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Lesson 4 of 7
Stain, mould, decay and drying degrade
Stain, mould, decay and drying degrade turn good boards into claims. They have different causes, and the cause decides who is responsible, so this lesson keeps them apart.
Moisture decides what can grow
The US Wood Handbook states that untreated lumber uniformly below 20% moisture content (MC) is immune to attack by fungi, and a Forintek review calls 20% a frequently quoted maximum safe MC. The same review puts established decay at roughly 22 to 24% MC and says spore infection probably does not occur below about 29%. Most wood-rotting fungi thrive between 20 and 30 degrees C.
Blue stain, mould and decay
Blue stain (sapstain) is caused by fungi living on the sugars in sapwood; they colonise freshly felled logs and green timber quickly. Teknos says damp pine sapwood is highly susceptible. Blue stain does not reduce strength, but it makes water penetration easier, and LWF describes it as a blue-black discolouration, often in summer-felled wood, that rules out visual-grade use although technical properties remain. For appearance products it is a grading criterion, not only a storage problem. DIN 4074-1, which grades strength, permits blue stain in all of S7, S10 and S13.
Mould is surface growth on damp wood; it needs an MC above about 20%, and the discolouration is often confined to the surface.
Decay destroys structure. Early brown or white rot reduces strength, and rot that began in the standing tree or the stored log cannot be reversed and is cut out.
How fast does it happen?
The Wood Handbook storage chapter reports that blue stain can develop in exposed log ends within 2 weeks after felling in warm weather, and can penetrate a log as fast as 1 ft (about 30 cm) a month, a maximum rather than a typical rate. Green timber at 25 to 30% MC, solid-piled in warm weather, can stain within a week. Antistain dips protect only for a limited time: 1 day at 27 degrees C or above, but 1 month at 10 degrees C.
A Latvian study of silver birch logs (1 m lengths, 2016) shows temperature at work. At about +18 degrees C first discolouration appeared after 6 days and all logs were discoloured after 23 days; at +6 degrees C the first signs came after 17 to 18 days and all logs after 68 to 69 days. Worked example. Cooling from +18 to +6 degrees C stretched the time to full discolouration from 23 to 68 days, about 68 / 23 = 3 times as long (own calculation). Stain is managed by speed and cool storage.
Stains made by chemistry
- Sticker stain: imprints under stickers from chemical or fungal action. Remedies are dry, narrow or grooved stickers and airflow of at least 200 ft/min (FPL).
- Brown stain in softwood: oxidative and enzymatic reactions of sapwood chemicals in the kiln. Remedies are a low dry bulb temperature, a large wet-bulb depression, steaming and antioxidants (FPL).
- Iron stain: blue-grey to black where iron reacts with tannins (oak, LWF Bayern). Avoid steel contact on wet green boards.
Drying degrade
The FPL kiln manual (North America) gives cause and remedy for each drying defect:
| Defect | Main cause | Main remedy |
|---|---|---|
| Surface check | Surface dries too fast | Higher humidity early |
| End check | Ends dry first | End-coat green ends |
| Honeycomb | Internal tension failure, core above FSP | No temperature rise until core is below FSP |
| Collapse | Excessively high early dry-bulb temperature | Low initial temperature, air drying first |
| Case hardening | Unbalanced residual stress | Conditioning |
| Warp | Differential shrinkage, grain, reaction wood | Good stacking, sawing pattern |
For uneven final MC the manual advises presorting on the green chain; otherwise sort after drying and redry the wet boards, which can add 25% or more to drying cost. Worked example. If drying costs 40 EUR/m3 (illustrative), a redried board costs 40 x 1.25 = 50 EUR/m3 to dry.
Boards at 20% MC or above are unseasoned, and softwood below 10% is overdried (FPL). EN 14298 assesses target MC, lot average and variation between pieces, with an optional case-hardening specification, but its scope text excludes checks, shake, warp and stain. A dry-quality report therefore does not answer a stain or warp complaint.
Key points
- Fungi do not grow in wood at or below 20% MC; stain and mould need moist, warm, still conditions.
- Blue stain keeps strength but can exclude a board from appearance grades; rot destroys structure and is cut out.
- Warm weather is fast: stain on log ends within 2 weeks, on solid-piled green timber within a week; cooling stretched full discolouration of birch logs from 23 to 68 days.
- Sticker, brown and iron stain are chemical and are controlled by dry stickers, low early temperatures and avoiding steel contact.
- EN 14298 covers moisture, not checks, shake, warp or stain; use separate evidence for those.