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 2.4 PHYSICAL PROPERTIES. The physical properties considered here include moisture content
and explanation of the wood drying process, dimensional stability, density and specific gravity, per-
meability and weathering.
2.4.1 Density And Specific Gravity. Density is the single most important indication of
strength in defect-free wood, and is an indicator of many characteristics, such as, ease of machining,
hardness, and others. Density is the weight of wood per unit volume; that is, pounds per cubic foot.
The moisture content should also be given because it will greatly affect the density. Generally, as
the density of wood increases, so does its strength. Dense woods also tend to shrink and swell more
than lightweight woods.
Specific gravity (Table 2-2) is another term commonly used when talking about the relative weights
of different woods. Specific gravity is the ratio of the oven-dry weight (i.e., weight of the wood at
zero percent moisture content) of a given volume of wood to that of an equal volume of water at a
standard temperature.
2.4.2 Permeability. The permeability of wood is the extent to which it allows fluid flow
through a porous medium under the influence of a pressure gradient. There is a good correlation be-
tween wood permeability and treatability.
The "treatability" describes the ease with which preservatives can be forced into wood under pres-
sure. As with the many other wood characteristics, treatability varies with the different wood
species (Table 2-3). Significant variability may also occur within a given species. Douglas-fir
heartwood is a good example. Coastal Douglas-fir is permeable, Cascade Mountain sources are
moderately impermeable and intermountain sources are impermeable or refractory (Figure 2-9).
Lastly, the permeability and, hence, the treatability of wood varies with grain direction. The penetra-
tion ratio of end grain to side grain for the preservative oils is almost 15 to 1 and 20 to 1 for the
water-borne preservatives.
2.4.3 Moisture Content. Because the amount of water in wood (moisture content) affects
wood treatability, durability and stability, wood moisture content is often referenced in specifica-
tions.
The moisture content of wood is the ratio of the weight of water in wood to the weight of wood
when it is oven-dry. The following formula is used to calculate the moisture content of wood.
Initial wt. - Oven-dry wt.
X 100
Oven-dry wt.
Moisture Content(%) =
2-16
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