Burls (Tree Knots) are Created by Trees Under Stress
A Burl is a particular growth that is a rounded formation on a tree trunk commonly, or branch. The tormented interior is a mass of dormant buds that failed to produce new branches. Still alive and growing, the bark-covered burl swells and protrudes forming a ball that is like a malignancy.
Burls are usually the result of environmental or artificially-induced stress (physical damage or trauma, etc.) Most burls however occur in the root system of trees and go undetected. Unless the tree falls over, the burl's presence goes unnoticed. Burls on trees are often infested or infected with mold, insects or some other malignancy yet continue to grow. Wood-turners (people who operate wood lathes) can tell you that even a really nice-looking burl can turn out to mostly hollow and porous when they attempt to turn it on a lather. Until the burl is opened-up and work on it begins, it is a bit of a gamble what you will find.
Uses for Wood Burls - Wooden Bowls
The size of burls can vary greatly, ranging from thumb-sized to several feet in diameter. Highly prized by wood sculptors and wood-workers, tree burls are often sought for the creation of specialized wood products such as lathed wooden bowls, wooden veneers for tabletops and furniture, picture frames and even interior trim for high quality automobiles. The wood structure of a burl closely resembles birds-eye maple, although the latter is formed in a process unrelated to the formation of burls.
The most stunning use of wood burls in my opinion is in woodturning; the art of creating wooden bowls on a lathe. For this, wood burls are highly sought after by wood sculptors.
Collecting wood burls is difficult as it almost always involves killing the tree. Unless the tree is selected for removal or the branch with the burl is culled, obtaining desirable burls suitable for use is far and few between for most hobbyist-woodworkers.
The burl if harvested green from the living tree, must be allowed to 'season' or 'cure' before it is usable. Some woodworkers would merely retain suitable rough-cut burl in an attic or garage for a year or so until it is suitable for turning.
Preparing a Burl for Woodturning into a Wooden Bowl
A burl must be made cylindrically-round in order to be more safely lathed. Here, we see a woodworker's innovative approach to preparing a burl for the lathe.
A round template is affixed to the burl as a saw guide, and the outline is cut using a heavy duty bandsaw. When attached to the lathe and rotated at high speed, the woodworker requires the piece to be of a relatively uniform cylindrical shape. This allows first for safety of the worker and for the tool guide to rest close enough to the spinning burl to effectively hold the chipping knives.
The burl is artistically carved into whatever shape the artist desires. In this case, a wooden bowl.
Wood Burl Structure
Working a wood burl on a lathe is very difficult because the wood grain can run in multiple and opposing directions. Encountering a porosity in the seemingly solid burl can also occur, and is dangerous to the machine operator. At speed on the lathe, breaking into a hollow cavity using the chipping knife is a possibility. Or, the burl itself could shatter along invisible fracture lines. This can cause the burl to disintegrate and fly apart violently, potentially injuring the machine operator.
Stunning Wood Burl Bowls
But with risk comes rewards. Artistically lathing a quality burl can produce stunning examples of art that are also useful. Here, a cherry burl is wood-turned into a small candy dish.
Sanding is accomplished by holding sheets of sandpaper against the spinning wood while still attached to the lathe. Polished to a luster and sealed for protection, the finished wood burl bowl creation is a one-of-a-kind post-modern primitive art piece that is almost too beautiful to describe.