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A burl is a wartlike, deformed growth on the trunk or root and sometimes even the branches of a tree, caused by (1) an injury to, or (2) and infection in, the tree just under the bark, or (3) the existance of an unformed bud which has all the genetic material necessary to grow a full branch, or even a whole tree, but which for some reason did not grow properly. In any case, the result is that the tree cells divide and grow excessively and unevenly in a process somewhat analogous to cancer cells in a mammal. Burls are sometimes called tumors on wood, although I'm not aware of their ever being fatal. Trees with burls continue to grow otherwise normally.
Continued growth follows the contour of the original deformity, producing all manner of twists, swirls and knots in the wood fiber. Usually, this results in wood that has a spectacular pattern that can be used to great effect in woodworking, and sometimes it is also accompanied by the creation in the burl of dormant buds which create "eyes" that make the burl even more spectacular when worked.
Burl wood is usually darker than the rest of the tree and in some cases (Paela comes to mind) may be a significantly different color altogether. Because of the diverse grain direction, burl wood cannot be relied on for strength, but that's of little consequence since burls are prized for beauty, not strength.
Burl wood can be difficult to dry without cracking. Sometimes there are bark inclusions in burls, and also sometimes gum pockets, either of which can cause surface defects when the burl is worked. In some species of wood, gum pockets are common in any burl found on the tree.
Burls come in all sizes and shapes from golf-ball and smaller to thousands of pounds of massive growth on the side of a large tree or in the root system. Burls as large as an SUV and weighing 10,000 pounds have been reported as have trees with hundreds of small burls. On really large trees, such as the redwood, burls commonly exist that are large enough to be used to create veneer and even smaller burls are sometimes sliced into veneer for use in craft items such as jewelry boxes. Burl veneer frequently does not stay flat after cutting and has to be moistened and clampled flat before and/or during application.
"Cat's Paw" and "cluster burl" are a couple of commonly identifed types of burl figure. Cat's paw is frequently found in cherry and cluster burls are found in a number of species. Most often, burls have no sub-designation and occur in a large number of species. Common burl species include redwood, oak, ash, maple, madrone, elm and walnut. Some exotics with very popular burls are mappa (poplar burl), thuya and imbuya, and there are MANY more.
a couple of burl oddities --- one shaped like a nose and one like a bear-like monster
misc burls on trees and a couple of vertical cross-sections of trees each with one burl
two trees with many burls on each and two bulbous burls
two large burl areas on redwood trees
A box elder tree with multiple burls and an enormous sitka spruce root burl (you can see where the tree trunk was coming out of the top and the roots out of the bottom
redwood burl veneer, bookmatched, and a vase made from a redwood burl. Note on the vase how pitch pockets have left small natural inclusions in the wood. Redwood burls can reach sizes that are enormous by burl standards and the large ones are usually made into veneer because they bring in a ton of money that way.
a thuya burl and a "box" (about 3" across) made from thuya burl. The burl pic can be enlarged.
an olive ash burl and a carpathian elm burl. Note the holes in the elm burl. They are particularly common in that burl but also fairly common in burls in general. Both may be enlarged. The carpathian elm burl is widely used in the dashboards of high end luxury cars because when finished it has a very rich look.
an amboyna burl and a collibah burl. The outer surface of the cullibah burl (top picture --- the bottom pic is the inside) is typical of the wart-like appearance of burls from the outside.
two karri burls --- both have been oiled. I've never seen one of these, but they certainly look gorgeous.
red oak burl veneer and a white oak burl. I have included these because anyone familiar with wood is likely to know what oak grain looks like and can immediately see that these burls look nothing like that. Burls are always swirly and have no uniformity in grain pattern --- that's what makes them so beautiful, but it also means that they have low tensile strength. That doesn't really matter, since they are used for looks, not strength.
a paela burl. This burl (about 6" across) is a completely different color from the wood of the tree on which it grows, which is a golden tan. These burls, which generally do not get bigger than about 10" across and are usually smaller, are highly prized for the creation of wooden jewlery such as earrings. The irregularity of the outside of this burl is typical of burls. The color on this is exaggerated (not by me).