Editor’s note: I’ve known Gene Wengert for just 26 years, but he’s been answering our readers questions faithfully, factually, and with wit for 45 years. As best we can tell, his first column ran in the May 1979 issue of FDM magazine. It was run initially as a two-part article with the second installment in the June 1979 issue of FDM. It proved very popular with readers and propelled Gene to industry celebrity, affectionately dubbed “The Wood Doctor.” Here, then is the second part of that first article: “Preventive Medicine for Furniture.”

Shrinkage problems in wood furniture can show up in various ways, not just warp. However, it is appropriate to stress that if wood is dried to the same moisture content that it will experience in use, and if it is manufactured and stored at this same moisture content, none of these problems will appear.

Symptoms, diagnoses, cures
A) Open joints in edge glued panels. Occasionally, especially in late fall or early winter, when a large panel is made from several smaller pieces of wood edge glued together, the glue joint will open, usually during initial processing subsequent to gluing. Almost without exception the cause of these open joints is shrinkage of the wood after gluing (i.e., the wood is drying further), often accompanied by a medium to low quality glue bond.

This shrinkage is most pronounced at the ends of a panel because end grain dries faster than edges or faces of the panel. As a result, the ends of the panels are shrinking rapidly while the center of the panel is not shrinking.

The key for elimination is proper drying to a low enough moisture content and plant humidification, although end coating with a moisture barrier, such as Nelsonite or applying masking tape to the ends can help by slowing down the change.

B) Open joints. Open joints can also occur later in manufacturing if two pieces of wood are joined so that the grain of one is at an angle of 90 degrees to the other (see Figure 2 in Part 1). Quality furniture manufacturers that must use such a design will design so that the one piece can move or float as the other piece changes size.

One common design is a door for a cabinet with a solid wood rail on all four sides and a veneer panel in the center (Figure 1). The panel should be attached at the top and at the bottom at only one spot if it is to be free to swell and shrink in width with changes in humidity, while the rails remain constant in length (i.e., the rails are longitudinal and wood doesn’t shrink or swell much in that direction.)

If the panel isn’t free to move, then when changes in relative humidity occur, especially abrupt changes, warping is likely and checks may appear. The same effect can be noted in a dowelled joint, especially when the horizontal member is quite wide (Fig. 2).

C) Open miter joints. Of all the ways available to join wood, the miter joint is most sensitive to changes in moisture. Consider a miter joint with 2-inch rails (Fig. 3). Now assume that these rails dry (loss: 3 to 4 percent moisture). They will shrink approximately 1 percent in width and the 45-degree angle will become slightly smaller and, as a result, there will be a gap at the heel of approximately 0.03” (Fig. 3).

D) End cracks. Small cracks on the end of a tabletop or other flat surface are almost always the result of too rapid drying of the ends. (Lumber dries up to 50 times faster from the ends than from the faces. Therefore, the ends shrink rapidly while the rest of the piece shrinks more slowly.) The key again is proper moisture control in manufacturing and storage with no abrupt changes in RH.

E) Finish checks and cracks. As the wood below a finish expands and contracts with the annual fluctuations in humidity, it stresses the finish. On a microscopic level, these fluctuations are most severe on wood with large rays, such as oak, and are often compounded by checks in the veneer that developed during veneer manufacturing. Depending on the strength and elasticity of the finish and the change in humidity, the stresses may exceed the finish’s strength and small cracks will be seen. Control is often difficult to achieve but it involves improvements in veneer quality, in crossbands for veneer, in surface preparation and in finishing.

F) Sticking drawers/doors. Fine furniture is made to exacting size standards, but because wood shrinks and swells, the size will change as humidity changes. In most homes in the winter, drawers and doors fit well (if not even a little loosely in unhumidified homes). Yet in summer as the humidity increases, the fit may become quite tight. Because the manufacturer designs for the expected to typical RH in a home, occasionally extreme climates may result in much higher or lower RH’s and this in turn results in problems.

G) Loose joints. Often chairs and other glued furniture items will develop loose joints — usually a dowel joint — after several years. Most joints when first made are stronger than the wood itself. Over time the wood or glue does not dry out, but rather the repeated stresses from the wood drying in the winter and swelling in the summer cause small failures that compound over the years. Adhesesives today are much improved over past adhesives, so we can expect much less trouble with loose joints.

All of these problems can be accentuated if wood is exposed to liquid water, rather than high relative humidity.

Business remedies
The first step in producing trouble-free furniture is to dry wood to a uniform moisture content — 5 to 7 percent MC in the winter (with strict adherence to the 7 percent maximum) and 6 to 8 percent in the summer. After drying, the wood must be kept at an RH so that it will not gain or lose moisture (30 percent RH winter, 36 percent RH summer with no more than plus or minus 5 percent variation for more than 24 hours.). And this humidity must be maintained in lumber storage, manufacturing, shipping, warehousing, and display for best results.

To control RH in an unhumidified area such as warehouses, heat can be added according to the following rule of thumb: For 30 percent RH (equivalent to 6 percent MC), heat the storage area approximately 20 degrees F above the outside average temperature. For 36 percent RH (equivalent to 7 percent MC), heat approximately 15 degrees F above the outside average.
For extremely dry areas (such as Denver or Phoenix) or for extremely humid areas (such as Miami or New Orleans), some adjustments in these humidity and MC levels may be required. 

Home remedies
When a homeowner receives a new piece of fine furniture, the safest thing to do during the heating season is to assume that it may have come from a higher humidity condition. Therefore, it must be dried slowly to reach equilibrium with the environment in its new home. Manufacturers and dealers should advise to avoid placing it in a hot room or in a location where it may receive direct sunlight or direct heat from a a vent or radiator (At the same time, however, don’t put the furniture in a cool location or in front of a humidifier, as the humidity in this area may be too high.)

After a week or two, it is usually safe to move the furniture to any desired area. Wax — only wax without water — is beneficial. Avoid any water on the furniture. A humidifier in the winter in the colder climates is also beneficial for furniture to avoid humidities below 30 percent. (Vapor barriers in house walls may be required.)

One helpful hint in tracing a cracking problem to its source is to cut a crack or check open and with a magnifying glass look to see if there is any sign of the finish inside the crack. If so, then it’s certain that the crack developed prior to finishing. If the crack has small particles of dirt in it, then it is likely that it is an air-drying check.

Another hint: When measuring the moisture of furniture, it is usually too late (i.e., the piece has already dried out) once the problem has developed.