To understand the dynamics of a board it helps to understand how a tree grows. Roots act as an anchor to hold the tree upright, and they also absorb mineral-rich moisture. The sap is drawn up through the outer layer of the wood, or cambium, which is just below the bark, to the thirsty leaves, where moisture evaporates. The leaves absorb carbon dioxide, and with the help of sunlight, convert it into nutrients for growth through photosynthesis.
The cells in the cambium are designed to carry or store sap in their early years, and are transformed in time into the strong backbone of the tree. During spring and early summer, when the tree is growing, the cells are relatively large and full of sap to feed the tree. They work as a series of tiny interlocking tubes, with the sap winding its way upward from one cell to the next. Later in the year the cells are thinner with denser walls, and are used to carry moisture and strengthen the tree. Though the sap and nutrients move largely up the tree, there is some dissipation toward the centre through cells known as medullary rays, which lie at right angles to the rings and can produce colourful figuring when exposed on quarter-sawn boards.
Just as new layer of early and late sapwood grows each year, so another layer, nearer the centre, is transformed into heartwood to support the tree. We can thank that transformation for the remarkable strength of lumber, and the annual buildup of growth rings provides us with a method for ageing a tree or even a board. The proportion of sapwood and heartwood in a tree remains constant throughout its life, while right at the center the cells decay and often suffer from fungal attack. And though it might ultimately kill the tree, that decay can cause all sorts of special effects in lumber, with oak turning darker brown and the heart of ash often acquiring olive streaks.
CHARACTERISTICS OF LUMBER
No two types of lumber are the same, each being distinguished by a complex mix of characteristics. These distinctions can be determined by the genetics of a species, and be common to all examples of that type of tree, or by the circumstances of growth, soil type or climate, and therefore particular and exclusive to one individual specimen.
HEARTWOOD AND SAPWOOD
The proportion of sapwood to heartwood varies from species to species. Prized for its purple-brown color, exquisite sweeping grain and even texture, English Walnut (Jugans regia) has a frustratingly wide, soft band of pale sapwood that is good for almost nothing. The contrast between the sap and the heart of English Yew (Taxus baccata) is just as distinct, but is used by many woodworkers as a feature.
Other woods, particularly the tropical species, have little discernible sapwood, and for some the transition between heartwood and sapwood is so gradual and indistinct that much of the sapwood can be safely used for woodwork. The heart of oak, for instance, is fairly resistant to insect attack, but the sapwood is too tasty to resist and is often riddled with holes.
Woodworkers never stop talking about the grain of a wood. They endlessly debate its varying qualities and challenges, forever comparing the benefits of one against the pitfalls of another. There are, however, three major issues to consider when studying the grain of a wood: the texture, the consistency, and the direction.
Most wood are described as being either coarse (or open) grained or fine (or closed) grained, though many are somewhere in between. Coarse-grained woods, such as oak, ash or walnut, have fewer, large cells to carry the sap, and this results in large, open pores on the surface of the wood that have to be filled in with some kind of compound to obtain that perfectly flat finish. Plenty of woodworkers prefer such woods, and highlight their characteristics by sandblasting the surface or wire brushing. Close-grained woods, such as birch or maple, have thinner cells, and many more of them. This makes them easier to bring to a high, blemish-free finish, though a soft-grained wood will not have much lustre (the quality of the wood that makes the surface shine). Yew, despite having a close, even texture and consistent grain, is very difficult to use because its grain is interlocking and shoots off in all directions.
The lines of grain you see on a board are created by the contrast between the large, thin-walled cells that are formed in the early growing season of the year and the denser, darker late wood that comprises narrow cells with thicker walls. In tropical forests, where temperatures are even throughout the year, the contrast between earlywood and latewood is only slight, and such lumbers are often favored for their even grain and for their consistency. That does not mean they are always easy to use, however, as the texture may be coursed or interlocked, or both. In temperate regions the contrast between earlywood and later growth is more significant, being both easy to the eye and occasionally frustrating to work. The contrast in density can make the lumber difficult to cut and plane, with tools jumping as they hit a harder layer, and saws and chisels diverted widely from their true course.
