Kiln Drying Wood: Essentials to Know

Wood as a material has been used by humans for centuries, and it’s still one of the most popular materials to use in construction, furniture, and even art.

While wood is a wonderful material, it does have some issues that must be addressed before it can be used.

One of the most common problems with wood is its moisture content. A healthy tree can have a moisture content of 70% or even higher.

This means that when the tree is cut down, the wood still has a lot of water in it.

If this wood is used for construction or furniture immediately as is, it will eventually warp and crack as the water inside of it dries out. This is why wood must be dried before it can be used.

There is no skip to drying wood. Even the wood that is used as firewood must be dried out, or it will not burn as efficiently. Thus, drying wood has become an important process, and there are a few different ways to do it.

The most traditional method of drying wood is air drying. This is simply the process of stacking the wood in a dry area and allowing it to sit for several months.

During this time, the water inside of the wood will evaporate into the air, and eventually, the wood will be dry. Air-drying is a very slow process, and it can take up to six months for the wood to be completely dry.

Therefore, the industry has come up with a faster way to dry wood, and that is kiln drying. In this post, we will discuss everything you need to know about kiln-drying firewood.

What is Kiln Drying?

Kiln drying is a process of using heat to remove moisture from the wood. The wood is placed in a chamber where it is exposed to high temperatures.

The hot air in the chamber pulls the moisture out of the wood, and as the water evaporates, it is vented out of the chamber.

Kiln drying is a much faster process than air drying, and it can bring the moisture content of the wood down to the desired moisture content in just a few days.

If the wood is planned to be used for construction, it is typically kiln-dried to a moisture content of around 10% or less. For firewood, the desired moisture content is 20% or lower.

For other certain applications, the wood may be kiln-dried to even lower moisture content.

The process of kiln drying is not only faster than air drying but also more controlled.

With air drying, the amount of moisture that is removed from the wood each day depends on the weather conditions.

If it’s a humid day, not much moisture will be removed from the wood. However, if it’s a sunny day with low humidity, the wood will dry out much faster.

With kiln drying, the amount of moisture that is removed from the wood each day is controlled by the operator. This allows for a more consistent and predictable drying process.

What are the advantages of Kiln Drying?

Kiln drying has a few advantages over air drying. Here are a few of them:

Much Faster Process

Kiln drying is a much faster process than air drying. It can take many months to air-dry a piece of wood, whereas it will only take a few days in a kiln.

This is important if you need the wood for a project and you don’t have months to wait.

More Control (on Temperature & Humidity)

Having more control over the drying process is also an advantage. With air drying, you have to hope that the weather conditions are favorable for drying.

With kiln drying, you can control the temperature and humidity inside of the chamber, so you know that the wood will be dried to the desired moisture content.

Increases Strength

Another advantage of kiln drying is that it increases the strength of the wood. Since the moisture content is lowered more evenly throughout the wood, the fibers find it easier to bond with each other, making the wood stronger overall.

Less Deformation

Another advantage of kiln drying is that it minimizes the chances of wood warping or cracking.

When wood dries too fast, it can cause the wood to warp or crack. This is because the water inside of the wood evaporates faster than the wood can adjust to the change in moisture content.

Kiln drying dries the wood more slowly and evenly, so there is less of a chance of the wood warping or cracking.

Good Machinability

Kiln-dried wood also has good machinability. This means that it is easy to work with using tools. The wood is less likely to split or chip when you are cutting or drilling it, and it will take a nicer finish overall.

Good Gluability

Kiln-dried wood also has good gluability. This means that it is easy to glue together using adhesives.

The wood will bond well with the adhesive, and the joint will be strong. This is important if you are using wood for construction or other applications where the wood needs to be glued together.

More uniform color

Another advantage of kiln drying is that it produces a more uniform color in the wood.

When wood is air-dried, it can sometimes produce blotchy or uneven coloring. Kiln drying produces a more consistent color throughout the wood.

Kills pests & bacteria

Kiln drying also kills any pests or bacteria that may be living in the wood. This is important if the wood is going to be used for construction, as you don’t want to introduce any pests or bacteria into the building.

Any mold or mildew that may be starting to grow on the wood will also be killed in the kiln.

What are the disadvantages of Kiln Drying?

There are a few disadvantages to kiln drying as well. Here are a few of them:

High Initial Cost

The initial cost of setting up a kiln can be quite high. You need to purchase or build a chamber that can withstand high temperatures, and you also need to purchase the equipment to heat the chamber.

