What is "burl"?
If you are shopping for a wood product, you may have encountered the term burl and you may have heard it in conjunction with the type of wood being used, for instance, maple burl. A burl is an abnormal growth in the wood, similar to a tumor or scab in a human.
Although it can happen on any type of tree, burls themselves are rare and prized in woodworking.
Normal wood grain runs in one direction, lengthwise along the trunk and branches of a tree. This gives wood certain characteristics that are so well known they have entered the language as metaphors. We all know that something which "goes against the grain" is disruptive or disturbing. In normal wood, you will see either rings or relatively straight grain and the occasional knot, depending on the cut of wood.
So how is a burl different? Burl growth is chaotic with no discernible grain direction and it is these irregular patterns of grain that can make burl wood so attractive, often creating beautiful wave and swirl patterns in varying colorations. The beauty of the burl grain is what makes it so desirable, but also very difficult for woodworking. With no predictable grain pattern, the woodworking is always cutting “against the grain,” which can be very challenging and hard on the tools.
What do burls look like?
Similarly to a scar or tumor on a human, burls usually look very different from the rest of the tree. A burl may form a rounded bump on one side of a tree, or partially or completely surround the tree. The shape may be smoothly curving or irregular with twists and fissures.
Sometimes burls grow among the roots, sometimes at ground level and sometimes hundreds of feet up in the air. Burls more often grow on the trunk than on branches. They vary in size from a hand span wide up to several yards across, weighing many tons.
Why do burls form?
Scientists suggest different causes for the development of burls. These include insect infestation, physical trauma, and pollution. Some people suggest that burls result from a genetic predisposition, perhaps with an environmental trigger. This is supported by the fact that where you find one tree growing burls, you often will find others within the seed cast territory of that tree.