Timber selection under a moisture-content conflict
A workshop is building 12 solid blackwood dining tables for a client whose air-conditioned restaurant holds a stable 45% relative humidity. The delivered boards read 14% moisture content on a calibrated pin meter, and the supplier says the timber was kiln-dried to 10% but has been stored in an open-sided shed for three weeks. The workshop has a dehumidifier kiln, a moisture meter, and four weeks before delivery. Boards are 32 mm thick, 180 mm wide, and already dressed to final thickness. The client has paid a 30% deposit and wants no visible shrinkage gaps on the glued-up tops.
1. Should the workshop glue up the tops now, or take corrective action first? Justify with the equilibrium moisture content logic.
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Do not glue up now. At 45% RH and typical indoor temperatures, the equilibrium moisture content of blackwood sits around 8–9%, so 14% boards are roughly 5–6 percentage points wetter than their service environment. Once installed in the restaurant they will lose that moisture and shrink across the grain. On a 180 mm-wide board, tangential shrinkage of that magnitude translates to several millimetres per board, and a glued-up top of six boards could show cumulative shrinkage of 15–25 mm - enough to open glue lines, cup, or split at the battens. The supplier's stated 10% is irrelevant because the meter reads the boards as they are now; the shed storage has re-wetted them. The correct action is to re-dry to about 9% in the dehumidifier kiln, equalise, re-measure, and only then machine and glue. Four weeks is tight but feasible for 32 mm stock if drying is scheduled immediately and stress-tested before machining.
2. The client refuses any delay. Outline the risk-managed alternative and explain why it is still a compromise.
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The only defensible alternative is to reduce the consequence rather than the cause. Options include: re-drying as far as time allows (even to 11–12%) and accepting residual movement; designing the top as narrower glued sections with designed movement gaps at the ends and slotted battens; sealing all faces including the underside immediately after machining to slow moisture exchange; and issuing a written variation that records the higher moisture content, the reduced drying time, and the client's acceptance of possible seasonal movement. None of this removes the underlying problem - the timber is still above equilibrium and will shrink in service. It shifts risk to the client through informed consent and to the design through movement allowance. The workshop should document meter readings before and after any drying, because if a dispute arises the records are the only evidence of professional judgement.