Controlling Wood Movement – Essential Facts to Consider in Furniture Making

It is important to consider two simple and essential facts while making furniture successfully. The first is that wood moves. This means that it takes in and releases moisture, resultantly it swells or shrinks. This swelling and shrinking may sometimes be hardly noticeable or it may be quite significant, depending on the type of wood and the amount of humidity in the air. The second fact is that nothing can stop this movement.

Wood Movement Across its Width

Wood swells and shrinks across its width but not substantially along its length. That’s because the basic cell structure of wood consists of a series of fibers packed together longitudinally, similar to a bundle of straws. As they absorb or release moisture, the tubes change in diameter, resulting in expansion or contraction across the total width of the bundle or, in our case, aboard.

One of the worst features of wood movement is warp, the result of boards expanding or contracting in uneven patterns, deforming from their original sawn or milled shape. Typically, extreme warping happens during the initial drying stage when the wood is shrinking rapidly. But small degrees of warp continue long after the wood is dry and can be a constant source of anguish to a woodworker.

Controlling Wood Movement

Recognizing Grain Orientation

Although wood movement can’t be stopped you can limit it to some degree. The way your wood is cut has a big impact on movement. The way a board is sawn from a log determines its grain orientation. There are three types of grain orientation you will come across in a sawn board:

• Plainsawn – also called flatsawn

• Riftsawn – also called rift cut

• Quartersawn

Each term denotes how the annular growth rings are oriented in a board. Knowing the grain orientation of a specific board is important because it will help you predict how the wood will behave during cutting, how it will react to moisture and finishes, and how it will look in a piece of furniture. The easiest way to determine the grain orientation is to look at a board’s end grain.

Quartersawn Board – The most Stable Wood

In terms of wood movement, a quartersawn board is the most stable because expansion and contraction occur in a relatively even manner over the width of the material. Riftsawn wood stands second in terms of stability, while plainsawn boards will often cup or twist disorderly due to changes in the direction of the grain. Quartersawn wood is your best bet when you need the most dimensionally stable material since any movement occurs more evenly across its annular rings with much less chance of warp.

Designing Furniture for Movement

Another way to control wood movement is to design and build your furniture to allow this movement to take place freely without compromising the shape of your project. For example, you can fit a wide panel to float inside a narrow frame—a typical scenario in frame-and-panel construction. The wider, more movement-prone panel can then expand or contract freely without pushing or pulling on the frame. Gluing or otherwise fixing the panel to the frame will sooner or later result in failure by either breaking the frame joints or splitting the panel. Good furniture design must accommodate wood movement to ensure that the joints and other constructions will survive.

Ideal Moisture Content-Range

It must be kept in mind that the moisture content of wood constantly changes in response to its environment. Ideally, you would construct and assemble your wood parts at the mid-range of expected MC (Moisture Content). To avoid trouble, it’s best to use wood with a moisture content between 6 percent and 8 percent. However, it is worth mentioning that our completed projects will be subjected to greater swings in MC, anywhere from 4 percent MC to 14 percent MC. This depends on where your furniture resides and the specific atmospheric conditions surrounding it.

Using Dry/Seasoned Wood to Control Wood Movement

To further control wood movement, work with “dry” or “seasoned” wood. This refers to material that’s been properly dried from its green, fresh-cut state to suitable moisture content. The drying process starts with air-drying, followed by a combination of air- and kiln-drying. The basic approach involves the initial removal of all the free water in the green wood, which is liquid and moisture trapped inside the cells. Following that, the wood is further dried by removing a portion of the bound water—the moisture that saturates the walls of the cells. It’s important to understand that as the bound water evaporates, the wood starts to shrink. This is why your drying schedule should give the wood enough time to shrink slowly. Otherwise, unequal stresses in the wood will lead to warping and cracking. However, drying too slowly will invite a fungal attack on the surface of the wood. Therefore, it’s necessary to carefully monitor the moisture content of the wood as it dries.

With material that’s been dried to the correct moisture content, you will minimize the amount of movement and make it more predictable. For details, consult our blog “Drying the lumber – why is it necessary?”

Controlling Your Shop’s Climate

Another important consideration is the ambient moisture content of the air surrounding your wood and, ultimately, your furniture. To prevent the various furniture parts from expanding or shrinking excessively, it’s important to work in a shop environment where the relative humidity (RH) isn’t too high or too low. Bringing your wood into the shop is a good start toward keeping it at an optimum moisture content, but ongoing measures in the shop itself are necessary to ensure that your material remains in this ideal range.

Knowing and controlling the climate in your shop is the best way to keep your stock in good condition. Your shop’s relative humidity (RH) level should be in a range of 30 percent to 50 percent at 75˚ F (24˚ C). A relative humidity of 40 percent would be ideal. Wood properly stored under those conditions should be in the 7 % MC range, ideal for turning into furniture.

Impacts of Finishing on Wood Movement

A good finish with multiple coats can limit a board’s movement. But no finish will stop this movement entirely because the finishes we use in woodworking are not impermeable to moisture transfer. On a microscopic level, the wood still “breathes” and will respond by expanding or contracting as moisture finds its way through the finish and into the cells and pores of the wood. However, following a proven finishing treatment will help greatly in controlling the amount of wood movement in your furniture.

Left to right; Plainsawn – Riftsawn – Quatersawn

Oak Boards