Wood species
Black cottonwood wood
Black cottonwood is a hardwood (Populus trichocarpa). At 12% moisture content it has a Janka hardness of 350 lbf (1,557 N) and it weighs about 24 lb/ft³ (392 kg/m³), 2 lb (0.93 kg) per board foot.
Also called: Populus trichocarpa.
Key numbers
- Janka hardness, 12% MC
- 350 lbf (1,557 N)
- Green: 250 lbf (1,112 N)
- Weight at 12% MC
- 24 lb/ft³ (392 kg/m³)
- 2 lb per board foot
- Specific gravity (oven-dry weight)
- 0.35 at 12% MC volume
- 0.31 on green volume
- Shrinkage, green to oven-dry
- 8.6% tangential
- Radial 3.6%, volumetric 12.4%
One source: USDA Forest Products Laboratory, the authority for species properties it measured
Why: Measured by the USDA Forest Products Laboratory; black cottonwood's density is worked from its specific gravity with the Handbook's own equation.
How much black cottonwood moves
A 12 in wide flatsawn board of black cottonwood that goes from 6% to 12% moisture content swells about 0.22 in (about 7/32 in, 5.6 mm). Quartersawn, the same board moves about 0.09 in (about 3/32 in, 2.2 mm). Flatsawn faces move 2.4 times as much as quartersawn ones, because the wood shrinks 8.6% around the growth rings but only 3.6% across them.
Worked with the Wood Handbook's dimensional change coefficients for black cottonwood (Table 13-5: 0.00304 tangential, 0.00123 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
- Grayish white to light brown heartwood; whitish sapwood merging into it.
- Grain and texture
- Fairly uniform texture, generally straight grained; odorless when well dried.
- Working and character
- Most strength properties are slightly lower than eastern cottonwood’s, and shrinkage is moderately high. Tension wood can leave a fuzzy surface that is hard to work.
- Uses
- Lumber, veneer and pulpwood; the lumber goes mainly into boxes, crates, baskets and pallets.
It grows on the West Coast and in western Montana, northern Idaho and western Nevada.
Adapted in our own words from the Wood Handbook, Chapter 2, p. 2-6.
Strength
Bending strength is the modulus of rupture (MOR); stiffness is the modulus of elasticity (MOE). Clear, straight-grained samples of black cottonwood 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) | 4,900 psi (34 MPa) | 8,500 psi (59 MPa) |
| Stiffness (MOE) | 1,080,000 psi (7.4 GPa) | 1,270,000 psi (8.8 GPa) |
| Crushing strength (compression parallel to grain) | 2,200 psi (15 MPa) | 4,500 psi (31 MPa) |
| Compression perpendicular to grain | 160 psi (1 MPa) | 300 psi (2 MPa) |
| Shear parallel to grain | 610 psi (4 MPa) | 1,040 psi (7 MPa) |
| Tension perpendicular to grain | 270 psi (2 MPa) | 330 psi (2 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
Buying and finishing black cottonwood
Work it out for your project
- Rank woods for your project with the species picker
Weigh black cottonwood against other woods on hardness, weight, movement and color, every figure from the Wood Handbook.
Why these numbers
Why these numbers?
The situation
The USDA Forest Products Laboratory measured black cottonwood at 350 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 × 0.35 × 1.12.
Arithmetic: exact by definition , sources [1]
The number
About 24 lb/ft³, or 2 lb per board foot (1/12 ft³), at 12% moisture content.
Arithmetic: exact by definition
Sources