Know Your Materials

The following is excerpted from “Shaker Inspiration” by Christian Becksvoort. 

Not too many woodworkers can claim five decades of business success, but Becksvoort is among them. In “Shaker Inspiration,” he shares not only his woodworking knowledge and some of his best professional techniques for producing top-quality work, but also the business advice that helped him establish and sustain his long career in a one-man shop. Plus, he shares measured drawings for 13 of his own well-known furniture designs and seven Shaker pieces that he’s reproduced.

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Before starting any craft or trade, it’s essential to know the material you plan to work with. Most of us know the rudimentary properties of wood: It’s a renewable resource; it can be soft like pine or poplar, or hard like maple and oak; it splits with the grain, but not across it; no two pieces are alike; it can twist, warp and bow. That, however, is just the beginning. To really know your material, you’ve got to become aware of the nuances. I know I’ve covered this in several Fine Woodworking articles, and in “With the Grain: A Craftsman’s Guide to Understanding Wood” (Lost Art Press, 2015), but it definitely demands a re-hash.

So let’s jump right into Wood Technology 101. Don’t roll your eyes if this seems too elementary. Everybody has to start somewhere, so bear with me. I remember the first project I built in high school. We were taught the use of hand tools (and were tested), power tools (tested), basic joinery (tested), safety (tested) and finishing. My first big project was a 2′-square plant table. I built it to withstand anything. It was glued, screwed and tattooed. A Christmas present for my mom, she put it in the window, right over a hot air vent. Within a week the top cracked down the middle. I asked my shop teacher what I’d done wrong, and he said, “You didn’t let the wood move.” Huh? It wasn’t until I took a wood technology course years later that it all made sense. We’ll get to that shortly.

Trees are divided into two groupsscientifically: gymnosperms and angiosperms. Gymnosperms are the conifers. They are the older of the two groups, more simple in structure, have uncovered seeds and generally have needles that stay on year-round (except some species, including tamaracks). Commercially, this group is called “softwoods,” although not all conifers have soft wood, yellow pine being a prime example. Conifers have only tracheids and parenchyma cells. However, they have no vessels, so they are called non-porous.

Angiosperms made a more recent appearance on the planet, and are structurally more complex, with vessels, tracheids, parenchyma and other specialized cells. They are porous and have enclosed seeds, broad leaves that usually fall in the winter (with exceptions including holly and various tropical woods) and have a greater number of species. Commercially these are the “hardwoods,” although this is misleading, because angiosperms include trees such as poplar, basswood and even balsa. Of the roughly 1,000 native North American woods, only about 30 conifers and roughly 80 deciduous species are used commercially in any quantity. This huge selection of native woods offers a variety of colors, textures, smells, grain patterns and uses. I would strongly urge woodworkers to “go native,” as opposed to importing exotics and hastening the destruction of the rainforest. I mean, how can you beat the purple-brown of freshly cut walnut, the dark red of aged cherry, the smell of sassafras or the lace pattern of quartersawn sycamore? Native woods are often local, more easily obtained, and less expensive.

Let’s take a look at wood anatomy. Figure 1-4 shows the basics. From the outside is the bark, beneath which lies the cambium layer, the layer of lateral growth. It consists of the phloem, which forms the bark toward the outside, and the xylem, which produces a new growth ring of wood each year. The first cells produced each spring are typically larger (best seen in ring-porous woods such as oak and ash), and make up the early wood, while those produced later in the season tend to be smaller and are referred to as late wood. The outer portion of the tree trunk constitutes the sapwood, which is made up mostly of living cells used to transport water and minerals to the leaves and branches, and move sugar from the leaves to the cells and roots. When a sapwood ring dies, it turns into heartwood. This happens every year, but at different stages and on a different time frame for each species. The amount of sapwood also varies greatly in species, from less than five rings in catalpa, black locust and chestnut, to maybe a dozen in cherry and walnut, and to 40-50 in the maples, while it may take close to a century for tupelo and persimmon to form heartwood.

