Glossary of Wood Terms Defined Below growth rings grain heartwood/sapwood
figure color character
density/specific gravity hardness strength texture stability
The moment when a saw opens a log is a defining one, often exhilarating. What will be inside? The figure, color, and character of the wood — individual as a fingerprint — are revealed.
Each tree will have its own history written in the wood. The things we love about wood and the ways we fashion it into objects of beauty and use are conditioned by life and growth in the tree.
Density, hardness, strength, texture, and stability are relatively consistent within a given species; the technical principles of craft can be applied. Figure, color, and character, however, are distinctively individual from tree to tree of the same species; this creates opportunities for artistry in selection. We feature and celebrate these individual and distinctive values in our furniture. No two rocking chairs, dining chairs, or other pieces of our furniture, will ever be alike. Each will be a thoughtful and harmonious composition of figure, color, and character all its own.
All trees, regardless of species, have common genetic requisites and traits. Three of these — growth rings, grain, and the color differentiation between heartwood and sapwood — are of particular importance to a visual understanding of our work and furniture.
The cells just under the bark of the tree are the only growing and dividing cells in the trunk. Fertility, rainfall, sunlight, temperature, competition, and health determine the number of cells that will be added in an annual cycle. These layers of cells form the concentric circles that we count to assign an age to the tree.
Wood cells are generally long and slender. Most cells form fibers running with the length of the stem or trunk. Although the term is used in other contexts, grain, technically, refers to the direction of these fibers. The grain direction and the fiber quality in any given piece, or species, of wood determine density, hardness, strength, texture, and stability.
Heartwood and Sapwood
As the tree grows the cells nearer the center of the trunk die. Death is accompanied by accumulations of minerals in the cell walls. These minerals produce color. The wood that has died and turned the identifying color of the species is called heartwood. In all species the living wood just under the bark is creamy white or tan and is called sapwood. The sapwood can be thin, one or two rings, or thick, many dozens of rings, generally depending on the species — but conditions can cause significant differences within a species.
While growth rings, grain, heartwood, and sapwood are common to all trees, the manifestation of them is different in every tree. We describe these differences with three terms: figure, color, and character.
The growth rings we see on the end of a log are the lines we see on the flat made by the saw. Because the growth rings are irregular in their development, the plane of the saw cut intersects these roughly concentric cylinders at various angles and orientations. The conic sections revealed might be hyperbolic, parabolic, straight, or any imaginable combination, depending on the path chosen by the sawyer and the peculiarities of the tree. These patterns, often called grain, are, more correctly, figure.
The heartwood of cherry is medium reddish brown, that of walnut, a dark chocolate brown with a purple cast. Each species has its characteristic heartwood color, but the depositing of minerals that produce color is subject to forces unknown. Overall color varies within a species, and various streaks and shades occur within a log. The contrast of the light sapwood with the darker heartwood is always distinguishable and sometimes pronounced in the woods we use.
As they grow, events in nature affect the dividing cells under the bark, resulting in anomalies in grain and color. For examples, the cells and fibers have swirled around a limb or bud, an injury has been healed, pitch or minerals have been deposited in an area--resulting in irregular figure, variable reflectance, iridescence, burls, knots, and dark mineral streaks. When attempting to produce a homogeneous product for a mass market, these natural anomalies are considered defects. We consider them marks of character.
When comparing species of wood, we also use the terms below. These refer more to the working and technical properties than the visual and artistic ones.
The amount of wood fiber per unit volume is an important aspect — for most species it predicts other characteristics. Density relates directly to weight, but because wood in use will contain a varying amount of water (see Wood and Moisture), weight is not a constant. A more meaningful number is specific gravity: the ratio of the weight of a given volume of oven dry wood to the weight of the same volume of water. The higher the specific gravity, the denser and heavier the wood. Usually the denser the wood, the harder and stronger it is, but not always. Usually the denser the wood the more it changes dimension with changes in humidity, but not always.
There are numerical measures of hardness, but for our purposes, I rank the woods we use by this subjective scale, garnered by experience: moderately hard, hard, very hard. The harder the wood, the more resistant to denting, wear, and abrasion.
Again there are numerical measures for the strength of wood — many numbers for many directions and kinds of loading. Again, I use a subjective ranking garnered by experience: weak, moderately weak, moderately strong, strong, and very strong.
The physical size and composition of the wood fiber and vascular channels (pores) determine the texture of wood. The woods we use are either fine or very fine in texture. Finely textured woods can be sanded smoother and finished more beautifully. If the wood pores are visible, the wood is called opened grain; if they are not the wood is called closed grain.
When subjected to reduced or elevated humidity some woods are affected more than others. (See Wood and Moisture) I rank the woods we use as moderately stable, stable, or very stable.