Q. I continue to receive questions about checking (also called cracks) in veneer laminated products, including laminated flooring, laminated raised panels in kitchen cabinet doors, laminated furniture., and full sized doors in homes, offices, hospitals, and motels. So, I will start with the basics and move forward.

A. A thin piece of wood veneer or a wood cell itself (a cell is like a miniature, skinny, soda straw usually 3 to 5 mm long) begins to shrink at about 28% moisture content (MC) and continue to shrink down to 0% MC. At 99.5% relative humidity (RH), the wood will be at 28% MC. So, 99.5% RH is also called 28% equilibrium moisture content (EMC). At 80% MC, the wood will be at 16% MC, or we say that 80% RH at room temperature is 16% EMC. (As you might expect, there will be a very small variation in this 16% MC value depending on species, but for North American species, differences are so small as they are, from a practical standpoint, ignored.) At 50% RH, a humidity level in many homes and offices during the summertime, the moisture condition of the air is 9% EMC. At 30% RH, a humidity level in many homes and offices in the wintertime, 6% EMC. Of course, 0% RH is defined as 0% EMC.

Shrinkage
As a rule of thumb, wood veneer shrinks across the grain (thickness and width) about 1/4% for each 1% MC loss under 30% MC. So, if freshly cut veneer is put into a dryer at 80% RH, the shrinkage from 28% EMC to 16% EMC is 1/4 x 12 or 3% shrinkage. (This is for flatgrain; quartersawn is about half of this.) Appreciate that the veneer at 28% MC and wetter is about one-half the strength of dry veneer. But, in most cases, the wood veneer, especially thin veneer, can tolerate this 3% shrinkage without cracking; it takes a lot of attempted shrinkage to break strong wood.

Consider the veneer in a product exposed in a home. If the veneer enters the home at 8% MC in the wintertime, it will dry 2% MC which will result in about 1/2% shrinkage — much less than when drying. Plus, this dry veneer is about twice as strong as the wet veneer. Does it make sense that a little shrinkage in a home with dry RH will cause strong, dry wood to crack while weaker wet wood with 3% shrinkage does not? Bottom line: Dry veneer with small MC changes is too strong to form new checks or cracks.

Laminated panel
Now, let us take a different approach. Consider a piece of veneer with no preexisting checks that is glued to a core, such as MDF. The veneer is now exposed to slightly drier humidity, so the veneer will try to shrink. Can you appreciate that the shrinkage across the face is small and exists across the face uniformly? So, the 1/2% shrinkage (or even more in rare cases) is not trying to occur in one spot, but is distributed across every spot on the face. How can we get enough force with dry wood and small force at any one spot to initiate a new check? The answer is that it is impossible, unless the veneer is a lot wetter than it is supposed to be.

Sometimes, I will see two adjacent checks, small in width but long. If we had this develop on veneer without preexisting checks, when the first check opened, it would relieve the stress in that area, so a second check nearby would not have enough force to develop. So, we know two parallel checks are preexisting and not new checks that developed after laminating the veneer to a core.

In the above discussion, we assume that the wood has its normal strength, and is not weakened by knots, bacteria, reaction wood, or other rare factors.

Source of checks
Next, let’s back up and consider how veneer is sliced or peeled. A rather thick knife is inserted between the the large cant or veneer log and the veneer sheet itself.  The thickness of the knife means that the veneer is bent quite a bit as it is cut.  To prevent breaking or checking the side of the veneer in contact with the knife, we often have a bar that exerts pressure on the wood to minimize or eliminate these so-called “lathe checks.”  Just in case there are a few checks, this face of the veneer with lathe checks is glued to the core.  This means the exposed face is check free.  (Sometimes, the side prone to lathe checks is called the loose side and the other face is the tight side.).
Incidentally, if you sand the tight side enough, you will sand off enough wood to expose the loose side checks.
If you sand the tight side too much, or glue on the veneer so the loose side is up, the exposed veneer face will likely have invisible checks. It is only when the panel is exposed to a dry humidity that the surface will try to shrink, which will re-open preexisting checks. These preexisting checks are tiny in width but may be several inches long. This opening of the check can crack the finish, making these tiny checks very obvious.

