Wood-Water Relationships: Part I
Craig Mckinley
Oklahoma State University
Printed in The Cedar Leader Oct-Dec 2006
At the recent Aromatic Cedar Association Annual Convention, ACA
President, Paul Todd, gave a brief review of drying redcedar lumber.
During his presentation, it became readily apparent that the
interaction of wood and water is a most complex topic, and one which
is not easily covered in a brief manner. As a follow-up to Mr.
Todd’s presentation, this newsletter article provides an
introduction to wood structure and its subsequent water-holding
capabilities. In future newsletters, we can explore this topic more
deeply.
Moisture content of wood is generally defined as the weight of the
water in the wood divided by the total weight of the wood and is
expressed as a percent. For lumber products, the wood weight is
always considered as being derived from totally dry wood. Because of
the dry basis for calculations, lumber can often have more than 100%
moisture content. That simply means that the weight of the water is
greater than the weight of the dry wood. However, in dealing with
fuel wood, the energy to be derived is often based upon the wet
weight of the wood (wood plus included water), because drying the
wood results in using energy that is not recoverable. This leads to
calculated moisture content values that are always less than 100%.
The important point is for the buyer/producer to know the basis on
which the moisture content is determined. Are you including the
water or not as a basis for calculations? It certainly makes a
difference
How much water can wood hold? The answer is a very scientific, “It
depends.” Imagine, if you will, that the minute wood structure,
involving millions of individual cells, functions very much like a
clump of soda straws stacked both on top and among each other. The
total water-holding volume of each straw (or cell) is mostly
dependent upon two things, 1) the total size of the cell and 2) the
thickness of the cell wall. For cells the same size, the thicker the
wall, the less water the cell can hold.
The amount of cell wall material relative to the total cell volume
is often expressed as specific gravity. Specific gravity is defined
as the density of any material (in this case, wood) to the density
of water. Water has a density of about 62.4 pounds per cubic foot.
If a piece of dry wood weighs 31.2 pounds, then its specific gravity
is .50. Specific gravity is simply a number and does not involve any
identifying units. As the specific gravity increases, the amount of
moisture that can be held is consequently reduced. For example,
woods with specific gravity of .45 can have as much as 130 %
moisture content on a dry basis, while woods with specific gravity
of .60 generally have moisture contents of 70% or less. Again, think
of soda straws that are either very large and thin-walled or smaller
and thick-walled. Which can hold the most water?
One of the more interesting notes relative to wood structure is that
the wood material (mostly cellulose) within the cell has about the
same specific gravity regardless of tree species. Cell wall material
has a specific gravity of about 1.53, meaning that wood itself,
without any open spaces in the cell, would sink every time. From a
product standpoint, it also means that, on a dry basis, all wood has
similar pulping yields. Research has shown that a pound of dry
cottonwood (low specific gravity), actually produces slightly more
pulp yield than a pound of dry oak (high specific gravity). Yield
per dry pound is a function of woody material and is not related to
the amount of air space in the cells. However, we do have to use a
larger piece of cottonwood to get that pound of dry wood
In this article, we briefly reviewed the concept of cell structure
and specific gravity. In the next article, we will look at some of
the other variables that affect the wood-water relationship.
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