If you work with wood, one of the most important things to keep in mind is how moisture interacts with the wood you are using. Making sure the wood moisture content is at an appropriate level is a key part of effective woodworking.
The ideal level of moisture content in wood meant for woodworking depends on two factors – your geographic location and whether the final product will be stored inside your home or outside. In general, however, acceptable levels of moisture content range from 6-12%.
In order to find the right moisture content for your wood, you will need to take into consideration the humidity of the location where your finished product will be used. To understand why you first have to consider how moisture will affect your wood.
How Moisture Affects Wood
Wood is a hygroscopic material. This means that it can absorb water from and release water into the air. A change in the humidity it the air around it means a change in the moisture levels of the wood.
As wood gains or loses moisture due to humidity levels, it also expands or shrinks as well. When humidity increases, the wood gains water and expands. When the humidity decreases, it loses water and shrinks.
The expansion and shrinking can severely affect the wood. As the wood dries and shrinks, it also cracks, warps, and splits. This can cause end-use problems after the project is complete. Ideally, shrinkage and expansion should be reduced as much as possible.
Your wood will shrink and expand naturally seasonally. However, this can be minimized in order to protect the wood and the final project as much as possible. This is where the moisture content of the wood comes into play.
When your wood reaches a moisture level where it no longer gains or loses moisture, it can be said to have reached its equilibrium moisture content (EMC). It is this that is dependent on the humidity of the location – as humidity changes, so does the EMC.
Once you know the EMC of your location, you can calculate the right moisture content for your wood – this should be within two percentage points of the EMC at the location the final project will ultimately be used. So, if the final location has an EMC of 8%, the moisture content of the wood should be between 6-10%.
Knowing this is especially important if the final product is meant for external use, outside the house. If you plan on using or displaying your finished project within your home, it is easier to calculate the right moisture content.
Most homes today are environmentally controlled with the help of heating and central air conditioning. This means the humidity levels stay relatively stable throughout the year.
Once you have identified the right wood for your project, you will then have to make sure to keep it at that level for the duration of the creation process. This ensures that wood will not warp while being worked and affect the final design and look of the project.
How To Measure Moisture Content In Wood
When it comes to actually measuring the moisture content in your lumber, there are two main ways to do this:
1. Oven Dry Testing
This is the traditional way to test the moisture content of wood. Oven dry testing involves drying a sample of the wood in an oven or a kiln over a period of time.
As the wood is being dried, the weight is checked in set intervals of time. Once the sample’s weight stops changing, the weight is compared to what it was before the start of the drying. The difference in weight is then used to calculate the original moisture content.
There are several drawbacks to this type of testing. It is a time-consuming process – properly done, it can take hours. If rushed, the wood can often burn.
Aside from this, the wood sample that was used for the drying process is often rendered unusable for woodworkers – the exposure to heat and the drying itself can lead the wood to warp and deform. Furthermore, oven-dry testing requires specialized equipment that not everyone has access to, especially if you’re not a professional woodworker.
2. Moisture Meters
This is the fastest way to check the moisture content in wood. There are several different varieties of moisture meters, but they generally fall into two major categories:
- Pin-Type Meters: These use electrodes to measure wood’s moisture content using the concept of electrical resistance. To use, just insert the pins into the material you want to test – the biggest drawback here is that they will leave pinholes on the wood’s surface.
- Pinless Meters: These meters use a specialized scanning plate that functions as a non-damaging electromagnetic sensor. You will need to ensure complete contact between the wood surface and the scanning plate to get an accurate measurement. Since these don’t leave damaging pinholes, they’re the ideal choice to use on more expensive woods.
How To Lower Moisture Content In Wood
Once you’ve measured the moisture content in the wood that you’re planning to use for a project, you can identify if the moisture is too high. If this is the case, you will have to dry your wood before getting started. There are several ways to do this:
- Air Drying:
This is the traditional method to dry your wood. You will need to pick a storage spot for your wood that is spacious, in a space that isn’t overly damp and is protected from rain (if it is being stored in the open). The wood should be stored under shade to prevent it from drying too rapidly, you can also seal the end grain with wax for additional protection.
The rule of thumb for this method is to allow for a year of drying time per inch of thickness of the wood. However, this can be sped up to between a few weeks and a couple of months by air-drying indoors and using a heating system, dehumidifier, or fan to quicken the process.
- Kiln Drying:
If the pieces of wood you are looking to dry are not too large, you can also dry it using a kiln. There are industrial kilns for drying large timbers. This is a much faster process than air-drying, and you are not at the mercy of weather conditions.
There are various types of kilns that you can use to dry your wood, but all operate under the same basic premise – a large, insulated chamber is used to introduce heat to the lumber. Kiln drying allows you to control the temperature and humidity levels.
- Microwave Drying:
Unless you can access an industrial microwave, this type of drying is only for small pieces of wood that can fit into a standard microwave that you have at home.
This can be a very effective process when it comes to drying wood quickly, but you will need to be careful. It can be easy to over-dry the wood, to a point of destroying it completely.
The best way to go about it is to microwave it for short periods and test the moisture content between each round of drying. Using a moisture meter is recommended.
How do you protect your final project from the effects of seasonal wood expansion/shrinkage?
Even at the ideal EMC, there will be some level of seasonal expansion and shrinkage in your wood. There are a few ways to protect your end project from being adversely affected by this, including:
- Finish all surfaces: Finishing will significantly slow down the rate of absorption or release of moisture. Make sure both sides are finished – finishing just one side will lead to greater warping.
- Construct projects keeping in mind inevitable movement: This is especially relevant when building furniture. There are several techniques that allow for seasonal expansion and contraction, protecting the final piece from the effects of seasonal warping.
What happens if the wood is too dry and the moisture content is too low?
Just as too wet wood can be a problem when woodworking, so too can too dry wood. Some issues that can arise include:
- Swelling: Overly dry wood naturally absorbs water from its environment. This can cause comfortable fits to become tight, and may also cause crowning or buckling in some projects.
- Brittleness: Working on overly dry and brittle wood can lead to splits, cracks, and other damage. Carving or turning overly dry wood can also lead to splintering.
- Tool wear: Tool can dull faster when used on overly dry wood.
While there are a variety of ways to ‘wet’ your wood if its too dry, you will need to be careful you don’t drench it. One way to ‘wet’ the wood carefully is to use a sponge and dab water into the wood, using a moisture meter periodically to check moisture levels.
While the right moisture level differs based on location and used, the generally acceptable levels of moisture in wood meant for woodworking is around 8-12%.
Before you start on your project, make sure that the moisture level in the wood is right. This will prevent damage caused by warping, expanding, and shrinking. If needed, dry the wood before use.
Making sure that the moisture content in the wood you are using is correct is key to a successful woodworking project.