The study of wood has become a science in itself. Students who wish to go deeply into this subject can obtain detailed information from the Agricultural Research Service and the Forest Service divisions of the Department of Agriculture, Washington, D.C. Some general information, however, seems appropriate here so that the reader may gain a working understanding of his material.
A tree can be said to have three component parts: the root, the trunk, and the crown. The roots absorb water and chemical substances from the soil which are carried by the sapwood to the leaves and branches of the tree. The bark of the tree is for protection and insulation. The growth and thickness of the trunk is brought about by cambiumnial cells situated just under the bark of the tree.
These cells eventually become sapwood which in turn becomes the heartwood as the tree grows in size and strength. In the so-called ‘heartwood’ trees, the difference in color of heartwood and sapwood can be clearly seen. A sawcut across a log of elm or yew will reveal the light, outer ring of sapwood and the dark, rich center of heartwood. The latter has now ceased to function as a carrier of sap and the cells are filled by resins and pigment. The function of this heartwood can be likened to a strong scaffold holding the tree erect. The sapwood is more vulnerable to attacks by fungi and insects whereas the heartwood is harder and more durable. The color difference between the two types of wood is not always apparent; beech, lime, silver fir are cases in point.
In temperate countries annual rings of native woods, marking each year’s growth, can be clearly seen, and usually by the naked eye. In early spring the growth starts rapidly and large spongy cells are formed. These constitute the soft spring-wood. As the summer progresses, the cells formed are smaller and harder, also they are slower in growth. It follows then that each annual ring has two parts, the broader spring-wood and narrower band of summer growth. When these annual rings can be clearly seen, as described, we speak of ‘ring porous’ woods. Examples of these are ash, elm, and oak. During the autumn and winter the tree rests and growth ceases. The rate of growth varies a great deal according to species, climate and altitude. In tropical countries trees such as ebony, continue to grow all the year round and there are no marked rings, only areas of growth.
Broadly, we can divide trees into two groups: ‘softwoods’, or those belonging to the conifae, like firs and pines, and ‘hardwoods’, belonging to the dicotyledonae, or broad leaved varieties. As in the case of all generalizations, careful study discovers subdivisions of this rule. The softwoods belong to a more primitive and simple type of tree structure, while the hardwoods are far more complex in form. These are botanical terms and do not refer to actual hardness or softness.
Seasoning and shrinkage
In air and kiln seasoning, time depends on the density, thickness and water content of the wood. It is not advisable to use green, freshly cut timber for carving. The cells and the cell walls of this timber contain water. It is the process of drying out the free water in the cells, and the partial drying out of water in the cell walls, which is known as ‘seasoning’. It is obvious that when this water—which is after all a part of the tree’s composition—is removed, shrinkage is bound to take place.
Therefore the process must be controlled if distortion, cracks and splits are to be avoided. The two main methods of seasoning are air seasoning and kiln drying. The former is a slow method. For instance, with 2-in. planks it will take over six months of good drying weather to reduce the moisture content of twenty per cent of their weight. For timber used in heated buildings it is necessary to reduce the moisture to about ten to twelve per cent. The process of seasoning wood in a modern kiln can be achieved in a matter of weeks. In the case of tropical woods, air seasoning is favored, for periods up to two years. These can then be finally dried by the kiln method. There are a few woods that cannot be kilned at all as the cell walls of the wood collapse and render the wood useless.
Very briefly, the kiln is a brick-built room with heating pipes generally in the ceiling. Fans are installed to keep the air moving and steam is introduced through a number of jets. The wood is stacked horizontally in such a way that the air can circulate freely. The planks themselves rest on ‘sticks’ or wood bearers of 1 in. X 1 in., spaced approximately 4 ft. apart for planks that are 2 in. thick or more. Regular tests of moisture content during kilning are made and the drying process is controlled by varying the humidity and the heat.
Timber deteriorates if left on the ground and exposed to the elements. In air seasoning the timber is stacked, as in kiln drying, in such a way that the air can circulate. The stack should not be more than 6 ft. wide. A well-drained site is chosen and brick piers are built not less than 9 in. in height and preferably more. These piers are approximately 9 in. square and are spaced 2 ft. apart.
Timber cross-members 4 in. X 4 in. are placed on the piers. Strength is given by laying further heavy bearers along the whole length of the stack. The planks to be seasoned are placed horizontally on the foundation, each plank separated from the other by 1 in. X1 in. sticks. It is important that the wood should be kept level and sagging prevented. Therefore, the sticks are placed at intervals of 2 ft. to 4 ft., according to the thickness of the planks. The stack is left open but covered by a sloping roof with a good overhang. The ends of the planks should be protected by bituminous paint. In reasonable weather, 2-in. planks of softwood stacked in the spring would be seasoned by the autumn of the same year. Hardwoods should be stacked three months earlier.
Conversion of timber
Conversion is a term used to describe the cutting of wood into planks. In figure 4 you will see the ‘through and through’ cut or slash-sawn log. This is the most economical form of cutting timber but there is a greater tendency to warping, as shown in B. In C and D you will see two methods of quarter cutting. The center picture shows the more common form when the center is ‘boxed out’, that is cut away. Quarter cutting is used in oak in order to show the silver grain or medullary rays.
The greatest amount of shrinkage takes place in the tangential direction of the log, which means along the line of the annual rings at a tangent to the circumference of the log. The radial shrinkage is only half as much. Figure 4, E shows in black shading the relative shrinkage of a slash-sawn and quarter-sawn board. Uneven drying causes uneven shrinkage and various splits and checks can develop. Two examples, star-shakes and end splits, are illustrated (Fig. 4, F and G).
Fungal decay and insect attack
In the ordinary way, fungal decay and insect attack are not likely to trouble the wood carver. However, you can, if you wish, treat your carving with a wood preservative when the work is completed.
Fungus, the chief cause of decay in timber, is a plant and needs organic material to maintain life. It must also have moisture and oxygen so that wood kept dry will not be attacked. Whereas it is highly unlikely that your carving will be attacked by fungi, insect pests can easily move from infected furniture to your work. The most common of these pests is the furniture beetle (anobium punctatum), often known as ‘wood worm’.
During June and until August the beetles emerge from the wood looking for new homes in any suitable cracks and crevices in timber. Both hard- and softwoods are vulnerable. The insects then lay their eggs. These eventually hatch into larvae a quarter of an inch long. With powerful jaws these grubs bore into the wood. This process goes on for as long as eighteen months when the grub pupates and the whole cycle commences again. It is possible to treat wood with chemicals toxic to both fungi and insects. There are a number of proprietary brands on the market.
Durability of wood
The durability of wood is not related to its hardness or softness. When the term is used in the timber trade, it refers to the wood’s reaction to climate, weather conditions, contact with the ground, immersion in water, etc. Wood carvings are, as a rule, under roof protection and it will therefore be only on occasion that a carver must consider durability as a factor in his work.Timbers grown in the United States, such as oak and elm, are durable woods. Some softwoods are extremely durable, as for instance red cedar, which is more durable than oak. Resins, tannin and aromatic oils in the wood form together an effective preservative.