Mastering the art of
The relationship between wood and moisture is dy- namic, not static. Even after green wood has been dried, it still absorbs or releases moisture depend- ing on the ambient conditions surrounding it.
These moisture content (MC) fluctuations in the wood can be
the cause of a variety of problems such as swelling, shrinkage,
warping, cracking, splitting and decay.
The keys to preventing the vagaries that can afflict wood
products and the structures they constitute are to understand:
( 1) how wood holds and releases moisture and ( 2) how to measure the moisture content of wood accurately. By understanding the science, one can take the necessary steps to mitigate
moisture-related deformities in wood.
Where water resides and how it moves through wood
Wood cells hold moisture two ways:
The moisture content percentage (MC%) represents the
• The cell cavity can hold moisture in both its liquid and
vapor states. This type of moisture in wood is called “free water.”
• The walls of the cell hold water molecules that have chemi-
cally bonded with the cellulose molecules. This is called “bound
water” because it’s literally bound with the wood cell.
combined total of both free and bound water. Wood also absorbs
or releases moisture in reaction to the relative humidity (RH).
A low RH pulls moisture from the wood, while wood sucks up
moisture under high RH conditions. In general, the MC should
be equal to the equilibrium moisture content (EMC) where it
will be used, typically 6-8%.
Methods to measure MC
There are two methods to measure wood’s MC. The most
accurate – and most time consuming and expensive – is the
oven-dry method, which starts by weighing cut sections of
wood as test pieces or “moisture sections.” This initial weight
is the “wet weight.” As the wood dries in the oven or kiln, the
moisture sections are removed periodically and re-weighed.
When the drying is finished, the final weight is known as the
wood’s “oven-dry weight.” The wood’s MC can then be calcu-
lated by subtracting the oven-dry weight from the wet weight,
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Find more tips & trends
Learn how moisture content impacts the qualities of the wood product.
By Jason Spangler