Q. We are seeing some small, end checks in a hard maple product that we make. The ends of the maple are exposed, so these checks are considered a defect. They show up in the customer’s home, especially when it is cold outside. Guidance please.
A. For some reason, end checks in maple almost always are fine, hairline cracks, making them difficult to detect in production.
If we were to measure the humidity in a home, we would find that the colder it is outside, the drier it is inside the home.
This is because when cold, outside air is brought into the home, heating the air to 70 F lowers the humidity to under 5% RH. Fortunately, cooking, bathroom showers, plants, and furnace humidifiers add moisture, as this low humidity would be uncomfortable to us. Even so, colder outside means lower inside RH.
I am sure you appreciate that wood shrinks in response to lower humidities and swells when the humidity goes up. Now the shrinkage and swelling would be less of an issue if the wood could actually move.
However, in a glued-up product, the wood does try to shrink in response to low humidity, but in many pieces, the adjacent pieces stop this movement. So, instead of shrinkage, we have ATTEMPTED SHRINKAGE, which then develops stress within the wood pieces. If the stress exceeds the strength, a crack or split develops.
Normally, the changes in humidity, and therefore moisture content (MC) in the wood are small and occur over many weeks. The wood can absorb the stress fairly well.
But, if the homeowner uses a wet cloth to wipe or wash the wood, the surface MC increases (unless the coating is repellent to liquid water).
Now we have wood going from wet to dry quickly, WHICH MEANS LOTS OF STRESS, which means a high risk of forming a small check. So, the coating on the wood is certainly one key.
In this case just mentioned, we are creating a new check. However, it is possible that the maple lumber has some small end checks. (End checks can be fully controlled by using a commercial end coating product before drying begins.) As these checks are very small in hard maple, we would expect that people cutting the lumber into parts will occasionally trim too short, so that a small crack ends up in the finished piece, although it is virtually impossible to see in most production operations.
With this existing crack, it is really easy, when the wood is exposed to low humidity, for very little shrinkage stress to re-open the check. So, properly end coating green lumber is really important.
One closing observation: If the homeowner allows liquid water to go into an existing crack, the swelling will close the crack.
But, when this water evaporates and the wood shrinks, the crack will be bigger than ever.
This crack will not close when the humidity increases in the summertime. Overall, the best advice we can give is to avoid contact with liquid water as much as possible.
Q. We are having some difficulty at times with our three-belt sander. When done sanding, the piece looks smooth, but in a few hours, the glue lines have a slight depression and our finish does not seem to absorb into the wood as well.
A. The issue is likely caused by using sandpaper beyond its intended lifespan. In other words, the paper is dull and needs to be replaced.
Dull paper generates heat — quite a bit of heat — especially with finer grits. Heat tends to seal wood surfaces. Heat also expands the glue in a joint. The squeezed-out glue is removed by the sander. Then, as the glue cools, it contracts, creating a slight depression.
Change paper more often than you think.
Q. I have heard about a chemical that stabilizes wood so it does not shrink and swell. Any idea what this about?
A. There is a chemical, polyethylene glycol 300, a liquid solution, that can be soaked into green wood; green wood is wood that has not been dried yet.
PEG, once soaked into the wood, bulks the wood so the wood cannot shrink when it is dried or when the MC changes in-use.
Obviously, it needs to go into the wood before any shrinkage begins. It is also necessary to make sure that the PEG is absorbed throughout the piece, not just in the surface.
So, the wood, on a microscopic level, must be porous. White oak, black locust, and Osage orange are examples of wood species that are too impermeable to be a candidate for this treatment.
Treating time varies depending on the solution strength and temperature, and the wood thickness. A couple of weeks for 2-inch pieces is common.
Once dry, the wood can be machined as normal and finished with polyurethane, and maybe other finishes.
CONFUSION AND SAFETY. Ethylene glycol is what we use as car antifreeze and it is poisonous to humans and others. However, if you add 300 of the ethylene glycol molecules (PEG-1) together into a new, long-chain molecule, then the properties change. It is, before mixing with water, not a liquid, but a gel. Many medicines use a molecule of 3000 ethylene glycol molecules in a chain (PEG-3000) or even longer, so it is healthful. Car antifreeze will not work in preventing wood shrinkage in drying and in-use.
Back to PEG-300. Sometimes PEG-1000 has also been used with wood. It is the bulking property that makes PEG work. Due to the cost of the chemical, especially small quantities, PEG-300 is used mainly to for expensive items such as walnut gun stocks or rounds, and so-called cookies or thick wafers crosscut from a log end. Sometimes boats and wood artifacts that are submerged are brought to the surface and treated with PEG before they dry and crack.
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