The following is excerpted from the third edition of Christian Becksvoort’s “With the Grain: A Craftsman’s Guide to Understanding Wood.”
It is, above all, succinct, easy to understand and perfectly suited for the furniture-maker. As important as what is in its 160 pages is what is not. It’s not a detailed analysis of cell growth. It is not a heap of tables and equations for figuring truss loads in residential construction. It is decidedly not a scientist’s approach to the material.
Instead, “With the Grain” contains the facts you need to know at the lumberyard, in the woodlot and in the shop. It gives you enough science so you understand how trees grow. It explains the handful of formulas you have to know as a furniture-maker. And it gives you a hearty dose of specific information about North American species that will inspire you. Becksvoort encourages you to use the trees in your neighborhood and makes the case that just because you cannot find catalpa at the lumberyard doesn’t mean it’s not a good furniture wood.
You’ll learn to identify the trees around you from their silhouette, leaves and shoots. And you’ll learn about how these species work in the shop – both their advantages and pitfalls.
Becksvoort then takes you into a detailed discussion of how wood reacts to it environment – the heart of the book. You’ll learn how to calculate and accommodate wood movement with confidence and precision. And you’ll learn how to design furniture assemblies – casework, drawers, doors and moulding – so they will move with the seasons without cracking.
This week’s excerpt picks up where last week’s (on wood structure and the classification of trees) left off…
Figure 1-7 shows the three faces, or planes, of wood. Cutting the wood parallel to the ground and perpendicular to the pith exposes the cross section, or transverse face. Such a cut is in the transverse direction and reveals the growth rings, pores, rays and pith.
Cutting in a plane parallel to and through the pith reveals the radial face and is in the radial direction. This cut shows the parallel edges of the growth rings and best shows the widest portion of the rays.
Again cutting vertically through the trunk, but parallel to the pith, reveals the tangential face, or plane, which displays the ends of the rays and the wide figure of the growth rings.
Wood is an anisotropic material: It has different and distinct properties in each of its directions. In the longitudinal direction, with the length of the wood fibers, wood has its greatest shock resistance and compression strength. Hence the use of longitudinal lumber in posts, legs and other weight-bearing members. In addition, wood shrinks an insignificant amount in this direction. Yet with all its compression strength, it splits in the radial and tangential direction. In the transverse direction (cross section) wood refuses to split, yet compresses perpendicular to the grain. In ring-porous woods such as oak and ash, the concentration of large, thin-walled vessels in the early-wood forms rings of weakness. Utilizing this property, basket makers beat ash logs to separate the growth rings, lifting them off in sheets.
COLOR AND LUSTER
Color in heartwood is caused by extractives. In most species the sapwood is a light, creamy tan color. Heartwood color varies tremendously among species, and even within a species, color is variable. One theory states that shading is influenced by the soil in which the tree grew. Because of the variables, color should only be used secondarily as an aid to identification. Other factors also influence color: exposure to sunlight, which hastens the development of the patina; and finishes, which alter the natural darkening or bleaching of the wood. Although color is an unreliable factor in wood identification, it is this variability, in conjunction with the figure of the grain, that makes each piece unique and adds to its aesthetic value.
Luster is the natural ability of the wood to reflect light. It has nothing to do with color or finish. Luster is most evident when the wood is planed with a sharp tool. This produces a more reflective surface with more sheen than sanding, which abrades the wood and clogs the pores. Occasionally luster can be used to identify wood. White ash, for example, is noticeably more lustrous than black ash.
TASTE AND ODOR
Extractives are also the cause of taste and odor in wood. Some odors are so distinctive they can be used in identification. Aromatic extractives are most noticeable in red and white cedar, black walnut, sugar pine and sassafras. A few woods such as catalpa and tulip poplar have a rather disagreeable odor. Because most of the odor-causing extractives are volatile, they can be detected only in green or freshly sawn wood. Taste is usually not pronounced in wood. Rather, woods are often chosen for their lack of taste and odor. Butter tubs, cutting boards and kitchen utensils are made of fir, spruce, maple, beech and basswood for this reason.
DENSITY AND SPECIFIC GRAVITY
Specific gravity measures the relative amount of cell wall material, and is an excellent index for predicting the strength of wood. It is expressed as the ratio of the density of a material compared to the density of water at 39°F (4°C), or:
The moisture content of the wood must always be specified, because the wetter the wood, the larger its volume and the more water it will displace. Figure 1-8 lists the specific gravity of common native woods at 12 percent moisture content (air dry).
