Q: There's a debate going on at our lumber yard between the salesmen and the kiln operator. Is it true that winter-cut northern Michigan hard maple is whiter than summer cut? I've heard a couple of the old timers say that back in the good old days they had a rule that you don't cut hard maple if the leaves are on the tree. The reasoning was that in the summer the sap is in the tree making the wood a caramel color. In the winter the sap would no longer be in the tree, making it a nice white color. Is there any validity to this?

 

A: There is indeed truth to what they are saying about winter-cut being whiter than summer-cut, but the reason for this occurring is incorrect. The reason that this happens is that the discolor reaction requires warmth. In the summer it is warm enough for the reaction to begin occurring in the log after the tree is harvested and in the lumber after the lumber is sawn.

In the winter it's not warm enough for the reaction to occur prior to entering the kiln. However, if you mistreat the lumber in the kiln by using 90 percent RH or higher at 90 degrees F or warmer for 24 hours, you can discolor winter-cut lumber. If you use temperatures above 125 degrees F early in drying, you can also cause the lumber to turn pinker or browner.

Incidentally, the sap content is the same year-round. It's only that in the spring and summer the sap is flowing.

 

Q: I'm writing because we're in a discussion about the role that water and moisture play in bending wood. I know that how well a piece bends is dependent on the moisture content of the wood, but what role does the moisture play in the plasticity of the sample? If you were to take a piece of wood that's oven dry and were able to cool it in a chamber with 0 percent RH, would application of heat alone be enough to bend the piece and maintain it's shape?

 

A: Heat does indeed make the wood plastic, which is what we need in order to bend wood and have it maintain its bent shape. (If wood were only elastic, it would bend, but then when the bending force was released, the wood would return to its original shape.)

Moisture extends the plastic range of wood. I can't say which factor, heat or moisture, is more critical. Both are important, but perhaps heat is a bit more important than moisture. Note that at 0 percent MC, the wood is so brittle that the plastic range is very small. Failure can occur as soon as the wood bends a little bit, even when using heat to plasticize the wood.

 

Q: We're trying to sand small wood parts in quantity and were wondering if it's possible to use a tumbler for this. Please reply as soon as you can. Thank you.

 

A: A small tumbler is ideal for developing a smooth surface on small parts, especially for harder woods such as maple and hickory. We would normally suggest that you build it yourself. The size might be about 3 feet in diameter and 18 inches wide. It would spin at 5 rpm, although this is variable, so allow for changing drive pullies.

You may also wish to consider adding a little bit of wax (nugget size) and enhance the coating of the wood. In fact, I have one client who did this and was able to eliminate solvent-based finishing. There are many different types of waxes, so you'll probably have to experiment a bit to find the best one for your products.

 

Q: We have some oak pieces that have developed some cracks running along the grain. Some cracks are near the ends and some are away from the ends. These are in church pews. The cracks seem to have appeared this year (November) even though the seats are 3 years old. Give me some guidance on what to do, please.

 

A: We're tempted to say that the oak is drying out and shrinking and that the shrinkage forces are causing the wood to crack. However, we do have to remember that dry oak is very strong so shrinkage forces, and therefore MC changes, would have to be large and rapid.

Further, I assume the wood is coated with a finish. The finish will retard the speed of the MC changes when the RH changes. Therefore, the bottom line is that initiating new cracks in a finished piece of furniture is really quite difficult, if not impossible in most situations. (I'm including the fact that a church often has wider extremes in humidity than a home or office.)

I believe that what we're seeing in your situation are old cracks or checks that are reopening. Perhaps if the church environment has changed so that the heat is on more than in past years, the drier air is causing the extra shrinkage that didn't occur in past years.

To confirm my conclusion, look inside the cracks for evidence of stain or finish seeping into the crack when the pew was initially finished.

As far as repair goes, you need a rigid material to fill the cracks. A soft filler will, when the RH increases, be squeezed out. Instead of just filling the cracks with epoxy or other hard filler, I suggest using a wood spline technique for repair. If appropriate, you might also use a butterfly patch to help distribute the stress.

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