Fibre saturation point: why wood moves

Water in wood exists in two forms: free water in the cell cavities, and bound water held in the cell walls. The moisture content at which all free water has gone but the cell walls are still saturated is the fibre saturation point — between 28% and 32% for most species, usually taken as 30%.

Why the line matters

Drying from 60% to 30% causes almost no shrinkage: only free water leaves, and cell wall dimensions are unaffected. Below 30% the cell walls start losing water, the wood begins to shrink and strength rises. Every check, warp and open glue line happens below this line.

What it means in the kiln

The first half of drying can be fast; the second half must be slow. Around 30% the schedule drops temperature, adds humidity and slows down — otherwise the surface dries while the core stays wet, and the mismatch produces surface and internal checks. That is why we record the whole curve rather than just the number coming out of the kiln.

What it means for you

After delivery, material moves toward the equilibrium moisture content of your workshop — as low as 6%–8% in a heated northern winter, as high as 14%–16% in a humid southern summer. So let the material stand 5–7 days in the target environment before machining rather than pressing it straight off the truck.