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Friday, January 29, 2010

So, Milton ... exactly what is a BURL ???

What are burls? Why are they prized?

Good questions, glad you asked.

A burl (aka "burr") is an aberrant, more-or-less rounded growth on a tree; you've no doubt seen at least one- it's a bump on the trunk (or possibly on a limb, or even on the roots)... it looks like a large wart, tumor, or canker. Picture a bubble in the sidewall of a tire. Below is a photo of an Elm burl.

Some speculate they're caused by some form of stress to the tree, such as insect or frost damage, or bacteria or fungus invasion, but not a lot is known about the causes. Essentially, a burl is a swelling, an area of cell growth that's happening more rapidly than the surrounding wood. In normal tree growth, cambium cells divide and enlarge to create new tissue (wood and bark); in a burl, the cell division or enlargement is accelerated. They sometimes may be caused by soil-borne bacteria (Agrobacterium tumefaciens) that affect the tree's growth hormones.

Often a burl will contain a mass of growth buds that never developed beyond small knots, and look like birdseye figure within its wood (note: true birdseye figure occurs in non-burl wood, such as birdseye maple). Burls are covered with bark as they grow, just like the rest of the tree. They do not kill the tree.
Burl on root of Black Birch

A hophornbeam burl

3-foot White Birch burl
The random, contorted grain within the burl often produces some spectacular results in items made from it. Relative to the number of trees there are, burls are not common. They're considered to be defects by many lumbermen, but are prized by woodworkers because of the beauty within them.

Here's a bowl turned from a Black Cherry burl.

Many tree species can produce burls, but not all are highly desirable. Those that have strong coloration and birdseye-like figure are probably the most valued. Those that lack birdseyes still probably have at least an interesting wavy, undulating grain pattern, due to the annual growth rings bulging out of the trunk and into the burl. Here's a bowl of Black Ash burl showing areas of birdseye-like figure, and an area of curly grain.

When a limb is cut off, or dies off, the tree will gradually heal over by enclosing the stub in new bark. The resulting bulge is mistaken by some to be a burl, but it is not- it's a branch stub. Below is a photo of a healed over Elm branch stub.

In my area of New England, the most sought after burl is probably that of Black Cherry, although there are many other species, including maple, oak, birch, spruce, willow, etc.

Members of the Cherry family are susceptible to Black Knot disease, which causes unsightly growths on twigs, and can kill the tree, or at least the infected branch. These growths are galls caused by a fungus (Apiosporina morbosa). Often, when these galls are on twigs, the twig dies off beyond the gall. Here are 2 photos of Black Knot cankers on Cherry branches.

Many trees are affected by cankers, which are areas of dead tissue, and tend to be more of a sunken form, whereas burls are swellings. Cankers are caused by fungi, bacteria, or physical damage that is later infected.

There are beautiful burl woods found around the world; some of my favorites (besides our North American ones) are found on Australian trees, particularly on their many species of Eucalyptus. Some grow to monstrous sizes, weighing thousands of pounds!
Here's one of my favorite Australian ones, the Red Mallee burl.

Although burls from some tree species are valuable, not all are. Many just don't have particularly interesting figure or color inside. Even within a given tree species, some burls may be quite attractive inside, but others not. For example, I've been disappointed with many maple and oak burls (sometimes after paying cash for them), but have been pleased with others of those species. You just don't know what's inside them until they're cut open.

I use a wide variety of burls in my turnings, and continue to seek out species I haven't experienced yet.

Have any burl stories to share?

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