Those of us who work with wood have probably at some point experienced the frustration of wood warping, cracking and changing shape. dackwoods This is something that experts have been studying for many years and is still somewhat of a problem in the lumber industry. Obviously, no one would willingly choose a defective piece of wood over a nice, straight one.
This is why you still see warped or otherwise defective 2 by 4s and such at home centers, that are removed from the neat wood piles after people dig through the stack to find straight ones. I imagine they return these defective boards to the lumber yard and promptly get a refund. This problem apparently costs the wood industry millions of dollars every year in the US alone.
As easy, convenient, and economic it is to work with wood, there are still these downsides to deal with – not to mention potential durability issues when untreated. Let’s take a look into the science behind why wood warps and cracks, so we can better understand and deal with such challenges. Properly dealing with wood defects requires first recognizing them and then knowing what to do about them.
I covered the basics of the various types of wood warping in another article “Dealing with Warped Wood”, but in this article we’ll get into the whys and wherefores of the science of not only wood warping, but wood cracking as well. There are several potential reasons for wood displaying geometric and structural defects, some due to internal knots and grain inconsistencies and others due to post-cut environment conditions.
Wood is an organic substance, and as such, is greatly influenced by its surroundings. More specifically, it is constantly trying to reach a state of equilibrium with the surrounding air. When wood is first felled, it is called “green wood”. Green wood, depending on type, can contain over 50% of its weight in water.
This water that is naturally contained within wood is divided into two categories: Free water and bound water. As per the names, free water is water that is only held within the wood via capillary action, and bound water is water that is bound within the wood via hydrogen bonding. When wood is dried, the free water will be the first to leave the wood.
Fibre Saturation Point of Wood (FSP)
This is a very important junction in the wood-drying process, and defines the moisture content when all free water has been removed from the wood. Up until this point, wood shrinkage and/or strength change will not be a concern. It is from this point that warping and changes in strength begin. FSP is generally achieved at 25% to 30% moisture content.
“Why does wood need to be dried anyway?”
If wood just shrinks and warps when you dry it, why do it? Well, as I mentioned earlier in this article, wood is constantly on a journey to reach equilibrium with the surrounding air. This means that if you don’t dry it in a controlled environment prior to use, the wood will most likely shrink or warp in your “uncontrolled” environment after you’ve cut and fit it all nicely somewhere, leaving ugly defects and causing all kinds of problems.
As a rule, wood will shrink as moisture leaves the wood, and it will expand as it enters. This is why you’d want to dry the wood in a similar environment as the one you’ll be using it in. Drying wood in a humid region and then transporting it to a drier one for use will defeat the purpose of drying it in the first place, as you’ll get shrinkage!
The general rule of thumb is to get the moisture-content of your wood as near equilibrium as possible before use, so it won’t need to stress itself to do so afterward. This is the basic reasoning behind wood-drying. (Another trick some carpenters use is to make a preemptive cut that is clean and uniform down the center of the length of larger lumber to relieve the stress.)
There are also other benefits to drying wood. A major structural benefit is that strength will generally increase with the drying of wood below FSP. Of course, there are some exceptions. If wood is dried out too much, it becomes brittle and weaker to impact. Below I will list the main benefits to wood-drying for your convenience and reference:
Decay and stain-causing organisms generally cannot thrive in wood below 20% moisture content. Also, many wood-loving insects can only live in green timber.
It is lighter, making transportation and handling easier and cheaper.
Dried wood, as opposed to green wood, has better thermal and electrical insulation properties.
Various finishes, preservatives, and glues react better on dry wood, lasting longer as well. Dried wood allows proper penetration of these substances.
Dried wood is generally stronger than green wood.
Potential Problems related to Wood-drying
Now that I’ve covered the good points going for dry wood, let’s check out potential problems.One of the biggest concerns of wood-drying, is doing so too fast to where it creates compressive stresses toward the center called case hardening. This is potentially very dangerous when the stress is suddenly released upon cutting. Wood will naturally dry from the outside to the inside.
But if this process isn’t controlled, the outer parts of the wood will dry faster than the inside. Because the inner parts of the wood are still saturated, it won’t shrink at the same rate as the outer parts. This will place what is called drying stresses on the outer parts of the wood, leading to case hardening and/or cracking and splitting. The solution to this is controlling the drying environment by either using kilns or partially sealing the wood with an oil such as mineral oil.
Mineral oil will only penetrate 1-2 mm into the wood and can simply be planned off after drying. Partially sealing the wood will ensure that it doesn’t air-dry too fast, thus greatly minimizing the risk of cracks and splits. You can also wrap the wood in a material that restricts the movement of moisture, but must treat the wood against fungal infection. Something like gasoline or even oil will do.