KNOTS IN WOOD

by Grandpa Cliff

Oct 25, 2005

WHAT IS A KNOT IN A PIECE OF LUMBER?

Answer:  It is part of a branch that got enclosed in the wood. 

How could that happen?  Well, let's think about a 3 year old branch about as big around as your thumb.  Let's pretend it has a small branch growing out of one side.  20 years later, the main branch will be much thicker because 20 years of wood have been added.  What happened to the side branch?  Well, it didn't move, so the main branch had to grow out around it.  Part of the branch was then inside the wood.  The next two images both show a branch in the trunk of a tree.  

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The Pith that is shown in the photo on the right is at the very center of the stem (trunk or branch).  It forms in the stem during the first year of growth, but it does not grow any larger after that.

  The drawing at the left shows boards being cut from a log.  What if there was a small branch

  sticking out to the left, right where the board is being cut?  When a board is cut through one 

  of the branches that are inside the trunk, the branch makes a round knot in the board.  The

  next photo shows Cedar boards with knots.  Notice the heartwood in the center board.

When lumber dries out, the wood and the knots don't always dry at the same rate.  That can cause the knots to crack (as above) or to separate from the wood and get loose.  Sometimes the branch inside the wood has rough bark.  When the lumber is cut, the saw can knock out some of the bark, which makes the knot loose.  Either way, once the knot is loose, it can fall out of the board and form a knothole.  

This is a close-up of a cracked knot in lumber.  

Notice the annual rings and the pith at the center.

This is a knot in a dead tree.  Notice the ring of bark around 

the dead branch, and the pith at the center of the branch.

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Here is a very interesting bowl made from wood of the Green Ash tree.  

You can see the bark, the sapwood, the heartwood, annual rings and a knot.

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This is a KNOTHOLE formed in the board of a fence 

after the knot fell out.

When lumber dries out, the wood and the knots don't always dry at the same rate.  That can cause the knots to crack (as can seen in two photograhs above) or to separate from the wood and get loose.  Sometimes the branch inside the wood still has its bark.  When the lumber is cut, the saw can knock out some of the bark, which causes the knot to become loose.  Either way, once the knot is loose, it can fall out of the board and form a knothole.  

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