For a furniture designer/maker like myself a burr (or burl in American) is one of the most prized parts of a tree, it often will have a swirling intricate highly figured grain, which can be difficult to work with, but looks incredible, with every burr been individual and unique; but what actually is it, where is it from on a tree?
A burr is a growth on a tree which has formed in an irregular manner, often found on the trunk, or even the roots. It might be a single growth on a tree trunk, or multiple profuse growths covering much of the tree. The Growth is often formed from swirling grain and small knots and buds which have formed into a growth over many decades.
The growth itself is often a reaction to stress, such as injury from damage, insect attack or bacterial, viral or fungal infection. The tree responds to the stress by growing around it to protect the tree, creating a highly figured rare form of wood which can become very large. Because of their rarity, burrs are often very expensive and will be used in furniture in both a solid and veneer form.
Many species of tree globally will produce burrs, including some of my favorites such as Oak, Ash, Elm, Beech, Maple, Alder Chestnut, and Walnut. Here are some examples of pieces made from two of my favorite burrs, Walnut Burr and Chestnut Burr.