its anisotropy: its properties are significantly different in its three struc-
tural directions.
All timber species have common attributes of stem form and
structure, and the fundamentals of wood properties can be discussed in
general terms. Consideration of particular woods, however, reveals that
certain groups or individual species require qualification. As a first level of
investigation, for example, we recognize the broad differences between the
hardwoods and the softwoods; more specifically, we recognize that one
species of pine may be strikingly different from another. The systematic
study of anatomy goes hand in hand with wood identification, and famil-
iarity with anatomical structure is fundamental to the understanding of
wood properties in general, as well as to the understanding of the impor-
tant similarities and differences among woods.
Of the problems arising in painting conservation, those involving
moisture-related dimensional change are certainly among the most chal-
lenging. Therefore, along with a brief review of pertinent chemical and
mechanical properties of wood, this article will emphasize wood-moisture
relationships, with particular reference to dimensional change.Specific gravity—that is, relative density—is perhaps the single most
meaningful indicator ofother properties of wood. It is closely related to
strength and surface hardness, as well as to resistance to tool action and
fasteners. Woods of higher specific gravity generally shrink and swell
more than woods oflower specific gra vity, and they present greater prob-
lems in seasoning.
Specific gravity is the ratio of the density of a substance to the
density ofa standard (usually water). In reference to wood, it is customary
to measure density on the basis of oven-dry weight and current volume.
Because of shrinkage and swelling, the volume of wood may vary slightly
with its moisture content. Density is expressed as weight per unit volume:
as grams per cubic centimeter or as pounds per cubic foot. Water has a
density of1 gcm^23 (62.4 lb ft^23 ). A sample ofwood having a density of
0.5g cm^23 (31.2 lb ft^23 ) is half as heavy as water and has a specific gravity
of 0.5. (Note that specific gravity is a unitless quantity.)
Among woods the world over, specific gravity ranges from less
than 0.1 to greater than 1.0. Among the more familiar woods, balsa
(Ochromaspp.) has an average specific gravity of 0.15; snakewood (Piratinera
guianensis) averages 1.28. Figure 1 shows a comparison of specific gravity
values for a number of woods, including those commonly found in painting
panels. The chart shows that the terms hardwoodand softwoodare mislead-
ing with regard to literal hardness and softness. It is valuable to understand
these contrasting terms as indicating botanical classification with reference
to different anatomical structure rather than to disparate physical and
mechanical properties.Many ofthe physical and mechanical properties ofwood are inherently
tied to its anatomical structure. Gross features of wood—that is, visual fea-
tures or those apparent with low-power magnification such as a 10 3 hand
lens—provide important indications of its properties. It is therefore appro-
priate to begin by highlighting the gross structure of wood.Physical Structure of Wood
Specific Gravity of Wood
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