Wood-Water Relationships: Part II
Craig McKinley
Oklahoma State University
Printed in The Cedar Leader January to March 2007
Moisture content of wood is generally defined as the weight of water
in the wood divided by the weight of the wood and is expressed as a
percent. For lumber products, the wood weight is derived from
totally dry wood. In fuel wood, the wood basis is usually that of
the wet weight (wood plus included water).
As wood is being processed and sold, water found within the wood is
normally removed to some degree. Two important factors to consider
in this removal are temperature and relative humidity. At 70 degrees
F and 60 % relative humidity, the moisture content at equilibrium is
about 11%. Thus, 11% can be considered as the lower limit of wood
moisture content if drying is accomplished under those conditions.
The time it takes to reach this equilibrium is also critical. Thus,
the use of dry kilns is based upon temperature, moisture content,
and time being carefully controlled.
How much water can wood hold? The most important and the most
obvious factor is the amount of space within the wood cells. A
convenient measure of the amount of wood in the cells (and
conversely, the amount of open space) is specific gravity. Specific
gravity is defined as the density of any material to the density of
water. As the specific gravity increases, the amount of moisture
that the wood can hold is reduced.
For eastern red cedar, a generally used specific gravity value is
.47. This means that dry redcedar should weigh about 29 pounds per
cubic foot. However, as less-than-dry material is brought to a
processing plant, cell water is also being transported. But how much
water is being hauled? In theory, wood with specific gravity of .47
could have a moisture content of over 100% (in this case, the dry
weight of wood is used for the basis). In reality, wood seldom holds
as much water as is theoretically possible.
The average moisture content of “green” redcedar is about 33% for
heartwood, with sapwood being a bit higher. Assuming an average of
35% moisture content, a cubic foot of redcedar would weigh about 39
pounds. Thus, for every cubic foot of dry wood, an additional 10
pounds of water must be considered. Something to think about.
Obviously, the wood-water relationship is an interesting topic with
a number of facets. The important thing to remember is that water in
wood must normally be transported and/or removed prior to processing
and marketing any wood product. Thus, it is worthwhile to carefully
study and consider the water found in wood, its effects and how it
can be managed.
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