Wood seasoning, preservation and composite wood

In short

Seasoning is the controlled removal of water from felled timber down to the moisture content it will live at in service; preservation is the loading of toxic or repellent chemicals into wood to stop fungi, insects and marine borers; composite (reconstituted) wood converts logs, small wood and residues into plywood, blockboard, particle board, MDF, LVL and glulam. Together they decide whether timber stays dimensionally stable, how long it lasts, and how far a limited forest resource can be stretched. The topic is central to B.Sc Forestry wood-science papers, IFS and state forest-service exams, ICAR/ICFRE JRF-SRF and recruitment tests for IPIRTI, IWST, FRI Dehradun, plywood and panel mills, railways and PWD timber inspection. Indian practice is codified in IS 1141 (seasoning), IS 401 (preservation), IS 399 (timber classification, durability and treatability) and the product standards IS 303, IS 710, IS 1659, IS 3087 and IS 12406.

Explained

Water in wood and why seasoning is needed

• Free water sits in cell cavities (lumina); bound water is hydrogen-bonded to cellulose and hemicellulose in the cell wall.
• Fibre saturation point (FSP): the moisture content at which free water has just gone and the walls are still saturated - about 25-30% for most Indian timbers.
• Above FSP the wood does not shrink; strength and shrinkage change only below FSP. Strength roughly doubles between green and 12% MC.
• Equilibrium moisture content (EMC): the MC wood settles at for a given air temperature and relative humidity; it decides the target of seasoning. IS 287 divides India into climatic zones and prescribes maximum permissible MC for each end use (broadly ~10-12% in the dry interior to ~14-17% in the humid coastal belt - read the table in the current edition).
• Anisotropic shrinkage: tangential ≈ 2 × radial; longitudinal is negligible (0.1-0.3%). This ratio causes cupping and diamonding.
• Benefits of seasoning: dimensional stability, higher strength and stiffness, lower weight, immunity to sap-stain and decay fungi (which need MC above ~20%), better glue bonding, paint and polish adhesion, and far better preservative penetration.

Methods of seasoning

• Air (natural) seasoning: stacked on a raised, levelled, well-drained foundation (about 450 mm clear), boards separated by dry crossers/spacers of uniform thickness, stack end-piled north-south with the prevailing wind, roof over it, ends coated to stop end-splitting. Cheap, no fuel; slow (weeks to a year or more), weather-dependent, floor-limited to the local EMC (~12-18%), risk of stain and insect attack.
• Kiln seasoning: heated chamber with controlled temperature, relative humidity and air circulation following a kiln schedule (progressively higher temperature and lower humidity as MC falls), monitored by dry- and wet-bulb thermometers and sample boards.
  • Compartment kiln - whole charge treated as one unit, schedule changes with time; flexible, suits mixed species. Progressive kiln - trolleys move continuously from wet end to dry end; high output, one species/thickness.
  • Ends with conditioning/equalising (steam spray) to even out MC and relieve case hardening.
• Solar kiln: glazed chamber with black absorber and fans; developed and popularised by FRI Dehradun and IPIRTI for small units - faster than air drying, far cheaper than a steam kiln, but weather-dependent.
• Water seasoning: logs immersed in running water to leach sap, then air dried; reduces attack and shrinkage but lowers strength and elasticity.
• Boiling/steaming: rapid, relieves growth stresses, slight strength loss.
• Chemical (salt) seasoning: surface dipped in hygroscopic salts (urea, common salt, diethylene glycol) to lower the surface vapour pressure gradient so refractory timber dries without checking.
• Special methods: vacuum drying, high-frequency/dielectric and microwave drying, superheated steam, and press-drying - fast and uniform but costly and energy-intensive.
• Refractoriness classes (IS 1141): Class A highly refractory (seasons very slowly, checks badly - e.g. sal and other dense hardwoods), Class B moderately refractory, Class C non-refractory (seasons rapidly without degrade - teak is the classic example). Seasoning schedules are assigned by class.

Seasoning defects

• Cup - curvature across the width (tangential shrinkage differences). Bow - curvature along the face. Spring/crook - curvature along the edge. Twist/wind - spiral distortion. All four are warps.
• Diamonding - a square section becomes rhomboidal when growth rings run diagonally.
• Surface checks / end splits - the surface dries below FSP and shrinks while the core is still wet; tension on the surface tears the rays.
• Honeycombing - internal checks when the core finally shrinks against a set shell; invisible from outside.
• Case hardening - shell in compression, core in tension; detected by the prong (fork) test - a U-shaped sample closes in if case-hardened. Cured by steam conditioning.
• Collapse (washboarding) - cell walls buckle from liquid tension when free water is withdrawn too fast above FSP; partly recovered by steam reconditioning.
• Loosening of knots, resin exudation, blue/sap stain and mould from slow drying above 20% MC.
• Cause in nearly every case: drying too fast at the surface relative to internal diffusion.

