What Causes Firewood To Pop?

Last Updated on April 20, 2022 by Grow with Bovees

Understanding Why Firewood Pops And Crackles?

What is the reason behind the sound of a fire crackling and popping?

Not only is it comfortable and soothing to listen to, but it is a good sign that your fire is properly ignited.

There are several contributing factors to the cracking sound heard when making a fire. Usually it is attributed to gas, moisture content or sap reacting violently to the heating up of the firewood. As it heats up, the substance inside the firewood is heated up to the point where it begins to boil.

As it begins to boil more vigorously, some substance will boil out of the wood, and it is forced out through tiny cracks in the bark of the wood.

These little explosions are mainly behind the noise we hear when our fire is popping and crackling. In this article we will endeavor to answer some questions about firewood crackle, and why fires pop.

Understanding the Combustion Process

As wood is burned, combustion occurs. This chemical reaction is what turns it into heat and carbon dioxide. More technically speaking, the cellulose and oxygen within the wood react with one another and produce heat, this is combustion.

When the combustion reaction is not consistent or unsteady and only little matter is converted, an excess of particle matter is released into the air.

Usually this can be seen as an increased amount of smoke or tiny embers exploding, making the popping sound one always hears. Inefficient combustion of incomplete combustion are both contributors to the sound of firewood popping.

How Good is my Firewood?

The ultimate expectation we have from good seasoned firewood is that it will create good heat. Depending on how well your firewood is able to produce heat and warm up your home one would be able to determine their value.

It also depends on how you are burning your firewood. For instance, if you have a fireplace that does not have decent air circulation, it does not make sense to use expensive wood types over cheaper ones as all the energy and heat produced escapes through the chimney anyway.

Nevertheless, if you decide to go for the more expensive variations of firewood, such as maple or ash instead of the cheap cuts such as cedar or pine, you might be left with more smoldering coals instead of a heap of ash that does not generate any additional heat.

If you want to use your firewood for cooking or barbecuing, then it’s better to choose a type of wood that generates good strong coals after being burned.

Hardwood or Softwood Wood?

If you are looking for seasoned firewood that you can use effectively indoors for a fireplace or a sauna, or even a wood stove, hardwood is definitely the better option.

Hardwoods are able to produce energy that is a higher yield than that of softwoods. This is due to a higher density and lower moisture content.

Hardwood burns at a much slower pace and releases smaller amounts of creosote as well as smoke. Additionally, if you use it regularly, it is heaps more economical.

In terms of which of the two woods is cheaper, hardwoods are considerably more expensive. The main reason for this is that hardwood trees, on average, take about three times longer to mature than softwood trees.

Also, hardwoods, pound for pound against softwoods, produce energy with a yield doubling that of softwoods.  

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One disadvantage that you have with hardwoods, is that they take quite a while to naturally dry, around one to two years. Because of this, kiln drying has become quite popular as a method to produce dried firewood.

Softwoods are equally useful as hardwoods when applied in the correct scenario. They season a lot faster than hardwood, making them perfect for kindling.

Softer woods like cedar and pine are excellent sources for this purpose. It lights up fairly quickly and smells great.

Other than kindling, softwoods are the perfect firewood to use for traditional open fireplaces and fresh air campfires.

Resinous in nature, they ignite very quickly and give off much larger flames and much louder crackles and pops. This is typically also better wood for heating a home.

A good blend of hardwood and softwood would be perfect for any situation. The softwood will help to get the fire started and the hardwood will ensure an efficient combustion.

What makes firewood wood pop?

1. Moisture

Moisture content that is trapped within the wood can cause a fire to pop.

Although your piece of wood may look and feel quite dry, it definitely has some moisture locked inside of it.

All trees need to take in water to survive. Hence, there will mostly be some moisture in the wood.

If your firewood is too wet and contains a high moisture content, it will restrict combustion and this will subsequently cause the wood to crackle and pop as you can hear the steam escaping.

Apart from the combustion process being inhibited, damp wood could also pop and crackle as the result of a build up of steam. Wood and water react very differently in this regard.

Where wood starts to convert into heat, water will begin evaporating and convert into steam. When the unseasoned wood is burning, the moisture content inside the wood will convert to steam, and the more heat is added, the higher the pressure of the steam trapped in tiny pockets within the wood.

When the steam becomes too much, the air pockets will rupture and burst, making the wood crackle. It’s always best to use firewood where the moisture content is between 10% and 20%.  

2. Sap

Popping and cracking noises can also be explained by a high sap content within the burning wood.

Similar to what happens to moist firewood, the sap is forced to boil when the wood is burning. As the sap starts boiling more violently, it will be forced out of tiny cracks within the bark of the wood – hence the popping and crackling sound.

Why Do Some Woods Pop More Than Others?

Some firewood pops with a higher frequency than others, there may be several reasons for this:

1. The Cut or Type of Wood

The cut and species of wood has a lot to add when it comes to the popping and crackling of the fire.

Softwood logs usually have more sap inside them and a higher moisture content in comparison to hardwoods.

The more sap is contained within a softwood, the more popping and crackling sounds will be heard. Kiln dried firewood is probably one of the best alternatives.

So if you want to hear more crackles and pops, look to using more soft woods. To minimize popping, hardwood logs are preferable.

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2. Moisture From Knots

Some woods retain more moisture better than others, subsequently, if the wood has more trapped moisture content in it, it might crackle more.

