Case Hardening

Case hardening is a drying defect, almost always from kiln-drying too aggressively, where the outer shell of a board dries and sets while the core is still relatively wet, locking residual stress into the wood. A case-hardened board can look and measure perfectly flat and stable off the shelf -- the defect only reveals itself when the wood is cut.

Rip a case-hardened board on the table saw and the stress releases as the cut opens: the kerf can pinch shut behind the blade (a real kickback risk) or the two halves can bow apart. A quick prong test -- cutting a short cross-section into a few prongs and seeing whether they stay straight or spring in or out -- catches it before it becomes a table saw problem.