Look inside
We can often learn more by using magnification to look inside the check by cracking the check open (maybe being a little creative and destructive to do this) and to look at the check from the side, using a very sharp razor to prepare the surface. A sharp razor will cut the fibers and not squish them. When we look inside the check, we might see finish, indicating the check was open when the veneer was finished. Sometimes we can see where the laminating adhesive has been pushed into the bottom of the check, indicating the check was open when laminating. From the side, we can sometimes see the spacing of a check where it was laminated to the core, again indicating the presence of a check prior to laminating.

Special case. Sometimes when the veneer manufacturer has small pieces of veneer, they will edge-glue them together into a larger sheet. This joint is often perfect and invisible. However, when the veneer is laminated to the core material at a subsequent manufacturing step, the heat in the laminator can melt the glue between the edges, causing a crack or gap. It is likely that the finish will bridge the gap, but when any drying of the veneer occurs, including the small drying we have every winter, this joint gap will open slightly, cracking the finish and making a noticeable crack. Unlike other checks that are wandering around, these cracks will be straight.

Caution
There are a few people who incorrectly blame the CREATION of veneer checks on the low humidity in a manufacturing facility. They then say that if you raise the humidity in the facility, you will not see any checks. In fact, this is true — preexisting checks will not show up at high humidity. They show up only in dry humidity in the wintertime. But here is the problem with this approach to a solution. If a facility raises the humidity in the winter, the opening of preexisting checks will occur in the dry customer’s home. That is, the surface fibers will try to shrink and will open the checks, requiring custom repair or replacement. Note that customers in humid coastal climates will likely not see the problem.

Now the plot thickens. A customer’s home changes its humidity from 9% EMC in summer to 6% EMC in winter slowly over three or four months. This slow change allows some shrinkage stresses to moderate or dissipate over time. What this means is that some homes or offices will not have noticeable opening of preexisting checks. 
So, a facility that exposes veneered products to a rapid change in humidity, such as from a somewhat normal-humidity plant to a very dry finishing room, has a better chance of reopening preexisting checks, than if the dry humidity were avoided or the change occurred more slowly. This same abrupt change in humidity would apply to 8% MC veneer sent to a dry home in the winter. But can we afford to humidify a finishing room? Can we afford to humidify a home or office above typical, customary dry, wintertime levels? Or can we make a minimum humidity level part of the requirement when someone buys a laminated veneer product?

My overall conclusion is illustrated with an analogy: When someone falls out a window and is badly hurt due to the sudden stop at the end, we can cure the problem perhaps by making the landing more gentle with lots of cushions, foam, etc., and hope. Or we could address the root problem by putting bars on the windows.

Q.  Is walnut wood, used for food utensils, toxic to humans?

A. I do not know with 100% certainty.  So, you may have “Stumped the Wood Doctor.”  But the evidence I have seen says “not likely.”  Yet when dealing with allergies, it seems likely, as a non-medical doctor,  that someone, somewhere is like allergic to wood, including wood dust, of any specific species, if not a group of species.
I have seen walnut salad bowls made years ago that are still used today.  I have seen walnut utensils, like salad tongs.  I have seen walnut used in cutting boards.  And then there is the nut itself, that I enjoy, although I have noticed that over the past 20 years most of the hair on my head has disappeared. I have not heard any evidence that walnut wood is toxic to humans., although we might always have a rare person allergic to the wood.  

A 2021 document from Extension at the University of Georgia supports the non-toxicity of the wood: “The fruit, leaves and roots of black walnut trees contain a chemical, juglone, that can have a devastating impact on the roots of other plants. In humans, ingesting even a small amount of pure juglone can cause a serious poisoning effect.  Inside the tree, juglone is a clear liquid — called prejuglone — that’s nontoxic.”