The moisture content of wood is a topic that many woodworkers don't know a lot about. Most assume that if they are working with kiln dried wood that came from a wood supplier somewhere in their area that the moisture content of the wood is around 6 to 9%, which it probably is, or at least was at some point. A moisture content of 9%, for many woodworkers, is considered to be "ideal" for furniture and most other woodworking projects. Ah ... not necessarily so.
First of all a question. What is the ideal moisture content for hardwoods for woodworking projects?
The answer is more complicated than a simple number like 6% or 12%. First of all a bit of background. The moisture content of wood is something called EMC (Equilibrum Moisture Content). What this means is that where ever wood stored, it will absorb or expel the amount of moisture it needs be equal to its environment. This means that if you live is Seattle and bring home a piece of wood from your wood store where it was kiln dried to 6% moisture content, if you store this wood outside in a covered shed (not open to the weather) this piece of wood will likely absorb another 6% of moisture and will be come "stable to its current environment" when the moisture content reaches 12%. If you stored this lumber inside your home with lower relative humidity and higher temperature, chance are it will not absorb as much moisture and will probably stabilize to something closer to around 9%. Now if you live in the mid west and store this lumber in home where the temperature in you home is still around 70 degrees but the relative humidity is like 10% (because it is so dry) this wood will like stay around 6% EMC.
Any woodworker who makes furniture, and wants it to stay together, and the finish to remain stable on it, NEEDS to know what the moisture content of the wood they are using. It's fundamental to woodworking. There are many different moisture meters on the market, but the pinless, such as shown below, although a bit more in cost, is an excellent tool for the workshop, and can be taken to the lumber store to check moisture content of lumber purchases, without fear of poking holes in new lumber.
Click the image to read more, or to order your own Moisture Meter from Amazon
So, lets get back to the answer of our question. The real answer of the question is, "what is the environment that the project will be stored or used in". If it is a piece of furniture and you live in Arizona where the ambient temperature is around 70 degrees and the relative humidity in your home is around 20%, then the ideal moisture content for wood is actually closer to around 5%.
The cells within every piece of wood are constantly in state of absorbing or expeling moisture in order for the wood to "stabilize" within its existing environment. As woodworkers, once we know this we can begin to compensate for the expansion and contractions of wood.
Wood expands and contracts almost entirely on its tangential and radial direction almost nothing along its longitudinal direction. This means most typical "boards" will expand and contract through their width and depth and almost nothing in their length. The amount they expand and contract depends on the environment they are stored and used in. We can slow down the process of expansion and contraction by sealing the edge grain, but we cannot stop it.
If you have a moisture meter like a you can check your own wood to see what it's moisture content is then use that information to plan your project more effectively. This way you can allow for future expansion and contraction by now knowing which direction the wood will flow in and what you can do compensate for.
copyright - Colin Knecht
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