Woodsense: Acclimating Wood

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This article is from Issue 27 of Woodcraft Magazine.

It’s happened to all of us. We build a beautiful woodworking project, pouring hours of planning, designing, and hard work into the construction, not to mention the investment in expensive lumber. Then it goes horribly wrong. Cracks develop in the ends of parts. Or a drawer warps and sticks. In most cases you can trace the cause to the changing moisture content levels in the wood, which result in shrinkage or swelling. It could be you rushed the stock into the machining and project-assembly processes without giving it time to acclimate to your shop and home. Or maybe there was just one board with high moisture content. To keep your wood (and you) from seeing the shrink, see how to manage moisture in project stock, and then follow our strategies for proper stock storage.

Managing Moisture

So you just purchased some lumber. Do you know the actual moisture content of each board? Do you know what it should be? Is it ready to use? A moisture meter provides a quick indication of lumber moisture content. Check every board. It takes only one board with a high moisture content to cause problems in your project. Do you really want to take that chance?

In short, moisture content (MC) means the amount of water in wood defined by the weight of the water expressed as a percentage of the oven-dry (all water removed) weight of the wood. More specifically: MC = (Weight of the water in the wood/Weight of the oven dry wood) × 100.

In other words, if a piece of oak measured at 10% MC, the math would look like this: (1 lb. water/10 lbs. oven-dry wood) × 100 = 10% MC.

Typically, a freshly cut tree will have moisture content in the range from 30% to over 100%. If you cut this tree into rough-sawn stock and air- or kiln-dry it below 30% MC, the wood begins to shrink. Wood will continue to lose or gain moisture (and shrink or swell) until it reaches an Equilibrium Moisture Content (EMC) with the surrounding air. At EMC wood experiences no net gain or loss of moisture, (and therefore no change in weight or MC) when surrounded by air of a constant relative humidity (RH) and temperature.

Most hardwood lumber for furniture, cabinets, and millwork in the US is kiln-dried to 6-8% MC. Kiln-dried softwood lumber is available for a wider variety of products and is dried to a wider range of target MCs of 12, 15, or 19%, depending on the intended end use. Most kiln-dried softwood lumber will require some additional conditioning for interior-use projects. Be careful with exotic woods, especially from tropical countries. They, too, may be at a higher MC than domestic hardwoods.

To acclimate wood for projects, you need to get it to the final EMC of the room where it will reside as furniture, and within constant RH and temperature. For most of the U.S., the target EMC range for wood to be used in the home is from 6-10%, based on an average indoor temperature of 70º F, and average RH in the 30-50% range. For the arid West and Southwest the range is from 4%-9%, while for more humid coastal areas the range is 8-13%.

To determine the EMC in your shop or home, keep wood scrap in either location for several months, and then measure its MC with a moisture meter. Compare those readings to the average MC of your project stock. When the average readings approximate a match, the stock is conditioned and ready for machining, joining, and finishing. If you need to make the shop environment match the home environment, you may need to invest in an inexpensive dehumidifier, humidifier, or an air conditioning or heating system.

Also pick up a hygrometer and thermometer. These instruments help you compare the humidity and temperature differences between your shop and intended location for the finished project. Both have a bearing on the EMC at any given time, from a rainy spring to a dry winter. Keep a log of this critical data to help you manage your stock more effectively. Use the Ranges in Moisture Content by State, Figure 1, to determine what is typical during the driest month and the wettest month, with the higher number representing the wettest part of the cycle.

HOW WOOD GETS STRESSED OUT

Removing water at moisture contents below 30% bullies wood big time. As shown at right, when wood dries, it shrinks mostly across a board’s width and thickness, not its length. Uneven drying, or stresses within the wood, sometimes due to poor storage practices, can cause stock to develop undesirable conditions that can include checks, cups, bows, crooks, and twists.

Strategies for Workshop Storage

In addition to looking after the EMC of your stock, other strategies come into play in the acclimation game, centering on storage. They include:

• Find a suitable lumber storage area that mimics the conditions where the project made from that material will end up. Create and monitor those conditions as described above.

• Do not stand boards up and lean them against the wall. As they acclimate, a bow could permanently set in.

• End-coat stored green boards with a product like Anchorseal, Green Wood Sealer, #148772 to promote slow, even drying, and minimize splits.

• Avoid areas with direct sunlight,

close proximity to a source of heat,

or high airflow.

• Keep stock off the floor by setting it in a rack or on blocks, allowing for adequate airflow. Consider stacking and stickering the stock as shown in Wood Storage, Figure 2. Cover the stack with weighted plywood to distribute even downward pressure on it (and to keep off rain/snow if the stack is outdoors). Keep the pile 8-12" off the floor or ground and space the stickers 12-18" apart. Stickers typically measure 3/4" thick × 11/2" wide and should be dry.

• Monitor the MC of all boards—whether air- or kiln-dried—with a moisture meter. Because boards at the top and edge of the stack will dry more quickly than those in the middle, reshuffle the boards from time to time for overall even drying and conditioning.

• Note that slower drying rates result in fewer defects and that those surfaces exposed to the greatest air movement will dry faster.

• Check MC of every board before use to catch any board with a moisture content that is too high. It only takes one board to cause problems with your project.

About Our Authors

(Jeff Slahor, research instructor/wood technologist, and Larry Osborn, research associate/wood technologist, work for the West Virginia University Division of Forestry.)

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