Wood species
Black locust wood
Black locust is a hardwood (Robinia pseudoacacia). At 12% moisture content it has a Janka hardness of 1,700 lbf (7,562 N) and it weighs about 48 lb/ft³ (773 kg/m³), 4 lb (1.82 kg) per board foot.
Also called: yellow locust, locust.
Key numbers
- Janka hardness, 12% MC
- 1,700 lbf (7,562 N)
- Green: 1,570 lbf (6,984 N)
- Weight at 12% MC
- 48 lb/ft³ (773 kg/m³)
- 4 lb per board foot
- Specific gravity (oven-dry weight)
- 0.69 at 12% MC volume
- 0.66 on green volume
- Shrinkage, green to oven-dry
- 7.2% tangential
- Radial 4.6%, volumetric 10.2%
One source: USDA Forest Products Laboratory, the authority for species properties it measured
Why: Measured by the USDA Forest Products Laboratory; black locust's density is worked from its specific gravity with the Handbook's own equation.
How much black locust moves
A 12 in wide flatsawn board of black locust that goes from 6% to 12% moisture content swells about 0.18 in (about 3/16 in, 4.6 mm). Quartersawn, the same board moves about 0.11 in (about 1/8 in, 2.9 mm). Flatsawn faces move 1.6 times as much as quartersawn ones, because the wood shrinks 7.2% around the growth rings but only 4.6% across them.
Worked with the Wood Handbook's dimensional change coefficients for black locust (Table 13-5: 0.00252 tangential, 0.00158 radial per 1% change in moisture content), which hold between 6% and 14%. Individual boards vary: the Handbook gives shrinkage a coefficient of variation of about 15% (p. 4-11).
Color, grain and working
- Color
- Narrow creamy white sapwood; heartwood greenish yellow to dark brown when fresh cut.
- Working and character
- Heavy, hard, strong, stiff and tough, with fairly low shrinkage and highly decay-resistant heartwood.
- Uses
- Fence posts, poles, stakes, mine timbers and rough construction; historically wooden pegs for ships.
Adapted in our own words from the Wood Handbook, Chapter 2, p. 2-8.
Strength
Bending strength is the modulus of rupture (MOR); stiffness is the modulus of elasticity (MOE). Clear, straight-grained samples of black locust tested by the Forest Products Laboratory (Wood Handbook Table 5-3b). Knots and cross grain make lumber weaker than these clear samples (Handbook Chapter 5).
| Property | Green | 12% MC |
|---|---|---|
| Bending strength (MOR) | 13,800 psi (95 MPa) | 19,400 psi (134 MPa) |
| Stiffness (MOE) | 1,850,000 psi (12.8 GPa) | 2,050,000 psi (14.1 GPa) |
| Crushing strength (compression parallel to grain) | 6,800 psi (47 MPa) | 10,180 psi (70 MPa) |
| Compression perpendicular to grain | 1,160 psi (8 MPa) | 1,830 psi (13 MPa) |
| Shear parallel to grain | 1,760 psi (12 MPa) | 2,480 psi (17 MPa) |
| Tension perpendicular to grain | 770 psi (5 MPa) | 640 psi (4 MPa) |
A dash means the Handbook prints no value; none is filled in.
Dust and health
- Wood dust is classed as carcinogenic to humans (Group 1) by the International Agency for Research on Cancer, chiefly for cancers of the nose and sinuses.
- NIOSH recommends keeping wood dust exposure to 1 mg/m³ of total dust, as quoted in OSHA's sawmill guidance.
This is what the named authorities publish, not medical advice.
Compare with
Why these numbers
Why these numbers?
The situation
The USDA Forest Products Laboratory measured black locust at 1,700 lbf of side hardness at 12% moisture content, on clear, straight-grained samples (Wood Handbook Table 5-3b).
One source: USDA Forest Products Laboratory, the authority for species properties it measured , sources [1]
What that means
Side hardness (Janka) is the load needed to press a 0.444 in (11.28 mm) steel ball halfway into the wood across the grain; a higher number dents less (Handbook p. 5-5).
One source: USDA Forest Products Laboratory, the authority for species properties it measured , sources [1]
So
Density at 12% is water's density × specific gravity × (1 + 0.12) (Handbook Eq. 4-14): 62.43 × 0.69 × 1.12.
Arithmetic: exact by definition , sources [1]
The number
About 48 lb/ft³, or 4 lb per board foot (1/12 ft³), at 12% moisture content.
Arithmetic: exact by definition
Sources