Wood deterioration - what preservation defends against

• Fungi: brown rot (destroys cellulose, wood turns brown and crumbly), white rot (removes lignin too, wood bleaches and becomes fibrous), soft rot (wet, high-cellulose attack in cooling towers and ground contact), sap-stain/blue-stain and moulds (disfigure only, no strength loss).
• Conditions fungi need: MC above ~20%, oxygen, mild temperature, food. Removing any one stops decay - hence "dry wood does not rot".
• Insects: subterranean termites (*Odontotermes*, *Microcerotermes*) which need soil contact and build mud galleries; drywood termites (*Cryptotermes*); powder-post beetles (*Lyctus*, *Dinoderus* - severe in bamboo and rubberwood sapwood); longhorn and pinhole borers; carpenter bees.
• Marine borers: molluscan *Teredo* (shipworm) and *Martesia*, crustacean *Sphaeroma* and *Limnoria* - attack piles and jetties in brackish water; demand the heaviest treatment.
• Heartwood versus sapwood: sapwood of every species is perishable; natural durability is a heartwood property from extractives (teak's tectoquinones, deodar oils).
• IS 399 classifies timbers by natural durability from graveyard (field) tests - broadly Class I (average life above about 120 months), Class II (about 60-120 months), Class III (below about 60 months) - and by treatability grades a to e, where *a* takes preservative freely and *e* is refractory even under pressure.

Preservatives and treatment processes

• Oil-type: creosote (IS 218) and creosote-fuel oil mixtures - cheap, durable, water-repellent, excellent for sleepers, poles and marine work; dark, smelly, not paintable, cannot be used indoors.
• Organic solvent type: copper naphthenate, zinc naphthenate, pentachlorophenol in light oil, tributyltin oxide; clean, paintable, dimensionally harmless - joinery, windows, doors.
• Water-soluble salts: CCA (copper-chrome-arsenic; the Indian formulation ASCU was developed at FRI Dehradun; IS 10013 covers the composition) - chromium fixes the salts so they become leach-resistant; CCB (copper-chrome-boron) where arsenic is to be avoided; ACC (acid-cupric-chromate); boron compounds (borax-boric acid, disodium octaborate) - very effective against borers and termites, but leachable, so interior use only; ZMA (zinc-meta-arsenite).
• Arsenical formulations are restricted or withdrawn for residential and playground use in several countries; Indian use is governed by BIS specifications and by registration under the Insecticides Act, 1968 and Rules, 1971. Copper azole and micronised copper systems are the modern arsenic-free alternatives.
• Non-pressure processes: brushing and spraying (least effective, surface only); dipping; steeping/soaking (days, for salts); hot-and-cold open tank - wood heated in preservative expands the cell air, then cooling creates a partial vacuum that sucks preservative in (very effective for sapwood and small lots); diffusion - green wood dipped in a concentrated boron paste/solution and close-stacked for weeks so the salt diffuses in sap (standard for bamboo, rubberwood, plywood veneers); sap-displacement (Boucherie) - preservative under a head of pressure is pushed through a freshly felled pole from the butt end, flushing out sap; the modified/pressure Boucherie is the standard Indian treatment for green bamboo and poles.
• Pressure (cylinder) processes, in a treatment vessel with vacuum and pressure pumps:
  • Bethell (full-cell): initial vacuum → flood → pressure (~10-14 kgf/cm²) → drain → final vacuum. Maximum retention, cells left full. Used for creosoted sleepers, marine piles, CCA for ground contact.
  • Lowry (empty-cell): no initial vacuum; air trapped in the cells expands on release and expels surplus liquid - good penetration, lower net retention.
  • Rueping (empty-cell): initial air pressure then preservative under higher pressure; the strongest kick-back - the most economical creosote process.
• Penetration and retention are the acceptance criteria, not the quantity pumped: retention in kg/m³ of dry salt or litres/m³ of oil, and depth of penetration shown by indicator sprays (e.g. chrome azurol S for copper). IS 401 prescribes the process, preservative and retention for each service situation (interior, exterior out of ground contact, ground contact, fresh water, marine); marine and ground-contact work need the highest retentions, several times an interior joinery figure.
• Golden rule: timber must be seasoned before pressure treatment (water in the cells blocks the preservative) - except for diffusion, Boucherie and sap-displacement, which need green wood.
• No cutting after treatment: sawing or boring exposes untreated heartwood; cut surfaces must be flooded with preservative.
• Fire retardants (ammonium phosphate/sulphate, borax-boric acid; IS 10553 series) and fire-retardant plywood (IS 5509) reduce flame spread - they do not make wood non-combustible.

