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Lesson 7 of 7
Natural durability, species at a glance and limits of this course
Heartwood, extractives and decay
Decay fungi need food, moisture, oxygen and a suitable temperature. The Wood Handbook (North America) describes untreated lumber uniformly below 20 % MC as immune to fungi, and Morris (Forintek, Canada) places established decay at roughly 22 to 24 % MC.
A 2024 critical review (Kirker and others, in Insects) calls heartwood extractives the principal source of resistance to wood-destroying organisms in naturally durable woods, with specific gravity, density and water exclusion contributing. Four points matter in practice:
- Do not count on sapwood. ATIBT sheets state that durability data concern heartwood only. Treat sapwood as non-durable in every species.
- Durability varies with tree, site, age and silviculture. Old-growth wood tends to contain more extractives than plantation wood.
- Extractives are not permanent. Water-soluble ones can leach out and UV degrades them.
- Extractives are not a detachable preservative. When stilbenes from pine and other species were impregnated into non-durable wood, their effect fell to roughly one hundredth of that in the original heartwood.
EN 350 and the species at a glance
EN 350:2016 (as summarised by ATIBT) classifies heartwood resistance to fungi from 1 (most durable) to 5 (not durable). The portal could not read an authoritative copy, so class names are not quoted. Values for Europe as stated in the species articles:
| Species | Heartwood | Sapwood and treatability |
|---|---|---|
| Spruce | DC 4 (LWF Bayern; Teknos) | Impregnation poor, class 3 to 4 (CIRAD) |
| Scots pine | DC 3 to 4 (Teknos, NIBIO) | Sapwood highly susceptible but easy to treat; heartwood very difficult (NIBIO) |
| Larch | DC 3 to 4 (LWF Bayern) | Sapwood class 5; treatability sapwood 2, heartwood 4 |
| Oak | DHK 2 in older EN 350-2; field trials class 5 in ground, 4 above ground | Sapwood generally treated as non-durable (trade view) |
| Beech | Class 5, not durable (Swedish Wood) | Not durable |
A laboratory class is not a field guarantee, and the current EN 350:2016 class for oak was not read.
Chemistry that reaches the machine
Extractives affect more than decay. Oak has tannins (about 6 % in fresh wood) and a pH near 3.9; wet oak reacts with iron to give blue-grey to black stains, and alkaline glues or acid adhesives can be difficult, so test first (LWF Bayern; Gabarro). Blue stain and mould feed on sapwood sugars, so sapwood needs prompt drying: in an Austrian study, Norway spruce logs stored in summer at the higher site (864 m) showed initial sap stain after 14 days (Bohm and others, 2023).
From structure to the saw and the kiln
| Feature | At the saw | In the kiln |
|---|---|---|
| Sapwood and heartwood | Keep sapwood out of exposed grades | Wide moisture spread in softwoods |
| Latewood contrast | Sanding dips, coating problems (larch) | Little by itself |
| Rays, large pores | Check and tear | Surface checks, honeycomb risk |
| Density | Tool wear, truck weight | Water to remove |
| Spiral grain | Twist, strength loss | Restrain the stack |
| Juvenile wood | Sort by diameter | Bow, crook, twist |
| Reaction wood | Read the log end | Segregate; collapse in tension wood |
| Tannins, resin | Iron stain, resin on blades | Darkening if too hot |
Limits of this course
This course was drafted with AI assistance from the portal's fact-checked articles and adds no outside facts. Most strength, shrinkage and moisture data are North American (USDA Wood Handbook, US extension guides, a Canadian lecture). European data are single studies, such as Czech density measurements, German LWF tables, Swedish spiral grain and Finnish density studies.
Not verified, so not given: European shrinkage tables beyond those quoted, commercial European kiln schedules, EN 350 class names, drying differences between ring-porous and diffuse-porous woods, and any universal juvenile-wood boundary. The grain-slope percentages come from a lecture that does not name the property, and reaction-wood shrinkage multiples differ between sources. Figures are examples, not specifications. Check them against current standards (EN 338, EN 350, EN 14081) and your contracts before using them for decisions.
Key points
- Heartwood extractives give durability; sapwood is never durable, and durability varies between trees.
- Europe: spruce DC 4, pine and larch DC 3 to 4, beech class 5; oak's laboratory class is better than its field trials.
- Extractives cannot simply be added back to non-durable wood.
- Anatomy links to the machine: rays and pores to checks, extractives to stains, density to water and weight, juvenile and reaction wood to distortion.
- Treat this course as a starting point: AI-drafted, regional, to be checked against standards and contracts.