Fiber Saturation Point
The fiber saturation point (FSP) is the moisture content at which the wood cell walls are fully saturated with bound water but the cell cavities contain no free water. FSP occurs at approximately 28-30 percent moisture content for most species. Below FSP, wood begins to shrink, harden, and gain strength; above it, properties remain relatively constant.
In-Depth Explanation
The fiber saturation point is a critical threshold in wood science because it marks the boundary between two fundamentally different states of water in wood. Above FSP, water exists both within the cell walls (bound water) and as liquid filling the cell cavities (free water). Removing free water from the cavities has virtually no effect on the wood's dimensions, strength, or stiffness — it just makes the wood lighter. Below FSP, all remaining water is bound within the cell walls, and removing this water causes the cell walls to thin, which is why wood shrinks. Nearly all of wood's dimensional change, strength increase, and stiffness increase occur between FSP and oven-dry (0 percent MC). This is why the drying process below FSP must be carefully controlled — rapid moisture removal from the surface while the core remains above FSP creates the internal stresses that cause checking, splitting, and case hardening. The FSP concept helps explain why green timber can be worked and shaped easily (it's above FSP and relatively soft) while dry wood is harder and more challenging to machine.
Usage Example
The kiln operator knew that the critical drying phase began as the lumber dropped below the fiber saturation point, when checking risk increased dramatically.