Almost all woods absorb moisture like a sponge. Of course, woods like teak and white oak are water resistant, and will not warp, twist or crack. This is why they are good choices for outdoor furniture, and in the case of white oak, a good choice for boatbuilding.
For the average woodworker, though, the sponge-like quality of woods is well known. When wood absorbs moisture, it expands. As it dries and the water is released, wood loses weight and shrinks. If you build something with wood that is not sufficiently dry, that shrinkage can ruin your piece, making assembled parts warp or joints fail.
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Air drying of wood takes time, lumber stacked and separated so that all sides of lumber are exposed. As a general rule of thumb, it requires 1 year for each inch of lumber thickness for wood to dry sufficiently to be worked. As we know, time is money, and that’s a big investment.
Where green wood is to be processed into usable board, a kiln is usually used to facilitate and quicken that drying time under controlled conditions. Green lumber refers to recently cut trees that have not had time to season, a term that has to do with the drying of its internal moisture. Green wood will be considered as having 100% moisture content as compared with air-dried, kiln-dried, or “seasoned” wood.
There are many types of kilns used for seasoning (drying) wood. They are large, insulated rooms with a controlled environment (humidity, temperature, and airflow) that hastens along the drying process. The theory of kiln drying is simple: the increased temperature and airflow, under controlled conditions that regulate the room’s humidity, evens out the drying process. By evening out the process, we mean as between the outer shell of the wood and its interior.
The outer shell, of course, will tend to dry more quickly than the internal core if a less controlled process is employed. Moisture pockets can be avoided, and a more even drying is the result.
Poor kiln management, though, like an improper schedule of controlled conditions, can lead to uneven drying. The outer shell can dry too quickly and faster than the core of the wood. As the core then dries, the push-pull between core and shell can cause the core to split and create a condition known as honeycombing. This is irreversible and (no pun intended) cannot be cured.
How Dry Does Wood Need To Be?
For the average home woodworker, it’s likely safe to assume the wood you picked up at the lumber yard is dry enough for immediate use in your shop. It’s been kiln-dried under the right conditions and safe for your project.
This means the wood moisture content is probably in the safe range of 6% to 8% and would be suitable for almost any type of project, including cabinet making, furniture, toys for your kids, and even restoration work on your boat. Moisture content above that range, though, would mean you’d want to wait a bit longer and give the lumber time for more drying.
Stacking the lumber in single layers separated by scrap straps so that all sides are exposed to the air, and a dehumidifier running in your shop might be in order. After a time of drying, it will be good to go for your project.
How To Know If Your Wood Is Dry
But, you ask, how will I know if I’ve given it enough time to reach that safe range of dryness? Good question. Or, maybe your project is particularly important to you, a kitchen cabinet job, or a fine piece of furniture, or something with extra-fancy joinery, and you don’t want to take any chances.
Fortunately, there are devices you can use to measure the moisture content of your wood. These moisture meters, as they are called, come in one of two protocols: pin-type and pinless. Let’s take a look at each.
- Pin-type moisture meters/sensors. These meters use electrodes inserted in wood and use electrical resistance to measure moisture content. Use a piece of wood from your stock, and insert the electrodes for several readings.
As we remember from our high school science classes, water conducts electricity; wood does not. Water offers no resistance to the electrical current from the meter, while dry wood does.
The greater the resistance, the greater the water content. The readings will tell you whether your wood falls into the dry range of 6% to 8%. Deeper penetration of the wood will give you an accurate wetness reading well into the wood and help identify the presence of any moisture pockets.
- Pinless moisture meters/sensors. Pinless meters do not penetrate the wood; rather, they have a sensor pad that reads the wood by contact with its surface. They typically have the ability to read moisture content to a depth of about ¼ – ½ inch. They offer the ability to take sample readings on large areas more quickly, though, as you do not have to stick electrodes into the wood carefully.
Which Is Better – Pin-type or Pinless Moisture Meters?
As you might guess, the pin-type meters have the advantage of determining the depth of moisture pockets, helpful information to have when deciding if further drying is needed and for how long. As noted, pinless do have the advantage of taking readings on larger areas of the wood more quickly. Each offers something of value in helping you make an informed decision, but if the wood can be poked a bit without interfering with your project, a pin-type can offer more detailed and specific information.
Are Moisture Meters Accurate?
When it comes to moisture meters, you get what you pay for.
The more expensive meters will tend to be of better measuring quality and accuracy. A top brand moisture meter can be accurate to within 0.01% of your wood’s wetness. Cheaper models will not come close to that accuracy.
If your project is one that has no tolerance for minor defects, spend up for your moisture meter. Fine cabinetry and furniture projects certainly fall into this category and demand that degree of accuracy.
Be honest: how many of you consider the moisture content of the wood you’ve just brought home from the lumber yard? True, you are likely safe in assuming it’s dry enough for your project. But if your project calls for great care in its finery finish, consider testing and add a pin-type moisture meter to your list of must-have woodshop tools.