This is a common question. Understanding the reasons why this occurs and the potential solutions requires or involves an understanding of the basics of wood shrinkage. These cracks, for the most part, do not involve or are not caused by incorrect drying. Rather, it is all about natural or normal wood shrinkage.
Theory
So, to begin, it is key to understand that wood shrinks as the moisture decreases and swells when the moisture increases. More specifically, a wood cell that may have 75 percent moisture content (MC) begins to shrink when it dries to 30 percent MC (some people say 28 percent MC and some tropical woods it is closer to 22 percent MC). The cell continues to shrink as it dries down to 0 percent MC. As a rough rule of thumb, for each 4 percent MC change (which is roughly 20 percent RH change), lumber or smaller wood pieces shrink or swell 1 percent in width and thickness. Data exists to calculate the actual amount for each species.
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So, now let us pretend that we are a wood cell on the outside of this 4 x 4 piece of wood. We have just been freshly sawn so we are now at 75 percent MC and are exposed to the outside humidity, which in most of North America averages 65 percent RH. As a cell, we have lots of water inside the opening (a cell is like a skinny soda straw, although much smaller; 3 to 5 mm long and 1/100 of that in diameter). As this liquid water inside is evaporated, no shrinkage happens, as the walls of the cell are still soaking wet.
At about 30 percent MC, the liquid water is evaporated and now the walls begin to dry. As the molecules leave the wall, the wall shrinks, essentially to try and fill the empty molecular space…or, at least, tries to shrink. The problem is that deeper into the piece, the cells still have liquid water and have not yet thought about shrinking.
So, at 65 percent RH outside, the wood cell is going to dry to 12 percent MC. With this drying of 18 percent MC (from 30 percent MC to 12 percent MC), the cell is trying to shrink around 4 percent in width. This means that the 4-inch face of the timber is trying on the surface to shrink 4 percent which is 0.16 inch or 5/32 inch. But the wetter cells underneath are not yet interested in shrinking (and the water in them is incompressible), so stress develops. Certainly, wood is a little bit flexible or “stretchable.” but 5/32 inch for a 4-inch wide piece is a bit too much.
The stress that develops exceeds the strength of the wood, so the piece checks or cracks. (A check is a small crack.) This crack now relieves the stress. In fact, it is common to find one large crack, but very few other cracks on the face unless drying is way too fast. This one large crack will continue to grow as the surface shrinks more.
Jumping ahead, eventually the core begins to shrink and the shrinkage of the core begins to pull the surface check closed making it often invisible at the end of drying, but it is not healed, so it can reopen later whenever the surface is exposed to dry air and it shrinks (even after finishing or even after being put into service). Special note: For this reason, visually inspecting a timber after drying may not allow you to determine the extent of surface integrity; you might have to cut a few test pieces.
What to do
So, there are two parts to this shrinkage problem: a) The amount of stress (which obviously is related to the amount of shrinkage, which in turn is related to the humidity) and b) The strength of the wood. Let’s look at both of them.
Stress
Shrinkage is normal for wood as its moisture decreases. There is nothing we can do to change this basic property, although some species, like teak, shrink very little, and others like oak shrink quite a bit. So, certainly one suggestion to minimize cracking or checking is to use a species with small shrinkage overall.
Another possibility is to expose the wood, when it is first drying, to a fairly high humidity. This will mean lower stress. Plus, if we hold this high humidity for a long time, the moisture further down from the surface will evaporate, minimizing the shrinkage differences and stress from the outside toward the inside. In fact, one option for some products is to drill a large hole through the center of the piece, end to end. This allows room for the shrinkage to occur, plus it dries the inside at the same time that the outside is drying.
A third option would be to impregnate the wood with a chemical that will inhibit shrinking by bulking the wood. In other words, we fill the hollow center of the cells with this rigid chemical, One popular chemical is polyethylene glycol with a molecule length of 1000 (PEG-1000, although sometimes PEG-300 has been used). Another is a liquid plastic (a monomer) that cures (polymerizes) in the wood and becomes hard. Generally, such treatments must be more than a surface treatment.
In summary, everything mentioned above is rather expenses and time consuming.
So, another option is to make a crack in the wood, prior to the start of drying, on a face that is going to be hidden. We can take a Skil-Saw and make a saw kerf about 2 inches deep, full length. This kerf will effectively relieve the stress fairly well on the three remaining faces, so with controlled drying, checking will be minimal or nonexistent.
Another consideration is to have the stress level reduced (use a slightly higher humidity) so that the lumber gets a few hours to rest. This happens automatically when air drying, as the humidity at night in most locations is close to 100 percent RH. So, air drying in a shed, maybe with curtains to avoid really low humidities and keep the sun and rain off the wood, will be helpful.
Strength
Obviously, the strength of the wood is an inherent property. But there are a few things we can do to avoid reducing the strength. Realize that once a check forms, it is easy for it to get larger: longer, wider and deeper. This is because wood is weak in this sort of failure—the technical name is cleavage. So, the effort should be directed at preventing checking join the first place rather than controlling checking once it has occurred.
First, the hotter wood is (for example 105F versus 115F), the weaker it is. So, drying at cool temperatures (90F or under) will help.
Second, when the surface was first sawn, a dull saw actually tears the fibers and so there are small cracks in the surface before any drying begins. (In fact, a band saw gives a stronger surface than a circle saw.) One possibility for avoiding this weakening from sawing is to plane the lumber or timber on all four faces with a knife planner before drying begins. This planing produces a substantially stronger surface. Of course, rapid drying at low humidities and warm temperatures can still develop stress that exceeds the wood’s strength, so this presurfacing must be accompanied by proper drying as well.
Third, some species, most notably oak and eucalyptus, are more prone to checking than others. One factor is the width or size of rays — lumber with wider and longer rays is more apt to check. Also, the site where grown is a factor with wetter sites (faster tree growth) increasing the risk.
In softwoods, compression wood is known to be much weaker, so checking would be more likely. Finally, there are bacteria that get into the living tree and substantially weaken the wood. (These bacteria also make the wood stink, have a higher initial moisture content, and result in shake in the lumber.)
Summary
Some of the more practical procedures to avoid or minimize checking and splitting on the faces of large timbers include:
Sharp saws at the sawmill
Planing prior to drying
Drying at low temperatures and high humidities
Avoiding bacterially infected wood
Drilling a large hole in the center
Cutting a deep saw kerf into a hidden face
Avoid check-prone species
Avoid compression wood in softwoods.
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