Composite / reconstituted wood

• Why: uses small logs, plantation wood, thinnings and residues; gives large, uniform, defect-free, dimensionally stable panels with properties engineered in; breaks the dependence on large-girth natural-forest timber.
• Veneers: rotary peeled (log on a lathe, cheapest, highest yield), sliced (decorative figure), sawn (thick, wasteful).
• Plywood: an odd number of plies (3, 5, 7...) with the grain of adjacent plies at right angles - this cross-banding equalises shrinkage and gives nearly equal strength both ways and high splitting resistance. Made by peeling → veneer drying → glue spreading → lay-up → cold pre-press → hot press (around 110-140 °C for phenolics, pressure of the order of 10-14 kgf/cm²) → conditioning → trimming and sanding.
  • Grades by adhesive: MR (moisture-resistant, urea-formaldehyde - interior), BWR (boiling-water-resistant, melamine-fortified UF or PF), BWP/marine (phenol-formaldehyde, with preservative treatment). IS 303 covers MR and BWR general-purpose plywood (veneer qualities AA, AB, BB), IS 710 marine plywood, IS 4990 concrete-shuttering plywood, IS 1328 decorative veneered plywood, IS 5509 fire-retardant plywood; IS 848 covers the synthetic resin adhesives and IS 1734 the test methods.
• Blockboard and batten board (IS 1659): a core of softwood battens (up to ~25 mm wide in blockboard) faced with veneers; light, rigid, used in doors, partitions and furniture. Laminboard has narrower strips (~7 mm) and is more stable.
• Particle board (IS 3087 for medium-density general purpose; cement-bonded variety has its own standard): wood chips/flakes + ~8-10% resin, hot pressed; cheap, smooth, but low screw-holding and swells badly if wetted.
• MDF / HDF (IS 12406): wood broken down to fibres in a refiner, resin added, mat formed and hot pressed; homogeneous, machinable edges, ideal for mouldings, routed shutters and laminating - but heavy, poor in wet conditions unless exterior-grade, and must not be used structurally.
• Hardboard (wet-process fibreboard), OSB (oriented strands in layers - structural sheathing, rare in India), LVL (parallel veneers, long structural members), Glulam (laminated beams, large spans, arches), Finger-jointed and edge-glued boards, flush doors (IS 2202 solid core, IS 2191 hollow core), decorative laminates (IS 2046), WPC/PVC boards (wood flour + polymer, waterproof).
• Modified wood: impreg (resin-impregnated, cured without densification), compreg (resin + high-pressure densification - shuttle blocks, electrical parts), staypak (densified without resin).
• Adhesives: UF (cheap, interior, releases formaldehyde), MUF (BWR), PF (BWP, dark glue line, boil-proof), resorcinol-formaldehyde (cold-setting structural glulam), PVAc (assembly), PMDI/isocyanate (formaldehyde-free), casein and soya (legacy). Formaldehyde emission classes (E1/E2 type limits) now appear in the panel standards.
• Quality control: glue shear/adhesion in dry, wet and (for BWP) 72-hour boiling cycles, mycological test, MOR and MOE, water absorption, swelling in thickness, internal bond, screw- and nail-withdrawal, resistance to moisture. Wood-based panels - plywood, blockboard, particle board, MDF and similar boards - carry mandatory BIS certification (ISI mark) under the Wood Based Boards (Quality Control) Order, 2024, which has been amended to phase in the covered products.
• Research and training institutions: FRI Dehradun, IWST Bengaluru, IPIRTI Bengaluru (now merged with IWST under ICFRE), with BIS CED 20 and CED 9 technical committees writing the standards.

