Wood species
Slippery elm wood
Slippery elm is a hardwood (Ulmus rubra). At 12% moisture content it has a Janka hardness of 860 lbf (3,825 N) and it weighs about 37 lb/ft³ (594 kg/m³), 3.1 lb (1.4 kg) per board foot.
Also called: red elm, Ulmus rubra.
Key numbers
- Janka hardness, 12% MC
- 860 lbf (3,825 N)
- Green: 660 lbf (2,936 N)
- Weight at 12% MC
- 37 lb/ft³ (594 kg/m³)
- 3.1 lb per board foot
- Specific gravity (oven-dry weight)
- 0.53 at 12% MC volume
- 0.48 on green volume
- Shrinkage, green to oven-dry
- 8.9% tangential
- Radial 4.9%, volumetric 13.8%
One source: USDA Forest Products Laboratory, the authority for species properties it measured
Why: Measured by the USDA Forest Products Laboratory; slippery elm's density is worked from its specific gravity with the Handbook's own equation.
How much slippery elm moves
A 12 in wide flatsawn board of slippery elm that goes from 6% to 12% moisture content swells about 0.23 in (about 7/32 in, 5.8 mm). Quartersawn, the same board moves about 0.12 in (about 1/8 in, 3.1 mm). Flatsawn faces move 1.8 times as much as quartersawn ones, because the wood shrinks 8.9% around the growth rings but only 4.9% across them.
Worked with the Wood Handbook's dimensional change coefficients for slippery elm (Table 13-5: 0.00315 tangential, 0.00169 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
- Light brown heartwood, often tinged red, and nearly white sapwood.
- Working and character
- A soft elm, with American elm: moderately heavy, moderately hard and stiff, with high shock resistance. Elm has excellent bending qualities.
- Uses
- Elm lumber and veneer go mostly into furniture and decorative panels.
The Handbook reports that the diseases killing American elm also affect the other elms, so far less severely.
Adapted in our own words from the Wood Handbook, Chapter 2, p. 2-6 to 2-7.
Strength
Bending strength is the modulus of rupture (MOR); stiffness is the modulus of elasticity (MOE). Clear, straight-grained samples of slippery elm 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) | 8,000 psi (55 MPa) | 13,000 psi (90 MPa) |
| Stiffness (MOE) | 1,230,000 psi (8.5 GPa) | 1,490,000 psi (10.3 GPa) |
| Crushing strength (compression parallel to grain) | 3,320 psi (23 MPa) | 6,360 psi (44 MPa) |
| Compression perpendicular to grain | 420 psi (3 MPa) | 820 psi (6 MPa) |
| Shear parallel to grain | 1,110 psi (8 MPa) | 1,630 psi (11 MPa) |
| Tension perpendicular to grain | 640 psi (4 MPa) | 530 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
Buying and finishing slippery elm
Work it out for your project
- Rank woods for furniture with the species picker
Weigh slippery elm against other woods on hardness, weight, movement and color, every figure from the Wood Handbook.
- Plan a finish for furniture in slippery elm
The finishes whose makers name that use, and a dated coat schedule from each product’s own sheet.
Why these numbers
Why these numbers?
The situation
The USDA Forest Products Laboratory measured slippery elm at 860 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 × 0.53 × 1.12.
Arithmetic: exact by definition , sources [1]
The number
About 37 lb/ft³, or 3.1 lb per board foot (1/12 ft³), at 12% moisture content.
Arithmetic: exact by definition
Sources