Considering that burls have probably been around as long as trees themselves and that they have been prized by woodworkers for centuries, there is a surprising lack of consensus on the identification and nature of burls. The following general description of a burl does appear to be widely accepted: A burl is an area of abnormal and irregular growth protruding from a tree.
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. This comes from the fact that wood is best cut "with the grain," so the tool meets a minimum of resistance and leaves a cleanly cut surface. If you cut against the grain, the grain will tear and the wood will be hard to work.
Any woodworker knows what this means. A cabinet maker will think of the hiss of the clean shaving as it comes off a plane that is following the grain of the wood. A turner will think of the ribbons of shavings that fly from a gouge that is cutting into the wood at the correct angle to the grain. A carver will know that the gouge will only achieve a clean cut if it is cutting in certain directions.
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.
The absence of grain means that burl wood can be cut equally well in almost any direction. This makes it particularly suitable for peeling as veneer. The highly expensive paneling on the dashboards of classic cars is often burl veneer. Burl veneer has also been used as inlay for marquetry and other fine furnishing techniques for generations.
On the Tree
What do burls look like? 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. The exterior of the burl may look dramatically different from the rest of the tree, often having a rougher and darker appearing bark.
Burls can occur on most species of trees. 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.
Sometimes a tree is so covered in burl that there is hardly any part of the tree that is left unaffected. These trees are rare and are usually referred to as "burl trees." The Blakely Burl Tree is one such tree.
The Mystery of Burl Formation
Why do burls occur? Firstly, it needs to be pointed out that those who are talking about burls are sometimes talking about something else entirely. The overgrowth that develops to heal a physical trauma to a tree is one example. Close examination of such a growth will usually reveal conventional growth rings, something which is usually absent with burls.
Others confuse figured wood, sometimes called fiddleback or bird's-eye, with burl wood. Figured grain is usually found in stress points where the tree flexes in the wind, such as at ground level or where branches fork. It is probably a form of interlocking grain that strengthens the tree.
Scientists suggest different causes for the development of burls. These include insect infestation, physical trauma, and pollution. Those who work with burls and particularly those who harvest the burls themselves tend to the idea 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. This has been the experience of the team who harvested the Blakely Burl Tree.
The pecan tree belongs to the hickory family. Its wood has been used in many applications for generations, particularly where strength is required. Although burls are frequently found on many types of trees, there are others that rarely produce burls. The pecan is one such tree.
Not one of the Blakely Burl Tree team, including tree surgeons and wood harvesters, nor any of the residents of South Georgia involved in the project, could ever recall seeing a pecan tree with a burl on it, let alone a pecan burl tree. There may be more out there, but so far none have been found by the team.
For more information, read an article on burl written by Mark Lindquist, from his book: Sculpting Wood