestyle but without actually investing in a chimney or fireplace.  But what exactly causes a fire to make this popping sound, and is it really desirable?  The answer doesn’t actually lie with the fire itself. The culprit is in fact the burning wood, and so in this article, we’ll take a closer look at how that happens.

When a log is placed onto a fire it begins to heat and to burn, but it is not as solid as you might believe. Inside the wood there is a labyrinth of microscopic pockets and pores which contain fluids such as water and sap.  As the wood burns these fluids heat up in the same way as they would if they were heated in a kettle on your stove and will eventually boil and then vaporise to create steam. This steam finds itself trapped within the tiny gaps in the structure of the log and will exert an increasing amount of pressure on the surrounding wood. Eventually, the wood gives way and will split to release this gaseous steam into the fire. The sound you hear is the sound of what is actually a mini-explosion that results in the wood rupturing.

Typically, logs have a somewhat less than uniform structure.  As well as having many microscopic pockets within, there are likely to be a series of knots and voids.  When combustion gases build up in one of these larger areas, they can generate sufficient pressure to cause a slightly greater explosion that can be strong enough emit a loud crack as it shoots a piece of burning wood debris some distance from your fire.  This is why it is always very sensible to protect an open fire with a screen or to ensure that your stove’s door is closed.

You won’t be surprised to learn that the wetter the firewood the more it will snap, crackle, spit and splutter which is down to the higher volume of water trapped inside the log. A wet log will have a moisture content in excess of 50%, which as well as causing the fire to splutter, hiss and crackle will also reduce the temperature of the fire in exactly the same way as if you had poured water onto it! If the temperature of the fire isn’t high enough to ignite the gases that exist, they will instead dissipate alongside any unburnt wood particles in the form of smoke and soot. So, although you may hear lots of lovely crackling, this type of wet firewood will fail to generate a decent volume of heat. It will instead generate a lot of smoke and leave unwanted sooty deposits in your chimney or stove.

Conversely, a high-quality kiln dried log which typically contains less than 15% moisture will crackle much less, if at all. It will however generate a far greater heat output and emit much, much less smoke and soot.

So, is that crackling fire such a great thing after all?  Well, probably not for those of us who like to heat our homes efficiently without enduring copious amounts of smoke and soot or indeed risking a house fire as shards of burning wood land on the carpet.  In short, while the snap, crackle and pop of a burning fire might sound appealing it is probably better confined to a Netflix video than a real fireplace!