Shisham Planks Straight Off the Mill: Drying a Six-Week Chair Order
A furniture unit in Bareilly takes delivery of 60 shisham planks (Dalbergia sissoo), each 1800 × 150 × 25 mm. The planks have stood for ten days under a tarpaulin in the yard; it is mid-July, the relative humidity is about 80% and the tarpaulin drips on the inside. The unit has an order for 12 dining chairs in six weeks and owns an 8 m³ capacity drying kiln with a recording hygrometer, a pin-type moisture meter and a small oven in the store room.
The meter reads 24–28% on the faces and 19–21% when the pins are driven to about one-third of the thickness. Two planks show end splits roughly 120 mm long and there is grey mould along the edges and between the stickers. The supervisor wants to start cutting and jointing this week so that the chairs can be polished in time; the foreman argues that ten days in the yard means the timber is 'already dry'.
1. Which of the two moisture readings should the workshop act on, and why are the face readings higher than the readings taken inside the plank? State the moisture content the timber must reach before it is machined for chairs used indoors in Bareilly, and describe how the workshop should confirm that figure other than by trusting the pin meter.
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Neither figure can be taken as the moisture content of the consignment, but the 19–21% reading from inside the plank is the more useful starting point.
Why the order is reversed. A pin meter estimates moisture from the electrical resistance between the pins, so it reads the wood immediately around them. Under a dripping tarpaulin the faces have re-absorbed water and the surface fibres are wet, so the meter reports a wet shell. In a stack that is genuinely drying, the shell is always drier than the core; here the order is the opposite (face 24–28%, core 19–21%). That tells us the timber is being wetted from outside faster than it is drying, and that the stack has no ventilation. The true average is therefore near, or slightly below, the core figure, and the face readings say nothing useful about the interior.
Target. The equilibrium moisture content of furniture in a Bareilly room swings from roughly 6–7% in the dry months to 14–15% in the monsoon. Wood should be machined near the middle of the range in which it will live - about 10–12% for these chairs - so that swelling and shrinkage are shared evenly and no single joint takes the whole movement.
Confirmation. Cut a 25 mm slice from sample planks, remove loose splinters, weigh to 0.1 g, dry at 103 ± 2 °C to constant mass in the store-room oven, and calculate MC = (wet mass − dry mass) ÷ dry mass × 100. Do this on at least four planks from different parts of the stack and repeat on offcuts during machining. Meter readings must be corrected for species and temperature and checked against the oven result - the meter is a guide, the oven is the reference.
2. The supervisor wants to machine the timber this week. Judge whether six weeks is realistic and set out a drying plan - stacking, air drying, kiln stages and equalising - that will get 25 mm shisham to the target without case hardening or fresh end splits.
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Six weeks is achievable only if drying starts immediately; it is not achievable if the timber is cut this week.
Time budget. A 25 mm hardwood plank at 80% RH and about 30 °C loses roughly 1–1.5 percentage points of moisture a week in a good outdoor stack, so air drying alone would need several months to reach 10–12%. The kiln decides the answer. From about 20% down to 10%, a correctly controlled kiln for 25 mm shisham needs roughly 10–14 days, plus 3–4 days equalising and conditioning. That leaves about three weeks for machining, assembly and polishing - tight but workable. The charge is only 60 × 1.8 × 0.15 × 0.025 ≈ 0.41 m³ against an 8 m³ kiln, so there is no reason to crowd it.
Plan.
- Re-stack at once under a roof, not a tarpaulin: stickers 25 mm thick at 600 mm centres, aligned vertically over the bearers, stack 300 mm off the floor, weighted top planks, free air on all four sides.
- Brush off the mould in open air, then end-coat every plank end with paraffin wax or bitumen paint. The 120 mm splits show the ends are drying far ahead of the body.
- Air-dry 5–7 days so the wetted shell comes down towards the core, with moisture sample boards inserted in the stack.
- Kiln in stages: start at 40–45 °C dry bulb with a wet-bulb depression of only 2–3 °C (about 85–90% RH). Hold until the samples reach about 18%, then raise the dry bulb by about 3 °C per step and widen the depression gradually to 15–20 °C, never letting the shell run more than 2–3 points ahead of the core; finish at 65–70 °C dry bulb.
- Equalise at about 55 °C with a 2 °C depression until the samples are steady at 10–12%, then condition at high humidity for a few hours to relieve drying stresses.
- Before machining, run a prong test: cut a fork about 100 mm long with 2.5 mm prongs. Prongs pinching inward means the case is set hard; prongs splaying outward means the case is too dry relative to the core. Either result calls for more conditioning, not more heat.
3. The chairs will be sold where white ants and powder-post beetles are common, and the buyer has asked what has been done to protect the wood. Recommend a treatment, say at what stage it should be applied, and explain how you would avoid interfering with the glue joints and the polish.
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Shisham is not immune to termites or to Lyctus powder-post beetles, and a chair is attacked first where end grain is exposed - the bottoms of the legs, the tenon ends and every bored hole. The treatment must therefore protect the inside of the joints, not only the outer faces.
Choice. In a small unit the practical options are a borax–boric acid mix (roughly 2 parts borax to 1 part boric acid, made up as a 2–3% solution in hot water), a proprietary water-based boron gel, or a solvent-borne copper naphthenate for parts likely to stay damp. Pressure impregnation is the only method giving full penetration in a moderately refractory wood like sissoo, but it belongs at the mill. In the workshop, a hot-and-cold dip or two liberal brush coats give a useful envelope of protection.
Stage. Machine and bore first, treat, re-dry, then glue. Treating rough sawn timber is wasted because machining removes the treated skin; treating after assembly leaves the tenons and mortises unprotected. Boron salts are water-based, so they raise the moisture content and can interfere with PVA adhesion if the wood is glued wet - hence the re-drying to 10–12% before assembly, and glue applied to freshly machined tenon cheeks where possible.
Sequence to recommend. Saw and bore → sand to 120 → brush two coats of boron solution, flooding end grain and the insides of the mortises → stack and dry back to 10–12% → final sanding, glue-up, polish. Record the treatment in the delivery note so the buyer has written evidence.