Woods to Know

Woods to Know: Black Locust

Black Locust (Robinia pseudoacacia)

by Peter MacSween

Early settlers in the U.S. struggled to survive when they first arrived at Jamestown in the early 1600s.  Being short of food and shelter as well as facing disease, they quickly learned to use whatever the forests of their new home could provide.  This is where the story of Black Locust begins.

Hard, heavy and with a heartwood that is extremely durable, Black Locust was quickly utilized for all sorts of applications.  Building timbers, fence posts, boxes, wagon wheels, fenders and flooring were all made from this tree.  The advent of rail travel created another purpose for Black Locust in rail ties.  It was also utilized by boat builders looking for a durable wood.

Black Locust is a medium sized tree typically growing 40 to 60 feet tall with diameters of 1 to 2 feet.  Exceptional specimens have reached 100 feet in height.  It originally grew in an area encompassing the Appalachian Mountain Range west to Oklahoma and then south to Louisiana.  It is now found throughout North America, Europe, and Asia where it has been planted as an ornamental - and for erosion control.  Given its tolerance to air pollution, Black Locust is also commonly planted as a street tree.

WTK_black-locust

Black Locust is a fast growing tree reproducing sexually and asexually via root suckers.  It is shade intolerant preferring disturbed environments.  It can quickly overcome prairie and savannah ecosystems creating new forests.  For this reason, Black Locust is often considered an invasive species.

Black Locust is a ring porous wood with large early wood cells and distinct growth rings.  The heartwood is a yellowish green to a dark brown and will darken with age.  The yellowish white sapwood is distinct from the heart and is not durable.  The grain is straight with a coarse to medium texture.  The pores are filled with tyloses and extractives which probably give the wood its durability.

It is a stable wood with low volumetric shrinkage during drying.  Black Locust can be prone to attack by beetles.  Young trees can appear healthy but as the beetle infestation takes hold, the trees become stunted and fail to reach a mature size.

There is little information on the working characteristics of Black locust.  Do to its small size and density it never became a popular cabinet making wood.  I suspect the availability of cherry, walnut, pine, etc. relegated Black Locust to utilitarian purposes.  If you do decide to work with it, its hardness and density will require sharp tools; however, it will blunt tools, so carbide tooling is a good choice.  I would watch feed rates when machining to prevent burning.  Nails and screws must be predrilled.  Black Locust glues well and will accept all finishes.  There are some concerns around toxicity.  It is good advice to avoid ingesting any part of the tree and this applies to animals as well.

WTK_black-locust-endgrain
Black Locust End Grain

Black Locust is a medium priced domestic, but it is not a commercially cut timber.  So if you want to work with it, you may have to do some searching.  Specialty lumbers dealers are your best bet.  The sawn lumber can be small, and wood derived from urban trees can be problematic due to uneven growth.  There can be insect damage as well.  Despite this, Black Locust would be ideal for turning projects, tool handles and other small projects where durability and hardness is prized.  Black Locust doesn't have the same fine woodworking pedigree as walnut or cherry, but it is worth celebrating for its durability and hardwearing properties.



 
Average Dried Weight48 lbs/ft³A measure of its weight at 12% moisture and an ambient temperature of 70°F.
Specific Gravity.77A measure of the ratio of its density compared to water (at 12% MC)
ShrinkageRadial: 4.6
Tangential: 7.2
Volumetric: 10.2
Radial (the amount of crosswise shrinkage);
Tangential (the amount of lengthwise shrinkage);
Volumetric (the total amount of shrinkage.)
T/R Ratio1.6A measure of the uniformity of tangential to radial shrinkage.
Janka Hardness1,700 lbfA measure of resistance to denting and abrasion.
Crushing Strength10,200 lbf/in²A measure of compression strength parallel to the grain.
Colour
Ranges from a pale greenish-yellow to a darker brown. Tends to darken to a russet brown with age. Can be confused with Osage Orange and Honey Locust in some instances.
GrainUsually straight.
TextureMedium.
Workability
Although the grain is usually straight, its high density and hardness can make it difficult to machine. Has a moderate blunting effect on cutting edges. Responds very well to both lathe turning and steam bending; glues and finishes well.
Uses
Fence posts, boatbuilding, flooring, furniture, mine timbers, railroad ties, turned objects, and veneer.
PriceContact your local lumber dealer.
For a list of Canadian retailers click here

 
  
Photos and Specifications Courtesy of: The Wood Database