Moisture in the Woods
Knowing the moisture content of your wood is an important step in achieving a finish that will stay problem-free down the road.
By Mac Simmons
“I am big on metering each and every board upon receipt. I know it takes time, but considering the price of wood these days, it’s a good habit to get into to protect your investment and make sure that you are paying for what you get. More and more readers tell me they have integrated metering into their lumber procurement systems. That’s music to my ears.
“...The battle to control moisture in our woods continues right onto the plant floor into the finishing room and onto final assembly and finishing. Wood being the breathing, living creature it is, it will shrink and swell if temperature and humidity are not properly controlled throughout every phase of the manufacturing process.”
Every woodworking and finishing workshop should include a class on “moisture in the wood,” emphasizing the importance of checking woods with a moisture meter so you know the percentage of moisture content in your wood.
It is only by doing a moisture check and knowing what the appropriate moisture content should be that you can control and eliminate moisture problems like warping, cupping, shrinkage, failing joints, checking and splitting. Once you know the allowed percentage of moisture, you also can avoid such finishing problems as blushing, pinholes, crazing, checking, adhesion and the retarding of stains, glazes and coatings.
Some of these moisture-related problems will show up during your finishing process. But they might not show up until months after your product is delivered. This is why doing a moisture check before you start fabricating or finishing is so important. (In addition, all refinishers should check their “stripped” woods for moisture content before finishing.)
Finishing materials are not “barriers” against moisture content changes — coatings only slow down the exchange of water molecules which exit and return to the wood, depending on the relative humidity of its location. In many shops, the first moisture check is done when the lumber is delivered, before it is fabricated. If your lumber is not stored in a controlled environment, a second check should be done right before finishing, as the moisture content changes under different conditions. Woods that are adequately dry generally are stronger and easier to work than wet woods.
Moisture meters give valuable information on the percentage of moisture in the woods. These readings will tell you if you need to allow extra dry time or indicate that it is safe to go ahead and start your woodworking or finishing process. There are two different types: pin meters (resistance) and pinless meters (dielectric).
Pin meters have two sharp probes that are pushed into the wood. Some are pushed in parallel to the grain; on others it does not matter how the probes are positioned. Some pin meters use external cable probes that measure the difference between the core and surface moisture. Pin meters leave two holes in the woods.
Pinless meters have a flat plate that is pressed against the wood. They quickly scan large areas and do not leave any holes. Pinless meters need a smooth surface, where pin meters are not affected by the wood’s texture or shapes.
Most meters use micro processing technology. Some display analog scales, digital LCD and LED indicators. Certain meters can measure moisture content as low as 0% or as high as 99%. Some models use a sophisticated low frequency electromagnetic wave, others use electrical resistance.
Most meters have a setting that calibrates for corrections in the structure and density of different species of wood, while others provide a chart or table that gives instructions for calculating the moisture content. Many meters allow for “stacking” to obtain the right thickness.
Meters vary in price and size (some will fit into your shirt pocket). I suggest that every wood finisher have at least one meter in the shop. Invest in a model that fits your needs and budget. The meters are sensitive, but if you handle them with care, they will last a long time.
(NOTE: This article did not discuss the relative humidity or equilibrium moisture content, because these values will change from location to location. However, these values are important to know. In the end, wood adjusts to its own moisture equilibrium, depending on the relative humidity of where it is stored, fabricated, finished and its final location. Your coatings and lumber suppliers can give you the correct moisture content for working your wood and for finishing in your particular location.)
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