In this month's installment of Field Notes Scott Bowe of Kemp Station discusses bird’s-eye wood, some of the most beautiful material a woodworker can use.
When I say “figure in wood” you may not be familiar with the phrase. But if I say bird’s-eye maple, an image of beautiful swirls pops into your mind. There are many other types of wood figure such as curly, tiger stripe, fiddleback, and quilted, but I would like to focus on bird’s-eye today.
First, we should cover some basic wood anatomy. Please consider the main trunk of a tree such as a maple tree. If we were to cut a thin disk perpendicular to the trunk three feet above the ground, you would immediately recognize a number of features. The pith is at the very middle of the wood disk. The pith is from the very first growing season when the tree reached three feet tall. Around the pith you find consecutive growth rings representing the wood growth from one growing season. In Wisconsin, growth only occurs from spring to fall, leaf on to leaf off, since the tree is dormant during the winter months. You will notice the bark, the protective covering around the outside of the disk. The heartwood portion of the disk are the oldest rings from the pith outward. The sapwood portion is represented by the newest 10 or 15 years of rings just under the bark. It is important to note that the vast majority of wood cells in a healthy living tree are dead. Their main purpose is for structural support of the tree. Only in the sapwood do we find living cells, called parenchyma cells, that function to store and conduct sap; however, the majority of the other cells are dead.
So where does the growth happen in a tree trunk? Wood growth occurs just under the bark in a microscopically thin layer of living cells called the cambium. All of the new wood cells in a tree form in this cambial layer. It is here that mother cells divide, the new daughter cells enlarge, then these new cells die forming new sapwood tissue. You can now see why a shallow cut around the perimeter of a tree will kill it. Cutting through the bark and microscopically thin layer of cambium just under the bark will destroy the tree’s ability to grow. This is common on smaller trees and shrubs when a rabbit will chew completely around the trunk under the snowline in the winter. Good for the rabbit, not good for the landscaping in your yard.
So what does any of this have to do with figure in wood? If all wood growth starts in the cambium, it stands to reason that all figure in wood starts in the cambium too.
Bird’s-eye figure are localized swirls of wood cells that look like a tiny eye. So instead of the cells forming vertically in the tree trunk, the cells curve and form these localized swirls. Bird’s-eye can range in size from 1/8 to 3/8 inches. I’ve heard some bird’s-eye referred to as Champaign eye, like the tiny bubbles in Champaign. Bird’s-eye can range in density from sparse to very heavy. Heavy bird’s-eye can cover the whole surface of the board.
So what causes bird’s-eye? No one has figured that out. I suspect it is a genetic trait. Maple trees that contain bird’s eye can be more prevalent in certain regions where the parent trees pass on these genetics to their saplings. I asked a sawmill in Langlade County that saws mainly sugar maple about the prevalence of bird’s-eye. They said about 3% of the sugar maple lumber that they saw has bird’s-eye. Interestingly, bird’s-eye can be considered a defect if it is mixed in with normal wood. Consider a maple kitchen cabinet door made of edge-glued maple staves. If most of the staves that make the door are normal, but one stave has bird’s-eye, the figured stave will look odd. For this reason, many sawmills will sort out the bird’s-eye lumber and sell it as figured wood, to separate it from normal wood.
Bird’s-eye is common in sugar maple, but it can be found in other species such as red maple, white ash, mahogany, American beech, black walnut, yellow birch, and even oak. Trees that grow in the Great Lakes region of Canada and the United States yield the greatest supply, along with some varieties in the Rocky Mountains.
As a novice woodworker, I enjoy working with bird’s-eye maple. Everyone can appreciate the novelty and beauty of bird’s-eye figure, one of nature’s beautiful and renewable natural resources.
For Field Notes, this is Scott Bowe from the University of Wisconsin-Madison’s Kemp Natural Resources Station.