Structure of wood



Wood is derived from a tree. It is made up of bundles of fibers or long tubes that run parallel to the stem of the tree. These are crossed by other fibers that form the medullary or wood rays. These wood rays pass from the center or pith to the bark and serve to bind the units together. Concentric rings are formed as a layer of wood is added each year. These are called annual rings.

Sections of a tree trunk

Medulla, or pith: This is the center of the tree. It is lighter in color and less strong than the heartwood.

Heartwood: This section of the trunk, located between the medulla and the sap-wood, gives us the best building material. Sapwood: The recent annual rings are contained here, between the heartwood and the cambium.

Cambium: This is the most recent annual ring. Bark: This external layer protects the tree.

Sawing the tree into planks

A tree is usually cut during the winter, when there is little sap in the wood. At this time the wood is less subject to fungus attack. After the bark has been stripped, the trunk is washed to prevent fungus, mold, or other growth. This process also helps to season wood.

At the end of the seasoning period the trunk may be sawed into planks in any of a number of different ways. One of the most practical methods is sawing parallel to the grain. This is called plain, or bastard, sawing. Quarter sawing, another method, is used for higher quality work.

Wood Changes

Planks undergo both warping and shrinking during the seasoning process. Shrinking is most noticeable at the outer edges of the plank, because the annual rings of the sap-wood are fresher and less dense. Warping refers to the general change the plank undergoes after being cut.

Seasoning of Lumber

It is essential that lumber be well seasoned before it is used. The usual methods are as follows:

Natural seasoning: In this method sawed lumber is exposed to free air after it has been carefully stacked.

Water seasoning: A somewhat quicker method of seasoning consists of immersing the lumber in running water for about one month. The water entering the pores of the wood washes out the sap. The lumber is dried in the open air.

Artificial seasoning: In this method the lumber is placed in a drying kiln, and a current of hot air is allowed to circulate continuously between the layers. For some woods steam may be used. This is the fastest method.

Selecting and Buying Wood

After the design has been selected and studied, the next step is the ordering of material. One method is to buy the lumber in standard lengths and cut the required pieces as listed. Another method is to ask the lumber dealer to cut the material into the sizes you need. There will be a minimum of waste whichever method is used, because standard lumber sizes have been considered in the planning of the designs.

Avoid using solid wood and plywood together in the same piece of furniture, particularly if a flush board is to be visible. If such a combination of materials is unavoidable, glue should never be used for bonding the parts. Plywood and solid wood react differently to drying glue and to atmospheric conditions, but screws or loose joints will permit shrinkage or expansion.

Another point to keep in mind is that both soft and hard wood shrink in the process of seasoning. Thus the wood is usually 3/4 in. narrower than the nominal thickness. This difference is of consequence only in fitting such parts as doors, shelves, or drawers. If the wood is of a different thickness from that specified in the design, adjustments must be made in the dimensions of the part to be applied. Therefore it is best to secure lumber of a thickness as close as possible to that specified.

Now you know something about wood, it is time to start on your construction.