Q: We have a large pedestal base for a table. The pedestal is about 6 inches in diameter, 30 inches long and made of 4/4 red oak edge-glued and laminated into the larger piece. The problem we are having is that many of the bases are cracking along their length. The cracks can be very small, but usually there will be one crack that is quite long and is wider than the others. We putty the cracks but will this be permanent? What can we do to prevent the cracks from forming? Will they get worse? We checked the MC of the damaged pieces and most were around 7 percent MC.
A: My number one rule for wood is that it doesn't change size or shape unless its moisture content changes. With the cracks forming, we have a size change, so we know that there is a moisture change. Moisture changes in wood because the air is drier or wetter than the wood.
In your case, cracks indicate shrinkage, so the pedestals are too wet for the air in your plant. Is your plant too dry or is the lumber too wet, or a combination of both?
First thing you need to check is the humidity in your plant. Buy an inexpensive electronic humidity measuring device from your local electronics store about $30. You should run your plant around 35 to 40 percent RH (7 percent EMC), as this is what your customers will have in their homes or offices.
Next, you need to check the MC of the pedestals. Here is the key: Check the MC before they crack. If you check the MC of a cracked piece, the crack indicates that the moisture has already left, so you will get low MC readings. You want the MC before they begin to dry and crack. This is critical! I would expect that the MC of the pedestals should be 7 percent MC, with no readings over 7.5 percent. Incidentally, you might use a pin meter to check these. That way, by taking MC readings as you drive the pins into the piece, you can check for an MC gradient. Hopefully the core is the same MC as the shell.
It's most important that the EMC of the plant air equals the MC of the lumber because when they are equal there is no MC change and therefore no size change.
Now it is foolish to think that the pedestal will never change MC during manufacturing or in use; it is going to happen. So, what can we do to accommodate a small MC change (and a small amount of shrinkage or swelling)? Here are three effective, practical approaches. Finish the pedestal with a thick vapor-resistant (not just water-resistant) coating. This will help minimize MC changes after manufacturing especially when the EMC of the air changes for only a short time the thicker the coating, the better.
Even with this coating, we still might get an undesired MC change. So, another beneficial procedure is to drill a hole along the length in the center of the piece. What happens when the wood on the outside (the shell) dries and tries to shrink is that the wood in the center isn't shrinking and effectively prevents shrinkage. As a result, stress develops and then a crack. If there is a hole, however, then when the shell wood shrinks, there is room for shrinkage it can "shrink into the hole." The larger the hole the better. (Coat the inside of the hole with vapor-resistant finish too.)
Another helpful item appreciate that oak is one of the highest-shrinking species in North America; switching, if possible, to a lower-shrinking species can help.
A fourth idea is to minimize, as much as possible, the exposure that raw, unfinished wood has to the air. In fact, you may want to build a small storage room in the plant to hold the pedestals. Control the RH in this room very precisely to minimize any chance for MC change.
I suspect that by implementing these first three fairly simple, practical procedures, you will have fewer than 5 percent of your pedestals requiring any repair.
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