Wood cuts and how they react to moisture
Many of you already know that wood moves, but you might not understand why, or how. The first thing to remember is that wood doesn’t move significantly in relation to temperature. It does, however, move in relation to changes in relative humidity, which temperature does have an effect on. Wood absorbs or gives off moisture, in an exchange with the surrounding air. If you keep relative humidity constant, over time the wood settles at a moisture content known as equilibrium moisture content (EMC). Moisture content (MC) is expressed as a percentage of the weight of water in the wood, compared to the weight of the wood when fully dried. Fully dried wood has 0% MC, known as oven-dry. So wood with an MC of 10% may weigh 11 pounds and have one pound of water in it. Thus, after being dried to 0% MC, it will weigh 10 pounds.
In reality, EMC almost never exists because the relative humidity (RH) in the air is constantly changing. As RH increases, the EMC is disturbed as the wood starts to absorb moisture from the air. It will settle on a new EMC if the RH stays at that higher level for an extended period. But if RH starts to drop, the wood will give off moisture, MC will drop and EMC will only occur if RH again stops changing, for an extended period. The interplay between RH and MC is almost constant, and the wood is rarely at EMC. The wood is constantly moving.
So, why do we care about changes in MC? Changes in MC are important because wood changes both its shape and size as MC changes. Wood expands in size when it takes on moisture (increase in MC) and wood contracts in size when it gives off moisture (decrease in MC). Let’s look at how the shape of each wood cut is affected by changes in the environment’s humidity.
Some cuts of lumber, such as quarter sawn, have greater stability. One reason for its stability is that it moves less in width as the MC or the RH changes. Another reason is that quartered wood changes less in shape.
Take a look at Fig. 1, to review the three basic wood cuts. Now compare this with Fig. 2, and notice how the three cuts change in shape as they are dried. It doesn’t matter whether you kiln dry, or air dry the wood, it will still change shape as MC drops.
The first example in Fig. 2 shows the shape change that occurs when flat-sawn lumber is dried. You may be used to seeing the U-shaped growth rings on the ends of lumber, and you may also be used to seeing those boards cupped across their width. But you might not have noticed that they are always cupped in the opposite direction to the curvature of the growth rings. This direction of cupping only occurs when drying lumber. Increase the MC of the cupped piece of wood and it will start “uncupping”. The rate of movement is different along (or parallel to) the growth rings, than it is across (or perpendicular to) the growth rings. It is the difference of movement, within the same board, that is the cause of this cupping phenomenon. That is why serious woodworkers mill their own lumber, using only dried, rough-sawn lumber. It isn’t because rough-sawn lumber is cheaper. It’s because the boards ought to be dimensioned and flattened after most of the drying has already occurred, so that cupped boards can be made flat just before your furniture is built. Assuming that you don’t expose your furniture to massive fluctuations in RH, the MC will not change a lot and the boards that make up your furniture will remain quite flat over time.
Now look at the second example in Fig. 2. Rift-sawn lumber becomes a parallelogram as it dries. This phenomenon is known as “diamonding” because the original rectangular shape becomes more of a diamond shape. Assuming you’re drying the wood, the edges of the board will always slant in the opposite direction to the angle of the growth rings. The same goes for the wider surfaces. Diamonding is simply another example of the wood cupping opposite to the direction of the growth rings, but with rift-sawn lumber it’s occurring in another direction. Now look at the quarter-sawn example in Fig. 2. Because the growth rings are vertical, the board barely changes in shape at all. If the growth rings are quite curved (which often occurs with smaller trees) then the edges of the board will also cup opposite to the curvature of the growth rings. More often, a wide quarter-sawn board has very straight growth rings on one side (at the outer area of the tree) and more curved growth rings on the other side (closer to the centre of the tree), based on nothing more than growth ring geometry within a tree. This results in bulging out of only one edge of the quarter-sawn board – the edge that was closest to the centre of the tree.
As you can see, the three cuts of wood differ, not only in terms of aesthetic qualities, but also in terms of how each cut’s shape changes with changes in relative humidity. You’ll notice these differences when dimensioning rough lumber with your jointer and thickness planer. When jointing flat-sawn boards with the concave side of cupped boards on the jointer bed, you will only cut the outer areas of the board for the first few passes. When dimensioning rift-sawn boards, the two large surfaces will be amazingly flat, with the edges requiring a larger number of passes to square them up. When dimensioning quarter-sawn boards, you’ll be surprised at how much less jointing and planing are required, in general, to square everything up.
In my next article, I will compare the rate of movement and the size changes that occur, for each cut of wood (flat-sawn, rift-sawn and quarter-sawn lumber), as relative humidity and moisture content changes.