If you only need to dry a small amount of wood, it may not be worth the initial investment.

Requires Electricity

Another disadvantage of kiln drying is that it requires electricity. The chamber needs to be heated to high temperatures, so you need a reliable source of electricity.

If you live in an area with high electricity costs, kiln drying can be quite expensive.

Requires Operator Training

The kiln can be as simple as a dehumidifier or a clothes dryer with temperature control, or as complicated as a computer-controlled system.

Most people who are kiln-drying wood using a simple system will have no problem operating it.

However, if you are using a more complicated system, you may need to hire someone who is trained to operate it. This can add to the initial cost of setting up a kiln.

What is the importance of air circulation when kiln drying?

The kiln chamber’s ventilation is critical. Because without removing the water vapor given off by the drying wood, the relative humidity inside the chamber will rise, and not allow the wood to dry any further.

The speed of air over wood impacts the drying rate and ensures evenness in the process. The kilns can achieve air circulation through a forced or natural draft.

If you are using a natural draft, then you need to make sure that the kiln is located in a well-ventilated area. The vents should be open so that the air can circulate freely.

If you are using a forced draft, then you need to make sure that the fan is powerful enough to circulate the air throughout the chamber. You also need to make sure that the air vents are open so that the air can circulate freely.

Fan rotation is a must in a forced-draft kiln to ensure even drying. The on-and-off cycles of the fan need to be staggered so that each piece of wood gets an equal amount of air.

How long does it take to kiln dry wood?

Kiln-drying wood can take as little as a few hours or as long as a few weeks. It all depends on the type of wood, the thickness of the lumber, and the desired moisture content.

Softwoods, such as pine and cedar if not thick, can be kiln-dried in a few hours. Hardwoods, such as oak and maple, especially when thick, can take a few weeks to kiln dry.

The higher the moisture content of the wood relative to the desired moisture content, the longer it will take to dry. Also, the lower the temperature of the kiln, the longer it will take to dry the wood.

A lot of times wood is intentionally dried at a lower temperature to prevent checking and case hardening.

Checking is cracks that develop vertically in the lumber, and case hardening is when the surface of the lumber dries faster than the interior, causing the wood to warp.

Finally, the air circulation in the kiln chamber plays a role in how long it will take to dry the wood. Kilns use forced draft to circulate the air will dry the wood faster than a kiln that uses natural draft.

What is casehardening?

Casehardening is a process that can occur when the wood is dried too quickly. When the water inside of the wood evaporates too fast, it can cause the wood to warp or crack.

This is because the water inside of the wood evaporates faster than the wood can adjust to the change in moisture content.

Casehardening can also cause the wood to become harder and more brittle. This can make the wood more difficult to work with and less durable overall.

How does kiln drying prevent casehardening?

Kiln drying prevents casehardening by drying the wood more slowly and evenly. This allows the wood to adjust to the change in moisture content more slowly, so there is less of a chance of the wood warping or cracking.

However, still, there may be some times to add moisture in the wood during the drying process. Kiln drying allows for the rapid addition of moisture to the chamber, which can help prevent casehardening.

Sometimes, when kiln temperatures are too high for stress relief, the steam can actually make things worse.

It’s worth considering whether or not to cool the steam with water or whether or not to cool the lumber before steaming. It is recommended not to go over a dry-bulb temperature of 82°C (180°F).

What is conditioning wood with a kiln?

Conditioning is the process of rehydrating the surface of the lumber in order to alleviate any stress caused by the drying and shrinking of the outer surface.

The outer surface of the wood will typically dry much faster if correct drying schedules are not employed, and the exterior will shrink more than the interior, and it will be stretched.

A lack of relief from this stress can cause the wood to distort, especially when being worked on if it is not released. At the end of conditioning, the wood is tested for stress to see if it is safe to work with.

Conditioning is done after the wood has been dried in order to make the wood more pliable and less likely to warp or crack. To condition wood in a kiln, the wood is placed in the kiln and the temperature is slowly increased.

During this time, the humidity also increased. This allows the wood to slowly absorb moisture back into itself.

Once the desired moisture content has been reached, the kiln is turned off and the wood is allowed to cool. This process can take anywhere from a few hours to a few days, depending on the size and type of wood.

Air-dried lumber is less stressed towards the conclusion of drying due to weather changes. Some hardwoods can be damaged by air-drying because of the lack of control over drying rates.

If you don’t utilize the wood right away when it comes out of the kiln, it will naturally condition over time. The end use of the lumber may not necessitate stress reduction every time.