Q: We live in a fairly humid environment in southern Florida. As a result, even though we target 6 to 7 percent MC for the lumber we use, the wood gains moisture in storage and during manufacture. Then when we ship it north, it dries and we have some problems mainly splits, but a little warp at times too. Can you suggest some possible approaches to correcting this problem of moisture gain? Thanks.

 

A: Certainly, the only way to address this problem is to store the lumber, as well as the work in process, in a controlled-humidity environment. For ideal, trouble-free conditions, 37 percent RH is your target. The temperature is not critical.

Storing lumber is easily done in a fairly tight warehouse. In most cases, making a small room enclosed in plastic (basically sealed tight to avoid moisture entry) is the reasonable way to store goods in process. Lumber could also be stored this way. Although the wood can spend two or three days out of the room, the work needs to go back in the room for an equal amount of time to return to the correct MC.

You have two options for controlling the RH. First, you can add heat to the air. Adding about 15 to 20 F to humid outside air will typically achieve the desired 37 percent RH. This is probably the most practical approach for lumber storage. In fact, if you have a black roof and use a fan to blow the heat down from the attic area, you will probably have enough heat. If the RH is too high, however, you will have to add additional heat.

In the plant, the best option is probably to use an electric dehumidifier that is commonly used to dehumidify basements. Put this unit in the plastic room. As there is no major source of moisture entering the room, the DH unit will easily achieve the correct RH conditions.

You do need to monitor the RH and MC. I suggest a $25 electronic RH sensor from a store such as Radio Shack. Also, so you can check the surface and shell MCs, you should use a pin-type moisture meter. (Special note: The core MC should always be at the MC it was when it left the kiln. Core MCs do not change very much or very rapidly, even when the lumber is stored at more humid conditions than desired.)

 

Q: I have some white oak veneer that has small cracks or grain separation on the surface. The cracks go in at a little bit of an angle to the surface. They appear to have a feathered edge. This edge will catch on the tack cloths (when wiping stain, for example) and lift up, and even break off, exposing unstained wood. The cracks will also result in cracks in the finish, perhaps not showing up until the customer gets the item. What is the problem? Note that we seldom see the problem when we get this veneer off the boat, but when we move it to our inland plant, they show up, especially in the winter.

 

A: This defect is indeed grain separation, and is also called lathe checking or feathering. It results from a fracturing or tearing that occurs when the veneer is manufactured. Such tearing, which occurs on the loose side of the veneer, is normally prevented by using a sharp knife, reasonable feed speed, power, preheating the logs and proper use of a pressure bar on the lathe or slicer (to keep the wood surface from pulling apart).

Definition: The tight side, which will not have lathe checks, in a slicer or rotary lathe will be the side that the knife touches first, or the side furthest from the stock being cut, while the loose side is the side that is touched second or is closer to the core or stock being cut. The loose side is the outer curved side as the veneer is being sliced; this outer curved side is being pulled apart more than the interior side.

There will almost always be a few veneer checks, but they should be quite small and infrequent. Often, when veneer is assembled into a panel, the loose side with the veneer checks is glued onto the core or substrate so that the tight side (free of checks) is exposed or is on the outside. Unless the veneer is sanded heavily (which would sand past the tight side without any checks and into the loose side with the checks), the checks in normal veneering with the loose side glued to the substrate will not be a problem. Of course, if the lathe checks are very deep, then it will be a problem no matter how the panel is assembled.

Once the lathe checks are formed, they are never "healed." Therefore, if a veneer panel with the loose side on the outside that appears to be okay initially is exposed to a drier humidity, the drying that results will cause some shrinkage and opening of these pre-existing checks. If the moisture increases due to higher humidities, the checks will tend to close. This is why you see the checks at your plant (a dry location), but not in a more humid area (such as at the dockside).

 

Q: I have a flooring project we have just completed for a local college. The floors which we glued down to the base have buckled and lifted several inches off the floor at the top of the buckles. The contractor was not controlling the humidity after installation. Readings indicate that the relative humidity was 85 percent with a temperature of 65 degrees. Recently, the moisture levels have since been controlled and are stabilized below 55 percent, but the buckles are not entirely gone. Am I correct that this a moisture problem due to the high RH?

 

A: The buckles are caused by an installed and well-anchored floor increasing its MC and swelling. A floating floor would expand in size and stay flat, but when the floor is well-fastened, the stress builds up until, eventually, the floor "lets loose" and a buckle develops. The question is "What is the source of moisture?" It could be a leaky roof, broken pipe, wet mopping or very high RH. I think that the high RH in this case is the major cause.

 

Q: I am dealing with a serious problem. We've been manufacturing a white pine dowel for use as a handle. What has happened is the handle is breaking too easily. The MC is 13-16 percent. Is there anything we can do to help with the breakage?

 

A: You need to consider using a different species of pine that can be up to twice as strong. The key item will be the density of the wood: the denser the wood the stronger, usually by a large amount.

With respect to strength, moisture content itself is not an overwhelming issue in your case. So lowering the MC will not increase the strength substantially.

Also, if the handle holder crushes the wood when the dowel is inserted, it will weaken the wood substantially. You want a good fit, but not too tight. I suspect that you have an extremely tight fit now so that it does not loosen as it shrinks. Specifically, your wood is a little on the wet side. The dowel will shrink as it dries to its final MC (perhaps 10 percent if stored in an unheated area and 6 percent if stored in a heated area) and become loose unless it is really tightly wedged in when first made.

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