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 that first article: “Preventive Medicine for Furniture.” —W.S.

Every furniture manufacturer wants to produce a product that will leave his factory in a condition that will never cause a customer complaint. Unfortunately, complaints do arise. Furniture may be returned to dealers for loose joints, splits, or warped components. Any return means loss of profit and, often worse, loss of goodwill.

Many problems with furniture in the customer’s home can be related to the manufacturing process and the basic wood properties. The majority of these problems can be associated with the fact that when wood gains moisture, it swells, and when it loses moisture, it shrinks.

The patient
Wood is an organic substance made up of cells shaped like hollow cylinders, much like a bundle of soda straws. The cells have a strong affinity for water, both liquid and vapor. Most of the cells in wood run vertically in the tree (or along the length of a board) and provide the mechanism for moving fluids (sap) in the living tree from the roots to the leaves and back to the roots.

There are a few cells (up to 20% in some species such as oak) that run horizontally in the tree, radiating outward from the tree’s center, that provide horizontal transport of sap in the tree trunk. These cells, called rays, are visually prominent in woods like oak and beech.

As a result of the orientation of these cells, wood properties are different in different directions. (See Figure 1.) The three “directions” are longitudinal, tangential, and radial; they are defined as follows:

The longitudinal direction is the along-the-grain or lengthwise direction. The most significant property in this direction is that the wood does not shrink or swell appreciably when the wood gains or loses moisture.

The tangential direction is the direction parallel or tangential to the tree’s growth rings. In most cases, lumber is sawn so that the rings are parallel to the wide face. This creates the cathedrals which give wood its pleasing appearance. Wood can shrink and swell up to 10% in the tangential direction as the moisture changes from wet to dry. This is significant; a board 10 inches wide can shrink 1 inch as it is dried!.

The radial direction is the direction from the center of the three outward. Wood shrinks approximately half as much radially as it does tangentially. This is because the radial shinkage is restricted by the rays. Because of this, radially sawn (or quartersawn) lumber is preferred for such applications as wood siding on houses, where low shrinkage results in increased paint durability.

Allergic reaction to humidity
Relative humidity (RH) is the amount of moisture in air — 100% RH indicates the air is completely saturated; 0% indicates the air is completely dry. Relative humidity is important with wood as (1) wood shrinks and swells as the RH changes and, (2) moisture content changes. This response of wood to RH is illustrated in the chart shown here.

In a typical home, the RH will average around 40%, ranging up to 100% on damp summer mornings and dropping below 10% in the winter in a heated home without a humidifier.

Wood responds to daily and yearly cycles of humidity very slowly. Therefore, in manufacturing furniture, wood is prepared for a home environment by drying prior to manufacture to a moisture content that is the expected average moisture content in the home. Typically, this is 6-8% MC and is equivalent to 30% to 40% RH. (Incidentally, wood does not dry out year after year; it is as dry as it’s going to be when it’s made, if it’s dried correctly.)

It is important to realize that the daily and yearly cycles of RH normally do not cause serious problems for properly manufactured lumber. The finish helps to smooth out the cycles. However, the thinner the piece of wood, the more likely are the RH cycles to cause warping. In addition, the abruptness of the change in RH can be a factor.

Although the RH can change over six months from damp in the summer to dry in the winter without much problem, such an abrupt change over several days can be a catastrophe. (It is for this reason that many furniture companies dry wood somewhat drier in the winter than in the summer, so that the adjustment, when the piece reaches the home, is small.)

At this point, we have established that wood shrinks and swells with changes in RH. But, did Washington and Jefferson have problems, too? The answer is probably not — at least not moisture problems related to relative humidity.

This is true because until recently homes were not built so tightly and thermostats were not set so high. This in turn meant that the relative humidity in a home was higher and varied less than it does today. Of course, the trend toward year-round climate control in homes is beneficial for wood furniture.

Symptoms of moisture allergy
Some examples of common problems created by differences in directional shrinkage are as follows:

(1) When a piece of wood is glued across the end of a board or panel with its grain at 90 degrees to the panel (Fig. 2), and if the moisture in wood changes due to changes in relative humidity (RH), the panel will change width. However, the piece of wood glued across the end will not shrink in this piece. Depending on various factors, such 90-degree construction will likely warp or will crack when RH changes.

(2) A wide board, flat sawn, tends to warp or cup as changes in RH occur because the face nearest the tree’s center behaves more like a radial face than the bark face. As a result, the bark side will shrink more as RH drops than will the other face. As a result, warping is likely, when RH changes.

Let’s look at a common situation.

Quickie quiz: An oak rail was manufactured at 10% moisture content. However, this was too wet and the piece dried to 4%. The holes were originally 2 inches apart and were 0.310 inches in diameter. After drying to 4%, describe the size and shape of the holes and the distance apart.

Answer to quickie quiz: The holes on the end grain will be approximately 1.96 inches apart (over 1/32 inch closer together). They will be smaller in diameter and oval shaped ranging from 0.307 to 0.304 inches in diameter. The holes on the edge grain will still be 2 inches apart and will be oval shaped ranging from 0.310 to 0.309 inches in diameter.

Why is this so? First, the wood will shrink as it dries. The shrinkage as moisture changes from 10% to 4% is approximately 2% in the tangential direction and 1% in the radial direction. In the longitudinal direction, there is no appreciable shrinkage. These shrinkages are applied to the hole spacings and diameters as appropriate.