Characteristics of Wood
Presented by Hershel Weiss
February 9, 2002 at Woodworker’s Supply
There two types of trees: deciduous, and coniferous. With a few exceptions, hardwoods come from trees that drop their leaves (deciduous) and softwoods come from trees that are evergreen and bear cones (conifers).A tree structure can be thought of as a series of hollow fibers or tubes running vertically. The fibers carry the necessary nutrients and moisture from the root system up the tree through the sapwood to the leaves. These nutrients are converted to sugars and starches through photosynthesis and are then carried down through the cambium layer just inside the inner bark, and distributed throughout the tree by means of rays. The rays run perpendicular to the tree trunk.
Growth rings result from early, rapid growth in the spring and slower growth in the summer. The lighter colored part is called sapwood whereas the darker colored portion is called the heartwood. As the tree grows, cells from the cambium layer changes to sapwood and an equal amount of existing sapwood cells convert to heartwood. The amount of sapwood remains constant through out the tree lifetime. Heartwood is more disease and insect resistant than the sapwood since it is dead and void of nutrients and may contain extractives toxic to insects and fungus. The pith is the center of the growth rings and is the remains of the original twig.
Knots are parts of limbs that are enclosed in the main body of the tree. An embedded knot results when a limb is severed and new growth rings in the tree trunk engulf its remains.
The amount of water in wood is referred to as Moisture Content and is expressed as the percentage of the wet wood weight compared to the oven dry weight of the wood. The ideal moisture content for woodworking is about 6 to 8%. Wood moves as a function of the moisture in the wood. There is negligible movement in the length dimension relative to the width. A board cut in the radial direction (perpendicular to growth rings) will change in width about half that of a board cut in the tangential direction (tangent to the growth rings).
A humidity indicator can be constructed of two thin wood strips of equal length, width and thickness. One strip has the grain running along its length and the other has a cross grain. The two pieces are then glued together. The moisture in the air causes a curvature of the assembly, which is proportional to the humidity.
By choosing the cut in clever ways, it is possible to make grain flow with table leg designs. This procedure is described in the August 1992 issue of Fine Woodworking.
For an excellent reference source Hershel recommends reading: Understanding Wood by Bruce Hoadley
Thank you Hershel, for a very educational workshop.