Beauty of burls
We were standing at the back of a container stuffed with exotic woods that had just arrived from Germany. I was working for a well-known specialty wood company at the time, and this container had something special in it: five large European walnut burls. Anticipation was high as we wondered about the burls and the figures hidden within them.
The burls didn’t disappoint. They were a minimum of three feet in diameter, each weighing hundreds of pounds. When we cut into them, we saw swirling grain overlaid with streaks of purple and greys. Scattered within were many “eyes” which to some are the definitive visual characteristic of burls.
Burls are lumpy growths found on the trunks, branches or roots of trees. They come in all shapes and sizes with the largest weighing up to 10,000 pounds. Redwoods are particularly known for their super-sized burls of up to 26 feet in length, often encircling the trunk of the tree.
Every burl is unique, but they do have some common characteristics. The wood grain within the burl is usually disorientated, swirling about and often interlocked. There is usually, but not always, adventitious buds present, the so-called eyes of the burl. Buds are typically found on the stems of trees where they develop into branches and leaves. Adventitious buds are bud tissue growing where they’re not expected to.
Part of the mystery of burls is that we don’t know what causes them to grow. Disease, bacteria, viruses and fungi have all been implicated. Some surmise that physical damage to the tree from fire, mechanical damage or even shading from light can initiate the abnormal growth.
Burls are unpredictable and that’s part of their allure. You don’t know what you’re going to find until you cut one open. Burls can be all swirling grain with few or no eyes. Other burls have clusters of eyes surrounded by irregular grain. These are appropriately called cluster burls and include the cat’s paw figure (yes, it does look like a print of a cat’s paw) found in domestic cherry. Burl figure reaches its apogee when the figure is all eyes. Amboyna burl, one the most prized, is often extremely dense with eyes, yielding a visual pattern that’s rich and leatherlike.
I’ve also encountered burls where there are other figure types present in addition to the eyes and swirling grain. Pigment figure is common, adding overlays of colour to the beauty of the burl. Curly figure is often present in maple and ash burls. In species where there are distinct colour differences between the sap and heartwood, woodworkers can exploit this contrast to create interesting patterns and images.
Many species produce burls. Ash, maple, walnut, cherry, redwood, oak and madrone are some of the domestic trees that produce burls. Exotic burls include amboyna, thuya, camphor and various species of eucalyptus. What makes a burl rare is finding those that are defect free and large enough to work.
Big Leaf Maple Burl
The outer surface of a burl (at right, in photo) is usually very rough and randomly shaped, and this big leaf maple burl is no exception. When sliced open (at left, in photo) a beautiful surface is revealed.
European Walnut Burl
Here are some of the offcuts from one of the massive European walnut burls Mac Sween worked with a few years ago.
From left to right: side view of the “eyes”; a small block of burl showing the eyes perfectly; and the bark side of the burl. The large sapwood / heartwood colour contrast makes for an even more striking look.
Turned Burl Bowl
Doug McGrath turned this manzanita burl bowl in 2013. If planned properly, turners can use the bark side of the burl as a bowl rim or incorporate other natural features of the burl’s shape into the design of a piece.
Australian Eucalyptus Burl
The outer surface of this burl is almost as amazing as the inner, cut surface. This Australian eucalyptus burl is somewhat large, but because its outer surface isn’t solid there’s much less usable material in this burl. This is another reason why burls are valuable.
Box with Burl Veneers
Shao-Nan Huang made this box with white ash burl veneer as the lighter panels and black walnut burl veneer adorning the edge of the lid. Burl veneer can be difficult to work with, but if you have the skills it creates a powerful visual statement.
Burls are particularly prized by turners. Spectacular wooden bowls and other decorative objects have been turned from burls. Custom pool cues, walking sticks and even tool handles have utilized a burl for their construction. Solid burls are now being slab cut for live edge tables.
Burls are also sliced into decorative veneers. Perhaps this is the most common use of burls. Veneering allows woodworkers to cover large surfaces where solid wood would be impossible. Large architectural panels, doors and boardroom tables are a few destinations. Burls also make dramatic boxes and humidors. Burl veneers are also found in high-end automobiles, private aircraft, yachts and elevator cabs.
While burls are primarily used for decorative purposes, briar burl has a specific application. It’s the preferred wood for the bowls of smoking pipes. Not only does it make an attractive bowl, but the wood of the briar burl also has some properties that are useful for a pipe. Apparently, the wood burns in a specific way that creates a charred layer that aids in the burning of tobacco, keeping it lit at the right temperature for smoking.
Working with burls will challenge your skill set. Solid burls are hard on tools as the burls can contain defects such as bark inclusions that can dull edges, and the swirling grain tears easily. The anatomy of burls means they dry unevenly. You can mount a burl on the lathe, and it can change shape as wood is removed. Woodworkers often turn in stages, letting the wood dry and settle between episodes of turning.
Challenges in using burls
Using burl veneer is also fraught with challenges. Burl veneer tends to buckle, so it will have to be softened and pressed flat before gluing on to a substrate. Defects can be removed using an irregularly shaped veneer punch. The resulting holes can be patched using the material produced by the same punch on a piece of matching veneer.
The grain on a veneer sheet can also bleed glue during pressing. Using a high-solids glue made for veneering can limit this problem. Some woodworkers dye the glue they use to hide any squeeze-out. It’s important to press your veneered panels only for the time needed to set the glue.
While the above obstacles are mostly technical, perhaps the biggest challenge is in the design process. Burls rarely fit the size requirements of a particular project. Veneers will have to be spliced together, and finding the right pattern can be tricky. When working with solid burls, you must remember that the eyes have a direction that yields the optimum figure. Finding the orientation that displays the best figure is challenging.
Despite these problems, burls remain endlessly fascinating to woodworkers. There’s nothing like cutting into a burl for the first time to see what lies underneath. A striking veneer can inspire you to come up with new designs and projects.