Knots falling out of pine
November 1, 2013 | 7:00 pm CDT

Q: Why do knots fall out when I dry pine lumber?

A: Knots that fall out were never attached physically to the wood, but were only held in place by their geometry and by sap (resin). Such knots develop when a limb dies and then the living tree grows around and encases this dead branch. Some people call these knots black knots because there is often bark around the perimeter that looks black.

The knots can fall out during kiln drying, when the knot shrinks more than the surrounding wood, or when some of the sap is evaporated in the kiln. After drying, shrinkage (such as when a water-based finish evaporates or when a piece dries to a lower MC than when it was kiln-dried) or dissolution of the sap by a solvent will cause the knots to fall out.

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Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.