Heartwood cells are dead, and often a different color than sapwood. This is due to a collection of extractives such as tannins, lignin, gums, fatty acids, waxes and volatile organic compounds deposited in the cells. These give the heartwood its distinctive color, smell and decay-resistance (or lack thereof).

At the center of the tree is the pith, a soft, spongy material formed behind the apical meristem. The apical meristem (not shown) is the “growing point” at the leader at the top of the tree and the ends of branches that give the tree height and the branches length. To put it more simply, the apical meristem grows the tree taller, while the cambium layer grows the tree wider.

Emanating radially from the pith to the cambium are ray cells, used in lateral transport of nutrients. These play a big part in the stability of quartersawn wood.

Wood Movement
I don’t want to spend too much time on what’s obvious to many of us: crooks, bows, warp, spalting, figured grain, burls and reaction wood. Check out “With the Grain” if you want to explore any of these terms a bit further. Let’s just jump into what’s really important: wood movement.

Wood movement is a major obstacle for many beginning and even intermediate woodworkers. The reason is that wood is an anisotropic material. That means that wood has different physical properties along different directions. As mentioned previously, it splits easily along its length but not across the grain. It has tremendous loadbearing capacity along its length (with the grain), but dents relatively easily across the grain. Figure 1-5 shows the amount of shrinkage that occurs in a red oak, from green (just less than 30-percent moisture content (MC)) to oven-dry (0-percent MC). Tangential shrinkage (think flat-sawn boards) is 8.6 percent, while radial shrinkage (quartersawn lumber) is about 4 percent, or roughly half. What’s going on to cause such difference? It’s mostly the ray cells (although the difference in early wood and late wood structure also plays a part), emanating from the center of the tree to the outside; the ray cells act like rebar in concrete. They actually hold the wood cells tightly in place and thereby reduce the amount of shrinkage. Now look at the bottom line in Figure 1-5, longitudinal shrinkage. It’s barely visible. Generally speaking, longitudinal shrinkage is about 0.1 percent, and is generally ignored.

Let’s put that graph into perspective. Suppose you have a red oak board that’s 12″ wide (30.5cm) and 100″ long (just more than 8′, or 2.5m). If it is perfectly flat-sawn, it will shrink 1.1″ (2.8cm), or just less than 9 percent, in width from the time it is sawn from a green log, until it is dried down to 0-percent MC. That’s quite a sizable amount. If that same red oak board were perfectly quartersawn, it would shrink only a smidgen over 1/2″ (1.3cm), or 4 percent. Either of those boards, flat-sawn or quartersawn, starting at 100″ (2.5m), will shrink only about one-tenth of one inch (.25cm) in length. That’s next to nothing in comparison, and virtually ignorable. So as a wooden rule of thumb, we say that wood moves half as much radially as it does tangentially, and doesn’t change in length.

Think of wood much like an accordion that changes in width, but not in length. That’s because the cell walls act like sponges, absorbing moisture when the humidity is high, and releasing it when the air dries out. It’s obvious that water is at the root of the problem. Eliminate changing moisture and you eliminate wood movement. So you’ve got a few options when working with solid wood. If you live in a museum where the temperature and humidity are constant year-round, movement is not an issue. You can encase the wood in plastic, or a 100-percent impermeable material and prevent moisture exchange. Or you can do what woodworkers have been doing for thousands of years: You can learn to deal with it.

Backtracking just a little, let me say a bit more about moisture content. Green wood can have from 45 percent of its weight as water (white ash, for example), to more than 200 percent of its weight in water (some cedars, sugar pine and redwood). That’s a lot of water. Much of the water is in the cell cavities. This free water doesn’t affect the shrinkage, only the weight. At about 30-percent MC, the free water has evaporated, and what remains is bound water, inside the cell walls. This is the fiber saturation point. Bound water is harder to eliminate, because it is trapped in the cell walls and it takes a fair amount of energy to drive that water out. That energy can come from either sunshine and wind, or gas, oil or electricity when kiln drying. Once bound water begins to leave the cell walls, they start to shrink. Likewise, the entire wood mass begins to shrink. Sort of like a sponge, more water causes the cell walls to expand, while decreasing water causes the walls to shrink.

