now-dried cells are defined by their walls, their central cavities apparently
empty in most cases.
Wood cells are typically elongated, varying from short barrel
shapes (sometimes large enough in diameter to be individually visible) to
extremely long fibers that are too small in diameter to be seen individually
without considerable magnification. Most of the cells—usually more than
90%—are elongated in the direction of the tree stem or branch. These
cells are termed longitudinalcells in relation to the stem axis. The remain-
der of the cells are raycells, elongated horizontally in the tree and there-
fore perpendicular to the longitudinal cells. Ray cells, arranged to form
flat, ribbonlike groups, radiate outward from the central pith of the stem.
If just the longitudinal cells could be removed without disturbance to the
ray cells, the rays in a tree stem would appear somewhat like bristles in a
giant bottlebrush.There is no single technique or method of wood identification best for
every situation or for every species. Surely the investigator begins the
process by taking advantage of any obvious features that may immediately
suggest an answer. Only a few woods, however, such as oak (Quercus spp.)
and beech (Fagus spp.), have unique visual features that enable fairly reli-
able identification. Among the other woods, visual features such as distinc-
tive heartwood color, as in walnut (Juglans spp.) and cherry (Prunus spp.),
or physical properties, such as the greater density and hardness of maple
(Acer spp.) as compared to the lightness and softness of poplar (Populus
spp.), are occasionally helpful. However, colors may fade or deepen with
age, density may be difficult to assess in panels that are framed or cradled,
and overall visual features may be obscured by gesso or by the painting
itself. Further examination requires magnification.Hand-lens examination
Beyond the casual observation of visual features discussed above, the next
step is to determine the orientation of the grain in the wood and then
tofind a location where the wood can be cut across the grain, such that
the longitudinal cells will be exposed in cross section. In painting panels
theselocations will be along two opposite edges of the panel. An area of
approximately 5–10 mm square will usually reveal important information.
The final surfacing cuts should be made with a razor-sharp instrument to
ensure that the wood tissue is cleanly severed so that cellular detail will be
visible. A surface so exposed is called a transverse surface, a cross-sectional
surface, or an end-grain surface.
The width and placement of the growth layers (growth rings) are
usually immediately apparent. In a few species of hardwoods, such as oak,
chestnut (Castanea spp.), ash (Fraxinus spp.), and mahogany (Swietenia spp.),
the cells forming vessels (called poreswhen exposed on transverse surface)
are large enough to be individually visible without magnification. And, with
magnification, even the smallest pores can also be seen. A 10 3 magnifier,
referred to simply as a hand lens, is most commonly used. Hand-lens exami-
nation also serves to separate the hardwoods, in which all longitudinal cells
appear uniformly small. Figure 1a–c demonstrates the appearance of typical
softwood and hardwood end-grain surfaces as seen under low-power
magnification.Wood Identification
Techniques
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