What are tree burls?

J. Crowe locust bowl

Our woodcarver Jerry Crowe uses mainly burls for his bowls.  What exactly are burls and how dot hey occur?

A burl (American English) or bur or burr (British English) is a tree growth in which the grain has grown in a deformed manner. It is commonly found in the form of a rounded outgrowth on a  tree trunk or branch that is filled with small knots from dormant buds.

A burl results from a tree undergoing some form of stress. It may be caused by an injury, virus or fungus. Most burls grow beneath the ground, attached to the roots as a type of malignancy that is generally not discovered until the tree dies or falls over. Almost all burl wood is covered by bark, even if it is underground. Insect infestation and certain types of mold infestation are the most common causes of this condition.

Cherry bowl with turquoise

J. Crowe elm bowl with turquoise

Burls yield a very peculiar and highly figured wood, prized for its beauty by many; its rarity also adds to its expense. It is sought after by furniture makers, artists, and wood sculptors. There are a number of well-known types of burls (each from a particular species); these are highly valued and sliced into veneers for furniture, inlay in doors, picture frames, household objects, automobile interior paneling and trim, and woodturning. The famous birdseye maple of the sugar maple (Acer saccharum) superficially resembles the wood of a burl but is something else entirely. Burl wood is very hard to work with hand tools or on a lathe because its grain is twisted and interlocked, causing it to chip and chatter unpredictably. This "wild grain" makes burl wood extremely dense and resistant to splitting, which made it valued for bowls, mallets, mauls and "beetles" or "beadles" for hammering chisels and driving wooden pegs.

Oak bowl by Jerry Crow

Oak bowl by J Crowe 

Some burls are more highly prized than others.  Some resemble an explosion in which the grain grows erratically, and it is these burls that the artist prizes over all other types. These spectacular patterns enhance the beauty of wood sculptures, furniture, and other artistic productions. Burls are harvested with saws or axes for smaller specimens and timber felling chainsaws and tractors for massive ones. (Info from Wikipedia)