Air drying will usually bring the MC down to the neighborhood of 12 percent, depending on which part of the country or world you are in, while kiln drying aims for about 6-percent MC. Unfortunately, the wood doesn’t stay at those levels, but is at the whim of the weather. Warm summer air holds more moisture so the wood swells, while colder winter air holds much less water so the wood shrinks. Forced hot air heat has even less moisture, and can bring the moisture down to kiln-dried levels. In essence, wood is always play-ing “catch-up” to the current weather conditions, trying to maintain equilibrium with the moisture in the surrounding air.

Don’t panic over the amount of initial shrinkage from green to oven-dry. For a piece of finished furniture inside a home or office, the maximum range of MC is between 6 percent and 14 percent. That cuts the wood movement down considerably, but not enough to ignore. It’s still a major issue when constructing solid-wood furniture, but it’s manageable, and managing wood movement is what separates antiques from landfill fodder.

Let’s take the problem of wood movement head-on. Here is what you’ll need: First, make yourself a copy of Figure 1-6, “Dimensional Change Coefficients,” and keep it in your shop. Laminate it so it will last for years. Alternatively, you can visit the Forest Products lab website (www.fpl.fs.fed.us) and look up the “Wood Handbook: Wood as an Engineering Material.” Lee Valley Tools has a small paper “Wood Movement Reference Guide” that allows you to dial in 75 different woods and check their radial and tangential change coefficients. Highlight the woods you use most often, or memorize their values. Second, you’ll need a moisture meter. This is a must have item for the serious woodworker. I owned one before I had a table saw. You can get a digital pin-style meter at a home supply store for $30 to $40. Top-of-the-line electromagnetic wave meters can run in the neighborhood of $500, but the cheaper ones will work just fine. Finally you’ll need a calculator.

This isn’t rocket science, just a simple calculation. Here is what it consists of: the width of the piece in inches or centimeters, multiplied by the current MC, and the expected change as a whole number (how far from the maximum MC off 14 percent expansion, or the minimum MC of 6 percent for shrinkage), multiplied by the Dimensional Change Coefficient for the species you’re working with, and whether it’s flat-sawn (Ct or tangential) or quartersawn (Cr, radial). Because not all boards are 100-percent flat-sawn or quartersawn, you can pick a number in between these values. A good guess works, although I always try to err on the side of a more conservative value, just to be safe.

For example, I’m making a cherry drawer 6″ (15.24cm) high, in midwinter, with wood that has an MC of 7 percent. It’s mostly flat-sawn. Worst case scenario, it will absorb moisture next year and reach a max of 14 percent. That’s a 7-percent change. Working from Figure 1-6, the coefficient for flat-sawn (tangential) cherry is .0025. On my trusty calculator, I multiply 6 (width) x 7 (change in MC) x .0025, which equals .105″, or just more than 1/10″ (2.7mm). With a dial caliper I don’t even have to convert to a fraction; I just set the dial to .105 (or 2.7mm), and make my drawer front that much smaller than the opening.

Fitting that same drawer in midsummer, when the wood has an MC of 10 percent (14 percent max minus 10 percent current equals 4 percent), the equation looks like this: 6 x 4 x .0025 = .06, or <1/16″ (1.5mm). In this case, the drawer can be a mite taller.

Wood movement won’t go away if you ignore it. It’s something I take very seriously. Every time I fit drawers, doors and backs, make tabletops or do any sort of cross-grain construction, I reach for the moisture meter and the calculator. It only takes a few minutes, and will prevent serious future headaches. It keeps you and your customers happy. If you know what you’re doing, knock on wood, and follow these simple calculations, you’ll never have a piece returned for a stuck drawer or split case side.