Black locust: Decay resistant, tough & durable

Black locust: Decay resistant, tough & durable

Black locust is a member of the legume family; it can “fix nitrogen” in the soil. It is native to the Appalachian Mountains, from Pennsylvania to Alabama. However, in the last century, it has spread to almost every state. With a widespread, shallow root system, it is ideal for thin soils where it will prevent soil erosion; it is often used for strip-mine reclamation projects barren due to mining debris and acid soils.

The tree has long thin bean-like pods. (Do not confuse black locust with honey locust. They are not related.) The flowers are very sweet smelling during the early spring, and the pollen and nectar are used by bees to produce excellent honey.

High natural decay resistance of the wood has resulted in frequent use of black locust for fencing. It also was the popular species for the pins that held glass insulators on the cross-arms of electric and telegraph/telephone poles. It also was the prized species for wheel hubs for wooden wheels, such as used on the western covered wagons. Xylophone keys are another use. Today, this wood has fallen into neglect within our industry (lumber prices are often low, especially for cabinet grades; dry firewood is a common use). It deserves better treatment.

The tree is short-lived, so it does not grow to large sizes; a large tree would be 24 inches in diameter at the base and 50 feet tall. Wide, clear pieces of black locust lumber are not common. A wood boring insect often invades the tree. A leaf miner often turns entire hillsides into brown-leafed, “dead-looking” trees for a week or so in the summer, but this damage is not fatal.

Processing suggestions and characteristics

Weight. This is a fairly heavy wood, more than 10% heavier than red oak. The weight, when dry, is 50 pounds per cubic foot or about 4 pounds per board foot.

Strength. Black locust is one of the strongest, hardest native American species.  For dry wood, the ultimate strength (MOR) is 19,400 psi, stiffness (MOE) is 2.05 million psi and hardness is 1,700 pounds. Mechanical fastening is difficult because of a tendency for splitting. Predrilling for nails and screws is advisable.

Drying. The wood dries slowly with some risk of checking, end splitting, and warping. Slow shed air-drying should be followed by kiln drying. Shrinkage in drying is moderately high. Overall shrinkage in drying from green to 6% MC is 7.2% tangentially (the width in flatsawn lumber) and 4.6% radially (the thickness of flatsawn lumber).

Stability. A typical final MC range is 6.0 to 7.5%, unless used in a humid location. It takes a 4% MC change to result in a 1% size change tangentially and a 6% MC change radially.

Machining and gluing. This wood machines with difficulty due to its hardness. This wood glues with some difficulty.

Grain and color. The heartwood is often quite green when first cut, but ages quickly to a russet brown color. The grain is moderately fine, but the annual rings are obvious and add character to the appearance.

 

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