Case Hardening
Case hardening is a drying defect where the outer shell of a board dries and sets in a stressed condition while the interior remains wet. When the interior eventually dries and tries to shrink, the rigid outer shell prevents it, causing internal tension that can lead to honeycombing or cause the board to warp or cup when ripped.
In-Depth Explanation
Case hardening occurs when lumber is dried too aggressively — when the surface dries and shrinks much faster than the core. In the early stages of drying, the wet core restrains the dry surface, putting the surface in tension. As the surface dries further, the cells set (permanently deform) in this stretched state. Later, when the core finally dries and tries to shrink, the set surface shell resists the shrinkage, reversing the stress pattern — now the core is in tension and the surface in compression. This condition is called case hardening or residual drying stress. The practical consequence is that case-hardened boards appear fine on the surface but have hidden internal stresses that are revealed when the board is resawn or ripped. A ripped case-hardened board will pinch the saw blade or spring apart, curving in ways that make it unusable. Kiln operators detect case hardening using the prong test (cutting a thin fork from the end of a sample board and observing whether the prongs curve inward). The treatment is a conditioning step at the end of the kiln schedule, where high humidity is applied to rehydrate and relieve the stressed surface.
Usage Example
When the woodworker ripped the 8/4 maple board on the table saw, both halves immediately curved away from each other, a classic sign of case hardening.