Key points

• FSP - 25-30% MC; shrinkage and strength change only below it.
• EMC - the MC wood settles at for ambient T and RH; the seasoning target, set zone-wise by IS 287.
• Moisture content - always expressed on the oven-dry mass, so it can exceed 100% in green sapwood.
• Shrinkage anisotropy - tangential ≈ 2 × radial, longitudinal negligible; quarter-sawn boards cup least.
• Air seasoning - cheap and slow, limited by local EMC; kiln seasoning - fast, controlled, reaches 8-12% any season.
• Refractoriness classes A/B/C (IS 1141) - decide the kiln schedule; Class A dries slowest and checks most.
• Case hardening - set shell in compression, core in tension; prong test detects it, steam conditioning cures it.
• Collapse - cell-wall buckling from too-fast drying above FSP; reconditioning by steaming recovers part of it.
• Honeycombing - internal checking, invisible outside, caused by a set shell; the most dangerous seasoning defect.
• Decay needs MC > 20% - keeping wood dry is the first and cheapest preservation.
• Sapwood is always perishable; natural durability (IS 399 Classes I-III) is a heartwood property.
• Treatability grades a-e (IS 399) - how readily a timber accepts preservative, independent of its durability.
• Bethell (full-cell) - maximum retention; Lowry/Rueping (empty-cell) - deep penetration with less preservative.
• Hot-and-cold open tank - the best non-pressure method; expansion then vacuum on cooling draws preservative in.
• Boucherie and diffusion - the only processes that need green wood; the standard for bamboo and poles.
• CCA/ASCU - chromium fixes copper and arsenic, making the treatment leach-resistant; boron is effective but leachable.
• Plywood - odd plies, grains crossed at 90°; MR/BWR (IS 303) and BWP marine (IS 710) differ by adhesive, not by veneer.
• MDF versus particle board - fibres versus chips; MDF machines cleanly, particle board is cheaper and holds screws worse.

Formulas and examples

Example 1 - moisture content and seasoning progress. A teak sample weighs 580 g green; after oven drying to constant mass it weighs 450 g.

• MC = (580 − 450)/450 × 100 = 130/450 × 100 = 28.9% - just at the FSP, so the wood has lost almost all its free water and has barely begun to shrink.
• The same sample air-dried to 480 g: MC = (480 − 450)/450 × 100 = 30/450 × 100 = 6.67% - drier than the EMC of most Indian interiors, so it will re-absorb moisture and swell slightly in service.

Example 2 - shrinkage of a seasoned board. A species has total tangential shrinkage (green to oven-dry) of 7.5%, FSP 30%. A flat-sawn board is 200 mm wide green and is dried to 12% MC.

• S = 7.5 × (30 − 12)/30 = 7.5 × 0.6 = 4.5%.
• Width lost = 200 × 0.045 = 9 mm → final width = 191 mm. Five such boards side by side in a panel lose 45 mm, which is why panelled doors use floating panels in grooves.

Example 3 - preservative retention check. 1 m³ of seasoned rubberwood weighs 620 kg before and 790 kg after a Bethell treatment with a 5% CCA solution.

• Absorption A = (790 − 620)/1 = 170 kg/m³ ≈ 170 L/m³.
• Net dry-salt retention R_s = 170 × 5/100 = 8.5 kg/m³.
• This is ample for interior furniture, but ground-contact and marine situations call for much higher retentions - compare against the figure IS 401 prescribes for the actual service situation before passing the charge.

Example 4 - kiln water load and heat demand. A charge of 20 m³ of timber of oven-dry density 600 kg/m³ enters a kiln at 60% MC and leaves at 12% MC.

• Oven-dry mass = 20 × 600 = 12,000 kg.
• Water at inlet = 0.60 × 12,000 = 7,200 kg; water at outlet = 0.12 × 12,000 = 1,440 kg.
• Water to evaporate = 7,200 − 1,440 = 5,760 kg.
• Heat for evaporation alone, at ~2,500 kJ/kg ≈ 5,760 × 2,500 = 14.4 × 10⁶ kJ = 14,400 MJ ≈ 4,000 kWh - before heating the wood, the kiln structure and the vented air, so real consumption is substantially higher.
• If the schedule takes 12 days, average drying rate = (60 − 12)/12 = 4% MC per day.

Example 5 - bending test on MDF. A 12 mm MDF strip, width 50 mm, tested in centre-point bending over a 300 mm span fails at 450 N after deflecting 6.0 mm at a load of 250 N.

• MOR = 3 × 450 × 300/(2 × 50 × 12²) = 405,000/14,400 = 28.1 N/mm².
• MOE = 250 × 300³/(4 × 50 × 12³ × 6.0) = 250 × 27,000,000/(4 × 50 × 1,728 × 6.0) = 6.75 × 10⁹/2,073,600 ≈ 3,255 N/mm².
• Both are comfortably in the range expected of general-purpose MDF; acceptance is against the table in the current edition of IS 12406 for the grade and thickness.

