Wet Wood, No Good! The Tricky Business of Drying Wood

Trees contain a large amount of water. When cut, the wood from trees retains a lot of this water, which is problematic when trying to get maximum use out of this resource.

This below display from back in the day at the Forest Products Laboratory (FPL) illustrates cleverly just how much water is really in there.

The buckets hold the same amount of water as the green lumber on the table.

The buckets hold the same amount of water as the green lumber on the table.

When FPL first opened its research doors, vast quantities of wood were lost due to improper drying procedures. Researchers began to study the complicated underlying mechanism of wood drying, and eventually developed proper drying methods to decrease manufacturing waste and increase the quality of wood in service.

Wood shrinks when it dries, resulting in dimensional change, but it does not shrink the same in all directions. Research has shown that this shrinkage difference results from the orientation of the cellular structure of wood and the structure of the cells themselves.

This illustration of a cross section of a log shows how a piece of wood removed from a specific area in the tree changes shape when dried.shrinkage

FPL research explaining moisture movement during the drying process, led to the development of effective methods of air and forced-air drying to minimize defects, and resulted in the publication of the Dry Kiln Operator’s Manual.

Properly dried wood has many benefits, including:

  • Minimized insect damage
  • Improved dimensional stability
  • Improved preservative treatment
  • Decreased warping, splitting, and checking
  • Improved machinabiity
  • Decreased weight, resulting in easier handling and cheaper shipping
  • Decreased susceptibility to decay, mold, and stains
  • Improved strength
  • Improved joinery, such as nail-holding ability and gluability

(Compiled from John Koning’s book Forest Products Laboratory 1910-2010: Celebrating a Century of Accomplishments.)