Layman’s Definition Of Wood Charring
Wood Charring is a process where wood is heated but is deprived of oxygen. The heat will char the surface of the wood while not producing a flame on its surface.
Gasses are still released from the wood due to this process. They are flammable and while they must be vented it is necessary to prevent them from catching fire.
Layman’s Definition Of Wood Burning
Wood is set on fire in order to burn. For art or decorative structural purposes, we desire only the surface of the wood to be transformed.
Methods have been created for this process such as traditional Shou Sugi Ban burning. Western processes have become quite simple such as using propane torches to achieve this outcome.
Is Wood Charring Or Wood Burning Better For Maintaining Wood Integrity?
Note that I will introduce a hybrid method later in this article that is focused on safety and still achieving wood strength. I caution you that wood charring can be a dangerous process, there is a safer way.
It is common among wood species that the average temperature for wood to ignite and burn is 572 degrees Fahrenheit or 300 degrees Celsius. Despite this, when wood is heated slowly changes in composition take place.
There are three polymeric components begin to decompose in the wood as it is progressively heated. These components do so at three key stages of temperature.
Hemicellulose begins to decompose at 356 degrees Fahrenheit or 180 degrees Celsius. Cellulose begins to decompose at 527 degrees Fahrenheit or 275 degrees Celsius.
Lignin can begin to decompose at 482 degrees Fahrenheit or 250 Celsius, but is dependent upon the wood species. Upper ranges for decomposition can require temperatures as high as 932 degrees Fahrenheit or 500 degrees Celsius.
Why is this information important to know? In order to maintain the greatest degree of wood integrity it is best to control the environment of how the wood is impacted.
In essence, the goal is to raise surface temperatures to a point where it is charred while keeping internal wood temperatures as low as possible. In return the internal structural integrity of the wood is allowed to be better preserved.
Wood burning on the other hand will create the artistic surface we wish for our decor or art without giving thought to internal temperatures. As a result the wood integrity can be compromised in ways we do not intend.
Wood Charring Can Increase The Heat Resistance Of Wood
Another bonus to wood char over wood burning in art is due to what takes place at the surface of the wood. The charring process itself allows for a unique formation.
As the wood is heated gas forms are produced by pyrolysis. Initially the first byproduct escapes as mostly hydrogen and oxygen. These are produced by the lignin of the wood.
The second process is a non volatile and forms enriched carbon. It is carbon to carbon bonds that are formed in graphitic carbon structures.
As this process continues the carbon will form layers. The result is these carbon layers begin to protect the deeper layer of wood preventing decomposition from taking place as easily.
Char in of itself does not lend any strength to the wood structure. Rather it acts as a thermal barrier increasing the woods heat resistance.
In more ancient times it was common for indigenous peoples to “fire harden” spear tips. They discovered that heat can cause the Lignin to harden and become resistant to damage.
Where you resource your wood from can be important. The purpose of its use must be accounted for. The method must match these two components.
Wood Burning Fails To Protect Wood Integrity
The reason why wood burning fails to protect the integrity of wood is simple. There is no temperature control to the process.
Surface temperatures can reach well over 800 degrees Fahrenheit allowing the internal temperatures to raise too high. When this takes place the internal transformation of the wood decomposes to a point where it is weakened.
Time Is Important To The Wood Charring Process
While it is generally recognized that the ignition temperature of wood is 572 Fahrenheit, much depends upon the state and species of the wood. How this plays a role in timing the wood charring process is important.
The US Department of Agriculture compiled research showing that the level of moisture in a certain species, in conjunction to duration of time wood is exposed to lower temperatures, can still create a flash point in wood.
An example of this can be seen in Red Oak. It was exposed to a temperature of 356 degrees Fahrenheit for 20 minutes. It then reached a flashpoint and caught fire.
How is this possible when the known temperature needed is 572 Fahrenheit? The answer is simple.
Hemicellulose begins to decompose at that 356 Fahrenheit temperature. It took 20 minutes at this temperature for the surface decomposition to reach a point where it was creating its own heat in the transformation.
How Do You Begin Wood Charring?
For our purposes in wood art the process will be slower than wood burning. There are two ways to achieve this process. The right way, and the poor mans way.
Wood charring requires that you create an enclosure that will be self-contained but gasses can vent off, no oxygen can be introduced to the environment. If oxygen is introduced the wood will flame.
Note that this can become difficult to achieve and also potentially dangerous. Thankfully there is an alternative way that is safer but also the poor mans way to char wood.
The difficulty for wood charring can be the volume of wood needed to be charred. An oven can be created to house the wood you wish to char.
Heat can be increased incrementally. Lower heat below 350 degrees Fahrenheit will aid in removing moisture. Incrementally raise heat to the three key stages of wood transformation.
As mentioned above these are 356 Fahrenheit, 527 Fahrenheit, and if needed depending on wood species go higher than 527. Note some species will ignite faster at lower temperatures due to this process.
How Much Time Is Needed Between Raising Temperatures?
In the poor mans method the amount of time needed between steps in raising temperatures can vary. Volume of wood and species will impact this need. Yet allow me to review general rules of thumb.
Most hardwoods that are dry will need about 20 to 30 minutes warming to remove moisture below 356 Fahrenheit.
After 356 Fahrenheit is reached you will need to monitor the process. Another 20 to 30 minutes will be needed to allow Hemicellulose to begin breaking down.
As you raise the temperature from this point you are looking for the surface to catch fire. Indeed this is falling short of charring, but why do I recommend this?
We are mimicking the charring process by using heat and the chemical transformation within the wood. It is best to go slowly allowing this process to work before reaching the flashpoint.
This allows the wood to undergo as many chemical composition changes as possible before it flames.
Once the wood flames temperature will begin to get out of control. Allow the surface to be charred by the flame and put the flames out as soon as possible.
Allow the wood to cool and you will have achieved the closest thing to wood charring as possible without specialized equipment.
Protecting Wood Integrity Is About Chemical Process And Temperature Control
Wood burning by way of propane torches holds nothing back in terms of temperature. The temps can run so high that it causes irreversible damage to the wood integrity and it can only be used for decoration.
To preserve wood strength we must allow surface chemical processes to begin. This is achieved by keeping the surface temperature at levels where it can take place.
The goal is to keep the deeper temperature of the wood cooler than the required process for Hemicellulose breakdown to occur. This is why once the surface flames the clock is ticking.
As surface temperatures rise so will internal temperatures. You wish to achieve the desired burning effect while protecting wood integrity.
Realistic Expectations In Wood Charring Or Burning
If you use this slow method or poor mans way of wood charring, it will be cost efficient. You will achieve more consistent results by “slow cooking” smaller batches of wood.
This process does lack speed. The trade off is you gain the ability to maintain natural wood strength. Thus the question comes back to what it is that you need in what you are trying to achieve.
In projects that are purely decorative where little to no structure is needed, propane burning would be fine.
Where projects would see weathering or carry any kind of load then the poor mans wood charring is the winner.
Safety Is A Key Concern To Be Constantly Aware Of!
When woodworking we always need to be safety conscious. When introducing fire it is exponentially so. If you create a genuine wood charring process please follow all local laws and permits.
Why? The gasses you create in the process must not only be vented, but when concentrated can explode. This is why I strongly advise the poor mans method.
The poor mans method of incremental temperature increases, essentially slow baking your wood, is the safest method.
Here you gain a little of the best of both worlds while not exposing yourself to unnecessary risks.