The Wood Shoppe - #4 Chessboards: The Anisotropic Nature of Hardwoods

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Jan 10, 2011, 8:39 AM |
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Let's take a look at some of the issues faced when creating a solid hardwood chessboard.  We will get into the proper harvesting and curing of hardwoods in upcoming discussions. For the moment, assuming that the lumber has been properly prepared, let's understand how it will behave.

Anisotropic physical properties refer to differences in physical behavior along different axes.  In the case of hardwoods, we find that dimensional shifts occur naturally within wood wherein the magnitude of change is significantly different in longitudinal, radial, and tangential directions.  Additionally, the combined effect of radial and tangential shifts, commonly referred to as transverse movement, is dependent on the size and orientation of the growth rings with respect to any specific cross-section of lumber. 

The driving factor for the dimensional change is primarily moisture content within the wood, and to a lesser degree, temperature.  Therefore, shifts in relative humidity cause wood to expand and contract, and it moves at different rates in different axes.

Note that a piece of untreated lumber will expand and contract at a faster rate than an identical piece with a moisture barrier such as urethane or lacquer, but these barriers do not stop the eventual dimensional shift.  It will happen.

The anisotropic phenomenon is further complicated by gradients.  For example, suppose the relative humidity shifts from 45% to 85%.  The surface of the lumber is exposed to an increased moisture level and begins to react accordingly.  It takes longer for the inner fibers to become exposed to the new moisture level, they are still happily existing at their prior 45% RH moisture content level.  Since dimension shift is occurring on the outside, but not the inside of the lumber, stresses occur throughout the section as a normal state of affairs.  In truth, unless the wood is in a controlled environment and held for extended periods of time at constant temperature and relative humidity, it will be in a state of constant flux.

It is not uncommon to experience dimensional shifts of 1/8" or more across a 24" section of lumber.  The designer is faced with making decisions on how this will be best managed to insure a high quality product that will accept shifts in humidity.  The customer should understand the phenomenon and avoid exposing any type of wood products to extreme changes in humidity for extended periods of time.  What is a preferred range of relative humidity?  40% to 70% with a targeted nominal 50 to 55% not only for solid hardwood chessboards but for wooden furniture in general.

Anisotropic properties are a major consideration in the design and construction of hardwood products.. Other factors will be discussed in upcoming sessions.

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