When your firewood log has any knots of cracks, this is an opening in the wood which can cause the firewood to take on more water or trapped moisture. Here, seasoned firewood will always pop less than when burning green wood, since unseasoned wood has a higher moisture content.

3. Size of the logs

Smaller, thinner pieces of wood tend to pop more often, as it doesn’t take as much pressure for the steam or sap to pop out through the bark.

Using bigger and thicker logs will take care of this problem.

4. Age of Your Firewood

If the wood has been stored outside for longer periods of time, it may have absorbed a lot of moisture in that time. This will make the firewood pop more.

Dry wood does burn better than wet or rotten wood because the wood heats up faster. The age of firewood used, especially with unseasoned firewood, can contribute to the cracking of firewood.

Getting More Or Less Crackle When Burning Wood?

Wood Making More Pops

Softwood logs such as cedar, Douglas Fir, spruce and pine, which are fairly dry and read under 20% on the moisture meter, will be the choice wood if you want your wood to crackle a lot.

This wood burns quite quickly though, and you will be required to continuously feed the fire.  

Seasoned firewood that is quite dry and has a high sap content will be popping and crackling nicely.

Wood Making Less Pops

Hardwood varieties tend to do less popping and cracking.

The main reason for this is that hardwoods tend to have a lower moisture content and also do not absorb moisture as easily once already cut down. Hardwoods like ash, oak and birch also tend to have a lower sap content which is advisable if you want to minimize popping.

Solutions to Popping and Crackling Firewood

1. Kiln Dried Firewood

Using the process of  kiln drying to dry firewood is a good way of minimizing the popping and crackling sounds made by fire.

As said, moist or wet firewood is placed into a heated kiln for drying.

Kiln drying heats up the firewood and this exposure to higher temperatures and heat aids in the release of trapped moisture and gas. Kiln-dried wood is of exceptional quality.

Probably some of the best seasoned firewood available. Air-dried firewood or kiln dried firewood can be just as efficient a choice of firewood.

2. Keep Your Firewood Dry

Regardless of what firewood you prefer to use for making a fire, less moisture content will always mean less popping and crackling sound, so it’s always a good idea to store your firewood in a dry, safe area.

If you need to store your firewood outside, and you don’t have a garden shed to keep your wood, it’s always good to use a tarp under your log pile and as a cover to protect from the rain.

If your wood is stored for longer periods of time outside, ambient humidity will eventually penetrate the surrounding wood over time, which will again lead to making your seasoned firewood pop.

It’s always a better idea to store your firewood indoors where possible, in the garage or close to the living room fireplace.

Storing it close to a wood stove will also ensure that it stays dry. A simple firewood rack will provide easy storage. Dryer woods are always better.  

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Do not store your wood in any areas that are humid or damp, such as crawl spaces or basements.

3. Build Bigger Fires

Building a bigger fire may be a great way of reducing the pop and crackle a fire makes.

In general, the bigger the fire, the less popping sounds. Using only a couple of small logs won’t produce much heat and thus slow down and restrict the combustion reaction.

The logs will start smoking excessively and this might cause further popping. Using fewer logs when building a fire, will definitely produce less heat.

Using bigger, thicker and heavier pieces of kiln dried firewood in our fire pit is a great way of getting the combustion process more intensive and strong, reducing the firewood crackle. Avoid only using small log pieces on your fires.

4. Dampers on Wood-burning Stoves

Traditional fireplaces or wood-burning stoves have a damper latch that controls airflow through the firebox. Every fireplace will have a damper, typically located just above the firebox.

The damper is responsible for securing a steady flow of fresh oxygen to burn effectively. Should the damper be closed, the fire might suffocate slowly or combustion might be slowed.

Since fires need a lot of oxygen, a closed damper will cause the temperature to drop and subsequently more popping sounds to occur.

Double check that the damper on your fireplace is opened all the way. Kiln dried firewood is also always best to use in fireplaces.

Woods to Avoid

Firewood that has been salvaged or wood scraps picked up at construction sites or at the local wood workshop can be a cost effective way of saving you a ton of money when trying to heat your home.

However, there are certain types of firewood and firewood products that should be avoided for your own health and the safety of the surrounding area.

There are plenty of alternatives to firewood that contain harmful chemicals, which could produce toxic fumes that could be poisonous, as well as chimney emissions that could be a hazard to the environment. Some might also potentially damage your metal stove or cause a flammable build up of creosote in your chimney.

This is a list of woods that you should avoid burning to keep yourself and your surroundings safe:

  • Painted wood
  • Varnished or treated wood
  • Pressure-treated particle board / hardboard
  • MDF (Medium density fiber) / plywood (engineered wooden sheets)
  • Driftwood
  • Wood with an extremely high moisture content / Green Wood

Wood Burning Safety Tips

The best deterrent for any danger is being mindful.

Taking care when feeding a wood stove or building an open fire is the best way to make sure everyone stays safe. Never use more firewood than is needed in the current situation.

If you are making a campfire be sure to clear the area of any flammable materials, such as dry leaves or grass, as to avoid a bush fire.

When operating any wood-burning appliances, such as fireplaces, fire pits or wood stoves, take care to comply with all the recommended protective clearances and ventilation guidelines.

When burning wood in the home, having an active fire alarm (carbon monoxide alarm) in the area of the fireplace is advisable, along with having several smoke alarms throughout your home. You will then be able to enjoy the firewood pop in safety.

References;

https://extension.psu.edu/what-is-the-difference-between-hardwood-and-softwood

https://extension.missouri.edu/publications/g1731