The direction of the grain differs from species to species, and sometimes from one tree to the next. The finest woods, such as white oak (Quercus alba) and black walnut (Juglans nigra), are favoured for their straight grain, created by cells growing in line with one another, all pointing upward to the sky. Even those species genetically disposed to straightness can be sent off in all directions by poor woodland management, and while forests comprising a jumble of flora and fauna are valued for their biodiversity, they do not always produce valuable straight-grained woods. Even when the grain is straight, this does not mean it is consistent or evenly textured, but any good sawmill should be able to keep the grain parallel to the length of a board. The woodworkers’ saw or plane can then cut or slice along the grain without suddenly tearing the wood as the grain alters direction.
Other woods, like chestnut, tend to have spiralling grain, which can be observed from the outside of the tree. This is much more difficult to manage, so it devalues the lumber. The most challenging lumbers, from the woodworkers’ perspective, have what is known as interlocking grain, with its direction almost impossible to predict. Just when you think you are planing happily with the grain, like brushing a horse, the direction suddenly changes and you find yourself working against the nap.
CONVERTING TREES INTO BOARDS
The challenge for the sawmill lies in maximizing the amount of saleable boards they can cut from a tree, aiming to find a balance between quality and quantity. The simplest technique is to slice the trunk into layers, anything between 1 inch and 4 inches thick, a method referred to as quartersawn because the tree is often milled in quarters to achieve this result, which takes considerable time and skill and will often waste wood. Quartersawn lumber is given a value for its stability and, in some cases, for the distinctive medullary rays that are exposed by the technique.
COMMON PROBLEMS WITH BOARDS
Wood moves and can deteriorate during seasoning, and afterward in the workshop, as it adjusts to local conditions. Watch out for problems when buying lumber; some can be resolved but others make the boards very difficult to manage.
Crown-cut boards often cup away from the curve of the ring as they dry, with the contrasting diameter of the rings on either side of the plank contracting at different rates. They flatten again in moist conditions.
TWISTING AND BOWING
When boards have been badly cut, poorly stacked for drying or comprise spiraling or interlocking grain, they may twist and bow along their length. Bowing can often be the result of poor storage or stacking.
CHECKING AND SPLITTING
Small cracks occur if the surface of a board dries to fast. This is also the cause of end splits, but these are more difficult to prevent and are very common. Cracks and defects within a board may have happened during growth and are unavoidable.
These cracks occur within the tree, usually with one major crack working its way from the heart outward, but others can also appear.
COMMON DEFECTS IN WOOD
The woodworker who wants an easy life chooses straight-grained boards that are referred to as clean, or free from knots or defects that can weaken wood and be difficult to cut or plane. There is nothing worst than preparing lumber for the workshop, only to discover a large knot just where you want to cut a joint.
Well-managed forests produce straight, healthy trees with few knots, diseased or other defects. Many woodworkers enjoy the challenge of working with defects and believe that such features bring character to a project and reflect a more organic approach to forestry. Indeed, some defective wood is in such short supply and high demand that it is available only as a veneer.
Wounds to a tree’s skin sometimes transform themselves into burls, made from hard, swirling scar tissue that is highly valued by turners and carvers, and for veneer. The interlocking grain makes burls very difficult to use for making furniture, and with gaps between the swirls, they are not always very strong.
Lumber is often graded by the frequency of knots: these weaken a board and make it difficult to work. They also tend to ooze sap or resin, and should normally be sealed with a knot sealer, usually made from shellac.
DISEASE AND AGING
Remarkable colors and patterns may be created within diseased wood, some but not all of them undermining the lumbers strength. Spalted beech features black lines, while old oak turns a deep brown color. Stains, often from poor seasoning, can afflict some lumber and care must be taken that the sticks in a stack of drying wood do not stain the boards with a chemical reaction.
RIPPLE, QUILTING AND FIGURE
Extraordinary wavy patterns can be found in some woods, particularly maple and sycamore. Surprisingly, the ripple, quilting and figured grain patterns do not greatly affect the workability of these woods. The much-prized bird’s-eye maple features tiny eruptions, like very small knots.
CROTCH AND ROOT
Some specialist workers of wood the parts other craftspeople will reject. Gunsmiths, for instance, choose the dense, interlocking grain of walnut roots for gunstocks, both for its beautiful effect and because it has the strength to take recoil. Crotch woo, produced where a large branch meets the trunk, can offer rich flaming grain and is often used for panels in cabinetmaking, while boat builders and house builders will handpick curved limbs for the ribs of a boat or for a roof truss.