As woodworkers, how much do we know about wood? Seems like a strange question, as we are working with it everyday. Let me clarify. I am not referring to a specific species but to wood as a material. General information that is typical of all species allows us to choose the character of the pieces of wood before we narrow down the species. Alternately, if you have chosen the species, you can choose a specific cut of wood to give the desired character.
Mahogany, quarter sawn
When we look at a beautiful piece of furniture, it is often difficult to identify what cut of wood produced the visual effect, regardless of the species. Maybe it would be worth taking a closer look at our favourite material and try to identify what creates the different visual effects. This will lead to an understanding of what influences grain pattern and figure. After all, if we aspire to design and make beautiful projects, we need to know as much as possible about this very important characteristic. When we refer to grain pattern or figure, what are we really talking about? Grain pattern or figure is created when we cut through the annual rings. Each of these yearly growth rings had a spring and summer growth, which varies with each species. Open grain woods have a very pronounced difference in the two growths, therefore they have a more pronounced grain pattern.
Grain pattern or figure is a very important characteristic because it will govern the finished appearance of our work. The accompanying diagram looks at the three locations of the pieces of wood being sawn from a log, and the type of grain pattern that will be produced. Board A is cut tangentially or flat sawn. You can see by the end grain and the side grain, that there is a wavy grain pattern on the faces of the board and a slightly straighter grain on the edges. Board B, which is quarter sawn, will have straight grain on both faces and wavy grain on both edges. Board C is rift sawn and has straight grain on one adjacent face and edge (closest to the center of the log) and wavy grain on the other adjacent face and edge.
You may ask what causes the wavy grain effect? Even though the log is drawn almost perfectly round and the annual growth rings are drawn to follow the shape, this is an unusual case. Most tree trunks are oval or irregular in shape and are thicker at the base than the top. This means when a straight board is cut from the log, the saw cut will not follow the undulating line of an annual ring. The saw cut is not parallel to the annual rings, therefore the result is a wavy grain pattern or figure.
The easiest way of visualizing this process is to think of a roll of paper towels as a log. Each layer of towels is similar to the annual growth rings on a tree. Imagine slicing the roll in a lengthwise direction, similar to cutting a board off of a log. Our roll of paper towels is the same diameter at both ends and each layer is the same thickness. Our cut, or slice, from end to end, would reveal the straight edges of the sheets that were cut. These straight edges of the sheets could be seen as a very straight grain pattern. Now on the other hand, if we were to deform the shape of the roll of towels to an oval in one direction at one end, and an oval in the opposite direction at the other end. This would create a shape more like the trunk of a tree. Now, when this shaped roll of towels are cut end to end, the emerging pattern on the flat cut surface would be wavy because the cut is no longer parallel to the layers of towels.
Beyond straight or wavy grain is the exposure of the medullary rays. As this name implies, they are arranged in a radial direction emanating from the center of the log. These rays are an extremely important characteristic in the oak species. These trees have large medullary rays which, when exposed, present a flame pattern across the grain of the wood. This grain pattern is revealed when boards are quarter sawn or radially sawn from the logs. This grain pattern was very popular in period furniture, fittings and flooring because of the way light catches the medullary ray character of this cut.
You have probably heard of bird’s-eye maple, curly maple, tiger maple and a variety of burls. Each one of these very different grain effects is caused by growth deformities in the tree. In the case of burls, when a tree is damaged because a branch is broken off in a storm, the tree grows a patch over the damaged area to protect the area without bark. Subsequent annual layers of this patch do not grow in the same shape as the trunk. The burl appears like a round bump on the side of the trunk. Each year abnormal twig growth occurs, giving the burl an almost mottled or pitted appearance.
Curly or tiger maple grain patterns are also the result of grain deformities. Spiral growth, erratic growth and tension within the growing tree create these beautiful grain and figure patterns.
Some woods, like mahogany, develop a predictable spiral growth cycle where subsequent cycles are uniform. This growth pattern produces a beautiful, even stripe grain effect when the wood is quarter sawn. This striped effect is best seen when the light hits the face of a finished board. The stripe has an alternating light/dark sheen that is very regular and quite beautiful.
Understanding what causes grain or figure patterns, gives us the ability to choose wood by cut. We will be able to predict the effect of these beautiful conditions in wood, by knowing where in the log the board was taken.
In the next article, I will go over the various cut locations and the effect on a board’s stability. I will also go over the tendency of wood to warp and deform as it dries.