ENCYCLOPEDIA OF WOOD

WATER AND WOOD

As the moisture content of a plain-sawn plank of 2-by-10 softwood lumber drops below the fiber saturation point (FSP), the wood shrinks across the grain. At 17 percent, the board is lA inch narrower than it was at its FSP. It loses another Vi inch of width when kiln-dried to an 8 percent level. Shrinkage depends partly on a species’ density; generally, denser woods shrink and swell more than lighter ones. Sapwood also tends to change dimensions more quickly than heartwood.

Moisture changes in wood can cause problems for a piece of furniture, some merely annoying, others quite seri­ous. A freshly cut log can contain water equal to twice its dry weight; made into a piece of furniture, it can turn stone dry. This capacity to hold different amounts of moisture under different conditions causes wood to swell and contract. If this property is not consid­ered by the cabinetmaker, a drawer that opens smoothly in the dead of winter can swell and jam shut in the humidity of summer. A perfectly square carcase cabinet can pull itself apart as humidity levels change from season to season.

The amount of water in a piece of wood is often expressed as a percentage of its oven-dry or water-free weight. For example, if a 50-pound block of wood drops to 30 pounds after oven­
drying, the weight of the shed water—20 pounds—divided by the wood’s dry weight—30 pounds—is the moisture content of the original piece: in this case, 66 percent.

Wood holds moisture in two ways: as free water in cell cavities and as bound water in cell walls. As wood dries, free water is expelled first. When this is all discharged, the wood reaches what is termed its fiber saturation point (FSP). At this point, the cell cavities are empty, but the bound water remains, permeat­ing the cell walls. For most woods, the FSP occurs between 23 percent and 30 percent moisture content depending on the species, with 28 percent the average. The key point to remember is that at the fiber saturation point, there is no dimen­sional change in wood from its freshly cut size. It simply weighs less. However,

if the wood dries further, falling below the FSP, it loses bound water from its cell walls. The cells shrink and so does the wood. As the illustration on page 80 shows, the more bound water a board loses the more it shrinks.

The only way to prevent wood from shrinking is to treat it with a chemical such as PEG-1400. (PEG is an abbrevi­ation of polyethylene glycol; 1400 is the chemical’s molecular weight.) PEG-1400 diffuses into the wood and replaces the bound water, keeping the cell walls ful­ly swollen. The treatment is suitable only for green wood, however, and is most popular for use with turning and carv­ing blocks.

Wood gains and loses moisture as the relative humidity in the air around it changes. If the relative humidity rose to 100 percent, a piece of wood would reach
its fiber saturation point and be at the same size as when it was milled. If rela­tive humidity fell to 0 percent the wood’s moisture content would drop to 0 per­cent. Because relative humidity falls between those extremes only a portion of the bound water is lost. Realistically, the moisture content range of most stock is 5 to 20 percent.

From season to season, the relative humidity in a given location can vary 80 percent or more. This is because relative humidity and temperature are closely intertwined. Warm air can hold more moisture than cold air. As a result, when cold winter air is heated, as it is in homes and workshops, its ability to hold mois­ture increases dramatically. If there is no added moisture available, the relative humidity plummets to extremely low levels. In contrast, hot summer air can

hold a large amount of moisture. But when cooled indoors, it can hold much less. The result can be fairly high rela­tive humidity. Both extremes cause changes in the moisture level of wood and in its size.

You can take several precautions to counteract the effects of changing humidity levels. If you store lumber indoors, try to keep the relative humid­ity fairly constant, using a dehumidier, for example, when the levels get too high. And although you may not be able to control the environment where your fur­niture will end up, you should build the piece to compensate for wood move­ment. When cutting a panel for a frame, for instance, leave a '/4-inch gap in the grooves that will house the panel. The extra space will allow the panel to expand and contract as humidity levels rise and fall.

Relating a wood’s equilibrium moisture content to relative humidity

Whether wood is in the form of a log, a kiln-dried board or a finished piece of furniture, its moisture content varies with the relative humidity of the air around it. As humidity rises, so does the wood’s moisture content, expressed in percent in the graph shown at left.

The moisture level of a piece of wood eventually reaches its equilibrium moisture content (EMC) after the hum­idity stabilizes. The EMC also varies depending on the temperature. The band shown in the graph covers EMC values for most woods at 70 degrees Fahrenheit. Those values decrease slightly at higher temperatures and increase marginally with cooling.

Tangential and radial shrinkage

Lumber does not contract uniformly; as shown by the dotted red lines in the illustration at left, tan­gential shrinkage—parallel to the growth rings—is about twice the radial shrinkage, which occurs across the rings. This difference accounts for the warping of boards and panels as wood expands and contracts with fluctuations in moisture content. Shrinkage along the length of a board is usually negligible. A 2-by-10 plank that shrinks Уг inch in width, for example, might lose less than Vie inch along its 8-foot length.

SHRINKAGE VALUES OF DIFFERENT WOOD SPECIES

Finding dimensionally stable wood

The chart at right shows the typical amount of shrink­age of various species in both the tangential and radial directions when green wood is dried to zero moisture content. (Values are shown in percent; a 10 percent value in the tangential column, for example, means that a 10-inch-wide board would shrink by that amount to 9 inches wide.) Although tangential shrinkage exceeds radial contraction in every case, no two species shrink by the same amount. The average is 8 percent tangentially and 4 percent radially. The key column of the chart is the third: The T/R ratio indicates the proportion of tangential to radial shrinkage. The lower the ratio, the less the differential between the two types of shrinkage and the more stable the wood. Species with relatively low ratios, like mahogany (1.4) and teak (1.8), are less susceptible to warping than woods with higher ratios, such as beech (2.2).