Where it is used

• Plywood and panel industry - Perumbavoor and Kozhikode (Kerala), Yamunanagar (Haryana), Bengaluru, Assam and Punjab clusters: quality-control chemists and production supervisors running veneer dryers, glue mixing, hot presses and IS 303/IS 710/IS 12406 testing.
• Railways, PWD and ports - creosoted or CCA-treated sleepers, bridge timbers, transmission poles and marine piles, inspected against IS 401 retention and penetration limits.
• Pest-control and wood-preservation firms - anti-termite pre-construction and post-construction treatment, treatment of doors, frames and furniture; roles as treatment-plant operators and BIS-licensed applicators.
• Bamboo and rubberwood value chains - modified Boucherie and boron diffusion plants under the National Bamboo Mission, supplying agarbatti sticks, boards, flooring and prefab panels.
• Furniture, flooring and joinery manufacture and export - kiln drying to 8-10% MC, formaldehyde-emission compliance and fumigation/ISPM-15 heat treatment for wooden packaging.
• Forest departments, research and teaching - FRI Dehradun, IWST/IPIRTI Bengaluru and state forest utilisation divisions working on seasoning schedules, durability graveyard tests, arsenic-free preservatives and panel products from plantation species.

Interview questions

• Q: What is the fibre saturation point and why does it matter? A: The MC (about 25-30%) at which free water has gone but cell walls are still saturated; below it wood starts to shrink and gain strength, above it dimensions and strength are unchanged.
• Q: Why must timber be seasoned before pressure treatment? A: Water fills the cell cavities and blocks preservative flow, so penetration and retention fall; diffusion and Boucherie are the exceptions because they rely on sap movement.
• Q: Difference between case hardening and honeycombing? A: Case hardening is a stress condition - a set shell in compression and a core in tension, curable by steam conditioning; honeycombing is actual internal rupture of the core and is irreversible.
• Q: How do you detect case hardening? A: The prong or fork test - two prongs cut from a cross-section pinch inward if the piece is case-hardened and stay parallel if stress-free.
• Q: Why is plywood made with an odd number of plies and crossed grain? A: Crossing the grain at right angles balances shrinkage and gives near-equal strength in both directions; an odd number keeps the face grains parallel so the panel stays balanced and does not warp.
• Q: What is the difference between MR, BWR and BWP plywood? A: The adhesive - urea-formaldehyde for MR (interior), melamine-fortified or phenolic for BWR, and phenol-formaldehyde with preservative treatment for BWP/marine grade to IS 710.
• Q: Why does chromium appear in CCA? A: It fixes the copper and arsenic to the cell wall as insoluble complexes, making the treatment leach-resistant; copper is the fungicide and arsenic the insecticide.
• Q: Boron is highly effective, so why is it restricted to interior use? A: Borates do not fix in the wood and leach out on exposure to rain or ground water, so they are used only where the wood stays dry.
• Q: Compare full-cell and empty-cell processes. A: Full-cell (Bethell) starts with a vacuum and leaves the cells filled, giving maximum retention; empty-cell (Lowry, Rueping) traps air that expels surplus liquid on release, giving deep penetration with less preservative.
• Q: A treated post is cut to length on site. What must be done? A: Flood or soak the cut surface with preservative - the untreated heartwood now exposed is the usual starting point of decay and termite entry.
• Q: Teak is durable but hard to treat - is that a contradiction? A: No. Natural durability comes from heartwood extractives, while treatability depends on permeability; IS 399 rates the two separately, and a durable heartwood is often refractory.
• Q: Why is collapse different from normal shrinkage? A: Collapse is mechanical buckling of cell walls from liquid tension when free water is pulled out too fast above FSP, producing corrugated surfaces, while normal shrinkage is cell-wall contraction below FSP.

Common mistakes

• Computing MC on the wet mass instead of the oven-dry mass, and then calling a value above 100% impossible.
• Assuming wood shrinks continuously from green; nothing happens until it crosses below FSP.
• Treating longitudinal shrinkage as significant, or taking radial and tangential shrinkage as equal.
• Confusing natural durability with treatability, and assuming a durable species needs no sapwood treatment.
• Calling brushing or dipping "preservation" to IS 401 standard - they give only surface protection and no stated retention.
• Reporting the volume of preservative pumped instead of net dry-salt retention and depth of penetration, which are the actual acceptance criteria.
• Quoting plywood grades as a veneer quality (AA/BB) when the MR/BWR/BWP distinction is the adhesive; and specifying MDF or particle board where a load-bearing or wet-exposed panel is needed.

Related topics

Wood anatomy and wood properties · Non-timber forest products · Timber mechanics and strength grading of structural timber · Forest utilisation, logging and sawmilling · Bamboo processing and bamboo-based panels · Forest products marketing and wood-based industries