Firewood Crackle

When you place a log 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 lot of very small 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 on your stove and will eventually boil and 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 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 very small pockets within them.  When steam build up in one of the 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 or if outside maintaining a minimal clear distance from the fire.

The wetter the firewood the more it will snap, crackle, spit and splutter which is due to the higher volume of water trapped inside the log. A green 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 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.

So, is that crackling fire such a great thing after all?  Probably not for those of us who like to heat our homes efficiently without a lot of smoke and soot or 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 an outdoor fire, than in a fireplace!