Wood Seasoning and Preservation
Scope
This domain tests why timber is dried, the drying methods and their defects, and how preservatives are applied so that service life matches the end use.
Why season
- It brings MC down to the equilibrium moisture content of the service place: about 6–12% indoors, 12–18% outdoors under cover.
- Above fibre saturation (about 28%) wood is dimensionally stable; below it, shrinkage begins.
- Seasoning raises strength and stiffness, improves gluing and finishing, resists fungal attack and reduces transport weight.
Air seasoning
Stack on bearers 150–200 mm above ground, with stickers (about 25 mm thick) every 450–600 mm, a weighted or shaded top, and a covered, well-ventilated shed. Rule of thumb: about 25 mm of thickness per year to reach 15–20% MC.
Kiln seasoning
Heated, forced-air chambers controlled by dry-bulb and wet-bulb temperatures. It is faster and controllable, but too aggressive a schedule gives case-hardening, honeycombing, checks, collapse and warping. Progressive schedules start with a small wet-bulb depression and widen it as MC falls. Water seasoning and chemical salt seasoning are older alternatives.
Preservation
- Pressure impregnation: Bethell (full cell) and Rueping (empty cell) processes; retention is expressed in kg of dry salt per m³ of wood.
- Non-pressure methods: hot-and-cold open tank, steeping, dipping, brushing, diffusion.
- Preservatives: copper-chrome-arsenic (CCA) and copper azole for exterior and ground contact, creosote for poles and marine work, boron compounds for protected interior timber, PCP. Paint and varnish are finishes, not preservatives.
Worked example
A charge of 0.25 m³ absorbs 5 kg of dry preservative salt, so retention = 5 ÷ 0.25 = 20 kg/m³. That comfortably exceeds a typical ground-contact requirement of 6–10 kg/m³, so the treatment class is more than adequate - and wasteful for interior above-ground use.
Traps
Confusing seasoning with preservation; believing kiln drying sterilises timber permanently (reinfestation is possible if MC rises); treating CCA as acceptable for food-contact surfaces or children's furniture.