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The Wood Shoppe - #7 Chessboards: The Importance of Proper Curing of Hardwoods

BrokenArrow
Feb 8, 2011, 4:10 PM 2

ABSTRACT:  In brief, kiln drying is a process of controlled drying of lumber that employs a tightly controlled schedule of temperature and humidity levels administered to the lumber through air circulation within a controlled environment, with the desired outcome of yielding high quality product with minimal defects.   In the instance of solid hardwood chess boards, proper kiln drying is an essential process element.             

                                                                                                                                 DEVELOPMENT:  When a tree is harvested, it is of course in a 'green' or saturated state.  To reiterate past discussion briefly, in this state cells are fully saturated along with cell boundaries.  You will recall that a critical limit called the 'fiber saturation point' is reached when the cell body becomes void of water, but the cell boundary remains in a saturated state.  As the moisture content continues downward, the cell boundaries begin to give up water.  This is the critical point where shrinkage begins.  (Hold this thought).

As the timber begins to dry, it does so from the outermost fibers (those exposed to free air), and then drying continues inward toward the core.  If left to cure naturally in its felled state, you can imagine that the zone just beneath the bark will begin to dry first.  As this drying occurs, the outermost zone (the shell) attempts to shrink, thereby creating two potential failure modes.  The first is called 'surface checking' which occurs if the level of stress exceeds the tensile strength of the wood.  The resulting 'checks' are radially oriented and occur somewhat uniformly around the periphery of the timber, continuing until the level of stress falls back below the characteristic tensile strength of the wood. 

Now imagine that the tensile strength of the wood is not exceeded, and surface checking does not occur.  It follows that our 'hoop stresses' apply compressive loading within the core of the timber.  The second potential failure mode is one of internal buckling of the wood cells, called 'collapse' which can cause extreme distortion of internal fibers as they eventually dry.

If neither of these failure modes occur, drying continues inward toward the core of the timber.  As the outermost fibers stabilize somewhat (a state called case hardening), the inner core continues to give up water, shrinking in the process.  This condition actually causes reversal of the core stresses from their initial compressive state, into a tensile state and conversely for the shell.  The resulting potential failure mode is termed 'honeycomb checking', in which tearing occurs within the core when wood strength limits are exceeded. 

The way to best manage these failure modes is to establish a controlled moisture gradient that minimizes the effects of stress during curing.  This process is called 'kiln drying'.  Timbers are sawn into rough cut boards of uniform thickness, then 'sticked' (stacked with sticks of uniform thickness separating the layers of boards to allow air circulation).  The sticked stacks of lumber are placed into the kiln, then the kiln is closed up to provide a controlled environment.  By introducing steam to control levels of both temperature and humidity, the lumber is subjected to a strict schedule of air circulation at various levels of temperature and humidity until the lumber is brought to the desired moisture content, typically in the 6% to 9% moisture content range.  Kiln cycles typically range from ten hours to a month or more depending on the type and thickness of the lumber in question.

In the interest of keeping the kiln cycle to economical time frames, case-hardening is normally permitted along with the potential for minor surface checking.  Note that the desired outcome is the prevention of 'honeycombing'.  At the end of the kiln cycle, high humidification is introduced to relieve the case hardened condition.

Proper kiln drying yields high quality lumber. 

In our next discussion, we will talk about equilibrium moisture content.

Hoadley, R. Bruce  1980.  Understanding Wood  256 pp.  Newtown, Conn. :  The Taunton Press

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