Knots falling out of pine
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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 (1942-2025) was popularly known as “The Wood Doctor.” He trained thousands of people in efficient use of wood for more than 50 years and authored foundational resources on wood technology. He worked at the University of Wisconsin-Madison and Virginia Tech. His popular "Wood Doctor's Rx" column has appeared regularly in FDM and FDMC magazine since 1978. Because so much of his advice was timeless, he asked that we continue to run his columns in memoriam.