oak is used to make tight cooperage (whiskey barrels), and does not accept preservative treatment
188.8.131.52 Pits. Individual cells in both hardwoods and softwoods are connected
together by means of pits that allow movement of liquids between cells. Pits are important in
softwoods since preservatives move from one cell to the next through them.
2.3 CHEMICAL COMPOSITION. The discussion provided here is a very general and elementary
review of the complex chemical structure of wood.
2.3.1 Cellulose. Cellulose comprises about 40 to 45 percent of the oven-dry weight of
wood. Long strands of cellulose molecules arranged more or less parallel to each other within the
thick walls of structural cells (fibers and tracheids) contribute a high tensile strength to wood.
2.3.2 Hemicellulose. Hemicelluloses constitute from 20 to 35 percent of the oven-dry
weight of the wood. The exact function of the hemicelluloses is not clear. Some possibility exists
that they serve as a temporary matrix before lignification.
2.3.3 Lignin. Lignin constitutes from 15 to 35 percent of the oven-dry weight of the
wood. It reinforces the cellulose portion of the cell wall, thereby contributing to the rigidity of
wood. Lignin is a complex polymer and its chemical structure is not fully understood.
2.3.4 Ash. The inorganic materials, or ash as they are often referred to, generally con-
stitute less than one percent of the oven-dry weight of the wood. The most common constituents, in
order, are calcium, potassium, magnesium, carbonates, phosphates, silicates and sulfates.
2.3.5 Extractives. In addition to the major structural components--cellulose, hemicel-
lulose, and lignin--most woods usually contain some type of extractives. Most extractives are lo-
cated in the heartwood and are water soluble. They are often responsible for the general darkening
of the heartwood, for the resistance of some heartwood to decay and insect attack, for odor, and for
good dimensional stability. The more important organic extractives include the terpenes, resin
acids, polyphenols, tannins, and tropolenes.
2.3.6 Natural Durability. Some of the extractives, pitch, oil and other extraneous substan-
ces deposited in the heartwood impart resistance to decay and insects, thus making some species
more durable than others (Table 2-1). Where the extractives do not have a toxic or repellent effect,
the heartwood is no more resistant than the sapwood.
In the past, old growth naturally durable woods such as cedars, cypress, redwood, chestnut and
some others were generally available and commonly used to prevent deterioration. Now, the
demand for durable products exceeds potential supply, or they are too expensive for general con-
struction purposes. As a result, wood properly treated with preservatives is more commonly utilized