Woodsense: Black Locust

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This article is from Issue 106 of Woodcraft Magazine.

In some ways, black locust might be considered a superhero (or maybe a supervillain). The tree the lumber comes from is one of the fastest-growing hardwoods, and the wood is one of the hardest, stiffest, and most rot-resistant species native to North America. It’s rapidly gaining traction as a “greener,” more sustainable alternative to both pressure-treated lumber and many of the rainforest species imported for decking. When properly dried, the wood is quite stable and relatively easy to work given its hardness. Even as firewood, black locust stands out as having one of the highest BTU values of any North American species. The tree is also quite hardy and adaptable to many climates. Strong, resilient, easy to work—super. But black locust has a sinister side. When planted, it can quickly gain a foothold and crowd out other native species. It also grows back quickly from stumps and roots after being cut down making it hard to eradicate. Because of these aggressive tendencies, some areas and even states consider it an invasive species and outlaw its propagation.

Whistle while you turn. These small black locust whistles proved to be a fun project for my beginning spindle-turning students. Everyone smiles after their first tootle. Despite its hardness, the wood turns well even when cutting against the grain.

Where the wood comes from

To the best of our knowledge, black locust (Robinia pseudoacacia) is native to the Appalachian region. But Native Americans saw the benefit to this species and transplanted it throughout what would become the coastal plains of Virginia long before Europeans began colonizing. Since then, the tree has spread across North America and around the world. In fact, some of the finest black locust lumber comes from plantations in Hungary, where the species has been selectively cultivated for centuries. Sadly, little of that is imported to the U.S. What you’re likely to find here comes from small (60-70' tall, 15-20" diameter) hedgerow trees. As a fast-growing “weed” tree, black locust is not listed on the CITES or IUCN Red List and is considered a species of least concern.

History in woodworking

Black locust’s desirable characteristics have been known for centuries. When the first Europeans crossed the Atlantic, they found Native Americans utilizing it for hunting bows made from the strong yet springy wood. These early colonists soon took advantage of the wood’s rot resistance, using it for the foundations of their houses in Jamestown. Later, during the War of 1812, one reason the Americans bested the English in the decisive battle of Plattsburg Bay on Lake Champlain had to do with black locust. The U.S. warships were held together with black locust trunnels (“tree nails”) that withstood cannon fire much better than the British ships with their oak trunnels. Afterward, Britain began importing thousands of black locust trunnels to refit its naval vessels. Since then, the wood has been used for everything from fence posts to furniture.

Working and finishing 

Black locust is primarily sold as solid lumber, though some veneer is available. The trees are small and grow crookedly, yielding boards typically shorter (8-12') and narrower (<10") than many other species. For best rot resistance, seek lumber without sapwood and early growth, as these parts of the tree will decay. Pricing ranges from $5-7 per board foot. Once dry, the wood is reasonably stable. But as the trees are crooked, the boards tend to warp when cut. (For more about this, see sidebar at right.) So you’ll want to sight down the edges of your boards to find the straighter pieces. Black locust lumber is quite hard and dense, but it mills and saws well. Standard precautions such as predrilling for fasteners are in order. The wood accepts both glue and finishes readily. It is also a good choice for bending with either steam or lamination.
Misnomer. Despite its name, black locust is more tan with a greenish-yellow cast. Its wide growth rings give the wood a bold, somewhat streaky character. Its heartwood is quite durable, but the sapwood quickly succumbs to insects and rot.

Graceful integrity. Locust is an ideal wood for three-legged stools. Its exceptional strength allows you to size the parts to look graceful without compromising structural integrity.

A black locust adventure

In early November, as I developed my outdoor café table (see p. 32), making it from black locust seemed like a good idea. My lumber dealer had plenty of 4/4 in stock but no 8/4 for the base. “No problem,” he said, “I just got a couple black locust logs and can cut what you need. I can dry it and have it for you by Christmas.” When I picked up the order, I was a bit dismayed at how bowed many boards were. According to the sawyer, some of the boards dramatically peeled away from the blade as the logs were sawn. After several weeks of acclimation time in my shop, I crosscut the pieces to rough length. By the next day, every board had developed end checks. It was discouraging. My supplier said, “In hindsight, it probably would have been better to let those logs air-dry for several months before attempting to cut and dry them.” Lesson learned. But what about my table? A week passed before I summoned the courage to cut the stock to its final size. Milling went well; the hard lumber didn’t even do appreciable damage to my steel jointer knives. Even better, the end checks proved to be surprisingly shallow, less than an inch deep in most cases. As I write this several days later, the checks haven’t reappeared (knock on, well, wood). Even more impressive was how the wood behaved as I beveled the tapered staves that make up the table’s base. To adjust the bevel angle, I ran the staves over the jointer. The cut on the first edge of each piece was with the grain, but when cutting the second side, the grain was definitely against me. Amazingly, even on the uphill cuts, the wood didn’t tear out at all. Nice. In the meantime, I had made the whistles and the stool shown here. I think locust’s hardness works in its favor when milling and making shear cuts on the lathe. Sharp spindle gouges and skews left a highly burnished surface requiring little sanding. And the lack of tearout was just shy of miraculous.


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