Ring-porous and valuable hardwoods II: robinia, sweet chestnut, walnut and cherry

Durable and decorative timbers

This lesson covers two of the most durable European woods (robinia and sweet chestnut) and two decorative woods (walnut and cherry, both semi-ring-porous). North American species with similar names (black walnut, black cherry) are different timbers; keep their data apart.

Robinia (black locust)

Robinia is among the densest and hardest timbers grown in Central Europe, with ring-porous structure, abundant tyloses and a yellowish-white sapwood only about 1-2 cm wide.

LWF calls it the most durable European timber, ahead of oak and sweet chestnut. But durability depends on age: in 16-week laboratory tests mature heartwood lost on average 0.7 % mass to a brown-rot fungus, whereas juvenile heartwood lost 10.1%. Juvenile wood (roughly the first 7-15 rings per LWF) and sapwood are not durable. Drying is the main difficulty: slow, with strong warp tendency; air-dry under cover with sealed ends, then condition.

Worked example 1 (sapwood share). Cross-section area scales with diameter squared. For a 20 cm log with a 2 cm sapwood band on each side, the heartwood core is 20 - 4 = 16 cm across, so heartwood is (16 / 20)^2 = 0.64, or 64 % of the area.

Sweet chestnut (Castanea sativa)

Chestnut is a medium-weight, relatively soft hardwood: density 0.59 g/cm3 (LWF) against 0.71 for oak, stiffness 9,000 N/mm2 against 13,000 for oak, Brinell hardness 15-23 against 23-42 N/mm2. LWF reports no EN 338 strength class and no building-timber grade.

Worked example 2. Chestnut stiffness relative to oak is 9,000 / 13,000 = 0.69, about 69 %; density is 0.59 / 0.71 = 0.83, about 83 %. Stiffness drops far more than density, so do not size chestnut as if it were oak.

• Heartwood class 2 under the 1995 edition of EN 350, with high variability between studies; thin sapwood (1-1.5 cm) must be milled off.
• Ring shake is the main yield loss: in one German district only 30-50 % of stronger timber was sawable.
• Dry slowly: collapse and honeycomb are risks. Keep it away from iron.

Walnut (Juglans regia)

Walnut timber comes from scattered cultivated trees; burl and butt logs carry the value. Sapwood is wide (5-10 cm) and non-durable.

Sources conflict on drying speed and collapse risk. The one numeric schedule (Tropix, 38 mm or less, "for information only") runs from 50 / 47 C dry/wet bulb at green to 75 / 58 C at 15 % MC; treat it as an unverified starting point. Many substitute woods are sold under misleading "walnut" names.

Worked example 3. For a 40 cm log with 8 cm sapwood on each side, heartwood is 40 - 16 = 24 cm across, so (24 / 40)^2 = 0.36, only 36 % of the area is the dark heartwood buyers want.

Cherry (Prunus avium)

Cherry has a visible sapwood band 2.5-5 cm wide (Bavaria), semi-ring-porous structure and golden to reddish-brown heartwood that darkens in light. Colour is the central quality criterion.

• European density about 0.57 g/cm3 (range 0.49-0.67), bending 85-110 N/mm2 (LWF, Germany); black cherry (560 kg/m3) is a North American species.
• Dry slowly: too fast causes warping and end checks.
• Indoor use only: class 3-4 in the literature, sapwood non-durable.

Checklist: valuable hardwood logs

• Measure sapwood width before pricing.
• Keep wet boards away from iron (oak, chestnut, walnut).
• Label species and region on every datasheet row.

Key points

• Robinia: 0.74 g/cm3, heartwood class 1-2, but juvenile wood and sapwood are not durable.
• Sweet chestnut: stiffness about 69 % of oak's; ring shake drives yield loss.
• Walnut: wide sapwood, conflicting drying evidence, J. nigra data are not J. regia.
• Cherry: colour and sapwood share decide grade; interior use only.