MEASURING THE MOISTURE CONTENT IN WOOD

Reading moisture content in thick stock

The metal pins on commercial resis­tance-type moisture meters are typi­cally about 1 inch long. Since the pins should ideally reach the middle of a board when taking a read­ing, they tend to be inade­quate for stock that is thicker than 2 inches. You can extend the reach of the pins by driving two finishing nails into the wood until the tips reach the middle and the heads protrude from the surface. Then touch the meter pins to the nail heads and take a reading.

SUILP IT YOURSELF

SOLAR KILN

The kiln shown below, with a roof and front wall of tempered glass, provides a natural drying cycle. During the day, warmed by sunlight, the wood dries; at night, the moisture in the wetter core of the stock migrates toward board surfaces, ensuring more even drying.

Build the kiln according to the amount of wood you plan to dry and the space you have available. If you are
reusing glass parts, such as used patio doors or storm windows, you may wish to base the size of the kiln and its framing on the dimensions of the recy­cled material. The kiln shown below and opposite is 5 feet wide, 16 feet long and about 8 feet high.

Choose a sunny location for the kiln, then level the surface and spread grav­el over it. Lay concrete blocks at 2- to З-foot intervals as a foundation, then

build a base frame of pressure-treated 4-by-4s on top of the blocks. The rest of the framing and rafters are con­structed with 2-by-4 stock; the floor, walls and door are made of Winch exterior-grade plywood.

Once the base frame is in place, nail the floor on top of it, then con­struct a stud wall frame for the front of the kiln. Cut the studs to length and nail a sole plate to their bottom

ends and a top plate at their top ends. Recess the front edges of the studs about Va inch from the front edge of the plates to provide a ledge for the glass panels. Make the gap between the center of the studs equal to the width of the panels, spacing them no more than 4 feet apart. Set the wall frame upright and nail the sole plate to the floor and base frame. Repeat the procedure to make and attach wall frames for the back and sides of the kiln, this time without offsetting the studs from the plates. Cut the studs for the side walls so that the roof will have а 4-ІП-12 slope (4 inches of

rise for every 12 horizontal inches).

Cut the roof rafters to allow a few inches of overhang at the front and back, then nail the rafters to the top plates, spacing them to fit the glass panels to be installed on the roof. Tack fascia boards to both ends of the rafters, leaving a small lip above the top edges of the rafters to hold the roof panels. Cover the opening between the fascia and the back wall with a l-by-4 board as a soffit. On the front of the kiln, this space should be left open. Next nail the walls to the out­side edges of the studs on the back and one side, installing hinges and

hasp locks on one side wall to convert it into a door.

To install the glass panels on the roof, set them on adjacent rafters, leaving ample space between the pan­els for screws. Then fasten down 1-by - 3 wood strips that overlap the edges of the panels to hold them in place. To accommodate the glass panels in the front wall of the kiln, cut notches in the bottom edges of the rafters, then slide the panels up into the notch, resting the bottom of the pan­els on the sole plate ledge. Screw 1- by-3 wood blocks to the front edge of the sole plate to support the middle of each glass panel.

To keep the air in the kiln circulat­ing, fasten a piece of plywood as a baffle to two adjacent studs on the back wall, leaving an opening between the baffle and the top of the studs for air to enter. At floor level, construct a frame on the front of the baffle for an exhaust fan. The fan will pull warm air down through the baffle and circulate it through the kiln. Install the switch for the fan on the baffle, along with a thermostat to start the fan when the air temperature reaches 80°F and a timer to turn the fan off at night.

To keep the lumber stack off the floor, nail down 2-by-2 support pieces spaced about 16 inches apart. Pile the lumber as you would for air-drying, leaving adequate space between adja­cent boards and separating the layers of stock with l-by-2 stickers.

If you cannot supply electricity to the kiln, leave additional space between the boards to ensure adequate air cir­culation. Drying of the wood may take several months; use a moisture meter (page 83) to check on the lumber’s moisture content periodically.

ESTIMATING WOOD MOVEMENT

A thin line of unfinished wood is a telltale sign of wood movement in this closeup photograph of part ofa frame-and-panel door. After the finish was applied, the humidity level in the room where the cabinet was stored gradually dropped, causing the wood to contract. A similar amount of movement in a carcase construction might have threatened the piece’s structural integrity. The frame-and-panel design, however, allows for wood’s natural swelling and shrinking. The panel floats inside a fixed frame with room for Vi inch of movement horizontally.

ЄНО? TIP

Making a moisture indicator

Keep this moisture gauge in your shop as a reminder of the relationship between humidity and wood movement. To make the gauge, cut a length of wood from the end of a glued-up panel, or bond a few wood blocks together edge-to-edge. Nail a metal pointer to one end of this arm, then attach the arm’s other end to a piece of plywood. Drive a screw through the pivot hole of the pointer into the plywood so that the pointer is parallel to the end of the arm. Leave the screw loose enough to allow the pointer to pivot. As the relative humidity fluctuates and the arm swells or shrinks, the pointer will swivel to either side.