WOOD STORAGE AND FURNITURE DESIGN
By Ross Roepke, Dec. 2002
This is an extension of the earlier article by Loyd Ackerman, and I will discuss some of the issues related to storing wood and using wood in furniture design and construction. I have heard woodworkers discussing whether kiln dried or air dried lumber was better for building furniture. I want to shed a little light on these subjects based on my experience and observations over the past 50 years.
For reference, air-dried lumber reaches an equilibrium moisture content (EMC) between 12% (in April) and 14% (in August) in this region of the country (Ref. Modern Woodworking, Nov/02) because the mean relative humidity is above 70%. The problem occurs when a larger piece of furniture is taken into the house where the relative humidity in the winter months may drop to 35% for extended periods. Then the EMC may drop to 6% or 8% depending on the region of the country. Then, as the wood dries, it shrinks; the amount depends on the type of wood, direction of the grain, and the protection provided by the finish.
This shrinkage takes a finite time to occur. And the destructive effects depend on the relative movement of two joined pieces and how much of that movement can be accommodated by stresses in the wood and in the glue joint itself. These factors may let the furniture survive for several years. But, the repeated cycling from summer to winter and long-term embrittlement of the glue has deleterious effects on joints. If you desire your furniture to survive for your great grandchildren to enjoy, special care is needed in the design of the furniture.
That design is probably more important than the EMC of the wood itself. If the 6% change in EMC is a concern, the design is probably not ideal. There are times, however, where cross-grain gluing is necessary such as on the rails of chair legs or table legs. I just read last year that more chair leg failures are caused by wood shrinkage than by misuse of the chair.
For small pieces of furniture where any cross-grain glue joints are short, or even for large pieces where there are no cross-grain glue joints, the wood shrinkage is not likely to cause serious joint stresses. Over the years, I have observed numerous cases where hobby woodworkers have built tabletops firmly attached to the rails or where they have glued a face board across the end of the tabletop to cover the end grain. In most cases, the tabletop will be split several places across the tabletop because of wood shrinkage. Even I have done this in my earlier years; but I learned from my disappointments. I now make it a point to not ever glue any cross-grain over 3 or, at most, 4 inches. Special attention is necessary to keep from doing this, both during the design and construction, because it is so easy to slap glue onto a piece where it can later create problems.
Now back to the kiln vs. air-dried lumber. We go to the lumber yard and buy kiln dried lumber with the concept that it is safe to build with. But, that lumber has likely been stored in an open shed exposed to the ambient humidity for maybe several months. Whether originally air-dried or kiln dried, the chances are that it is at an EMC of 12% or higher. Or if you bring it home and keep it for several months in your unheated shop, it matters little what treatment it has had in the past.
Lumber at 12% EMC is adequate for making small pieces, or where there are no cross grain joints, or any time wood shrinkage will not produce joint stresses. The only safe way to build furniture is to design it so that variable EMC levels are not critical to it’s endurance.
There are other ramifications of using high EMC wood though that are quite subtle. A glue joint can sometimes be felt because the glue extends through the finish after the wood shrinks. Or, the glue may not cure as fast or it may not provide a full strength joint. And sometimes stresses or distortions are produced in a piece of wood, such as in stress wood, as it dries.
I used to store my wood supply in my garage or in a lean to. About 10 years ago I bought a 12 X 20-ft barn from Belvidere and installed racks for wood storage. I use a dehumidifier to keep the relative humidity to about 35% most of the year. However, a dehumidifier is ineffective below 60 deg. F so I turn it off over the winter months. There are desiccant dryer systems that work in cold weather but they are quite expensive.
The article in Modern Woodworking recommends using heat to control the EMC. To maintain the EMC at 7%, the storage area should be heated 22 deg. F above the outside morning temperature of the day. This reduces the relative humidity and thereby controls the EMC. I have not done this because I have not yet insulated my barn and it would be prohibitively expensive to heat it now. Now I depend on the dryness of the wood to help maintain the relative humidity of the building. On warm days in the winter, I start the dehumidifier, when I think of it! I need to get a thermal switch to turn it on automatically.
I only learned much of this recently so have not benefited from the information. So I pass it along to others so they can receive some benefit from my experiences. Please feel free to ask if you have any questions.