2

F

  of civilization, wood has played an indispens-

able role in human survival. It is therefore not surprising that wood

retains a prominent place in our cultural heritage. In the decorative

arts wood has routinely been utilized because of its aesthetic virtues. In

contrast, when wood is used for painting panels, where the surface appear-

ance is obscured, the choice ofwood reflects the universal availability of

the resource as well as the working and performance properties of the

timber. As an engineering material wood is strong and stifffor its weight

and has density and hardness in the range suitable for conversion with

hand tools. Wood is chemically stable when dry, and its surfaces offer a

compatible substrate for paint application. The use of wood is not without

its pitfalls, however, and requires the understanding that it is anisotropic—

that is, it exhibits properties with different values when measured in

different directions—as well as hygroscopic—adsorbing and releasing mois-

ture readily. It is also dimensionally unstable and subject to deterioration

by fungi and insects.

It is fundamental when exploring the complex nature of wood to

remember that wood comes from trees and that usable timber is found in

tens of thousands of tree species the world over. Pieces of wood large

enough for painting panels are normally from the trunks or stems of

mature trees. While many features of wood structure are common to all

tree stems without regard to type of wood, it is not surprising that among

such a diverse resource deriving from so many different species, awide

array ofcharacteristics can be expected—such as the twelvefold variation

in density from the lightest to the heaviest woods.

Trees are living plants, and wood is cellular tissue. Understanding

wood therefore begins at the cellular level, and it is both appropriate

andimportant to think of wood as a mass of cells. Woody cells evolved

to satisfy the needs of trees—on the one hand to serve as good structural

beams and columns, on the other hand to provide systems for conduc-

tion of sap and storage of food materials. The cells specialized for these

mechanical and physiological functions are primarily elongated and

fiberlike and parallel to the tree-stem axis. The alignment ofthese

longitudinal cells in wood determines its grain direction. The stem of a

tree “grows” in diameter by adding cylindrical layers of cells, which we

recognize as growth rings. The combination of the axial direction of

longitudinal cells and cell arrangement in growth rings gives wood tissue

Chemical and Physical Properties of Wood

R. Bruce Hoadley