Wood Preparation, Seasoning and Preservation
Conversion
Fell in winter when sap is low. Through-and-through (plain) sawing is quick but yields flat-sawn boards that cup; quarter sawing is slower, gives radial faces, better stability and figure, and is kept for high-value stock. Add machining and shrinkage allowance to nominal sizes.
Seasoning
Air seasoning: stack on level bearers, stickers about 25 mm thick every 450–600 mm, weighted top, roofed, open to wind but not rain. Roughly one year per 25 mm of thickness, down to 12–18% MC. Kiln seasoning - compartment, progressive or solar kilns - reaches 8–12% for furniture, but too fast a schedule causes case hardening, honeycombing and collapse. Moisture content is found by the oven-dry method (105 ± 2 °C to constant mass) or a moisture meter. Equilibrium moisture content depends on atmospheric humidity; Assam's humid monsoon air pushes EMC up, so boards must re-acclimatise in the workshop before assembly.
Defects
Warp - bow, cup, twist, spring; checks, shakes, knots, reaction wood, wane.
Preservation
Durability ranges from perishable sapwood to sal heartwood. Treat by brushing, dipping, soaking, diffusion, hot-and-cold bath, or pressure impregnation with CCA/ACQ/creosote. Sapwood is permeable, heartwood largely is not. Bamboo needs boron treatment (borax + boric acid) because it has no natural durability.
Worked example
A 50 mm sal plank: air-season about two years, then kiln to 10–12% and check with a meter at three points before machining.
Traps
"Thicker timber dries faster" is false; fast kilning is not better; stickers omitted so boards stain; end grain left unsealed and splitting; sapwood mistaken for durable wood.