Water has a specific gravity of 1.0, whereas pure wood material (with no cell cavities) would have a specific gravity of 1.54, about 50 percent higher than water. All wood has cell cavities filled with air, water or extractives. Live oak has the highest specific gravity (.88, at 12 percent moisture content) of any North American wood, while Northern white cedar has the lowest at about .31.
Density is defined as mass per unit volume, and is usually expressed as pounds/cubic foot (lb/ ft3), or grams per cubic centimeter (g/cc). Density also varies with the moisture content of wood. Water weighs 62.4 lb/ft3 or 1g/cc. Wood with a specific gravity of .50 would have a density of 31.2 lb/ft3, or .5g/cc. A cubic foot of solid wood material (with no cell cavities) at 0% moisture content, having a specific gravity of 1.54 would weigh 96.1 lb/ft3, or 1.54 g/cc. Density is a good general indicator of hardness and the amount of shrinkage and swelling to be expected for a given species. As a rule of thumb, the denser the wood, the more movement can be expected.
Density is easy to approximate by floating a long, thin piece of wood upright in water. The ratio of the length below water to the total length, times the weight of water per cubic foot equals the density at that specific moisture content. For example a 14″ (35.5 cm) is floated upright, and 8.5″ (21.6 cm) is below the waterline, then
GRAIN AND FIGURE
A knowledge of the basic makeup of wood helps in the understanding of grain structure. Grain is simply the alignment of the wood cells. The term is used in describing the arrangement, direction, size and appearance of the fibers, vessels and rays. Strictly speaking, figure is the appearance of the grain as it is exposed on the surface of a board. In the lumber industry figured wood refers to certain types of irregular grain patterns. By simply splitting a piece of wood, one can determine whether the grain is straight or wavy. Grain around branches, roots and crotches will be wild, wavy and extremely unpredictable and difficult to work.
In some trees the grain does not grow perfectly up and down, but tends to spiral around the trunk. Spiral growth is present to some extent in both hard and softwoods, but unless the spiral is severe, it is considered normal. In some species the spiral reverses itself every few years, growing first clockwise, then counterclockwise. This interlocked grain is very difficult to split, chisel and plane. The grain goes in opposite directions and appears as fuzzy stripes (Fig. 1-9). Interlocking grain is found in elm, sycamore and black tupelo. Finished, it is sometimes known as ribbon grain.
fuzzy on sawn sycamore.
Wavy grain patterns are the result of longitudinal cell growth in waves, which gives a washboard appearance when the wood is split (Fig 1-10). This occurs most often in maples, birches, and, to some extent, in ash and cherry. In maple, when the waves are small and tightly spaced, the grain is called tiger-stripe or fiddleback. This abrupt oscillation of the grain is known as chatoyance, (from the French chatoyer, meaning to shimmer) and results in dark and light areas, depending on the direction of view. Chatoyance is not limited to figured wood. Even straight-grained door frames made from the same piece of wood will exhibit differences in color when the rails are turned 90° to the stiles.
Very rarely, areas of interconnected ovals and grooves will result in blister, or quilted grain in maple. A more common figure is bird’s-eye (Fig. 1-11). This is the result of small indentations in the grain, more or less like dimples, usually evident under the inner bark. Because the cambium layer is the source of new growth, the bird’s-eye figure continues as each new ring is formed. Bird’s-eye varies in size and distribution. This figure is most common in rock, or sugar maple, but is sometimes found in ash and birch.
There is as of yet no satisfactory explanation as to what causes blister, tiger-stripe or bird’s-eye figure. Suggestions such as climate, soil, stress, viruses or genetics have so far been discounted.
Crotch grain, or feathering around root and branch crotches, is caused by distortion of the grain and crowding and twisting of the annual growth. Crotch grain (Fig.1-12) is one of the most common figures encountered. It occurs frequently in walnut, ash, oak, birch, cherry and to some extent in all hardwoods.
Another specialized pattern is pigment figure. Actually, it is not a grain pattern, but rather streaks of color independent of the growth rings. Pigment figure is caused by uneven distribution of extractive deposits in the heartwood. An excess of dark deposits is most dramatic, but occasionally a lack of extractives may cause a lighter area in the heartwood known as false or included sapwood. Pigment figure is common in black walnut, Eastern red cedar and sweet gum, and is sometimes present in cherry and even pine.