Relative Seasonal Stability of Various Woods used for Pendulums

Question from reader:  How accurate is your Number Six Clock?

My Number Six clock is quite accurate.  It is usually nearly spot on, but sometimes can run within about a minute or two a day depending on ambient atmospheric changes.

If you maintain a constant heat and humidity in your home, a pendulum clock will maintain better accuracy.  If your home has huge heat and cooling swings, or huge changes from humid to dry air, then any pendulum clock will also show less accuracy, this even includes clocks with metal pendulums.

The pendulum is the timing unit of any clock. A clock's accuracy is determined by the pendulum, and not by the gears.  However, the stability of the material used for the pendulum can affect the clocks accuracy.

A very stable wood used for a pendulum shaft is going to be a more accurate pendulum than a wood that changes a lot with changes in the ambient heat and humidity.  So if accuracy is top of mind, we would like to know the "Relative Seasonal Stability" of the woods we choose for our pendulum.

The way the wood is cut from the tree also has an effect on the stability of the final pendulum.

There are three ways wood is cut from a log; 1) Flatsawn (AKA Plain Sawn), 2) Riftsawn & 3) Quartersawn.  The stability of the final product from those cuts is also in that same order.

Flatsawn (the kind of wood that most of us have readily available) has the least stability to changes in heat and humidity.  Riftsawn is quite stable and can give better results when used as a pendulum, but the very most stable cut of wood is the Quartersawn.  Here is a picture that describes the three different ways wood is cut from a log.

With that being said, even Flat or Plain Sawn wood make excellent pendulums because the grain of the wood is almost assuredly going vertically throughout the entire length of the pendulum.  Flat or Plain Sawn wood has about the same stability to atmospheric changes as a solid metal rod.

Flat or Plain Sawn wood make excellent pendulums, and except for some plywood pendulums, Flat Sawn wood is generally the only type that I ever use on any of my clocks.

Here is a chart that shows the relative stability of wood by species and cut.  Since most of use can only get (or afford) Plain/Flat Sawn wood, compare the examples of White Oak (9.1) and and Honduras Mahogany (4.1).  The Flatsawn Mahogany has more than twice the Relative Seasonal Stability of the Oak.

So according to this chart if you had a choice of Oak or Mahogany, and you were shooting for the best accuracy from your pendulum, you would absolutely prefer your pendulum being made from a close grained wood such as Mahogany.