Wood is a complex organic material composed of cellulose fibers and lignin. A complex hydrocarbon that forms the upper level of vascular plants of the Earth. There are three basic types of trees: Gymnosperms, Angiosperms and Cycads. Gymnosperms are the softwoods, conifers or exposed seed trees. Angiosperms are the hardwoods, usually deciduous with seeds enclosed in an ovary such as a fruit or nut tree. Cycads are more primitive trees such as the ginkgo and the palms. The hardwood / softwood distinction becomes blurred when you consider that balsa is a hardwood and fir is a softwood, yet fir is much harder than balsa wood. The broadleaf / needle distinction also has problems when you consider that holly is a broadleaf conifer. The evergreen / deciduous distinction becomes a problem when you consider the larch, looses its needles every year and some tropical trees are in fact green year round. The gymnosperms and angiosperms grow by adding wood to the outside of the tree, while the cycads add growth to the inside. What I will be discussing here are angiosperms and gymnosperms as these are the major commercial woods utilized in woodworking. Generally I will be talking about softwoods (gymnosperms) and hardwoods (angiosperms) and this has nothing to do with the hardness or softness of the wood.
Wood is composed of cellulose a complex sugar and lignin is the polymer that glues it all together. As wood is made as trees grow, it is added on in annual rings. The cambium (lateral meristem) is where all growth occurs. Some of these are distinct and others are almost indiscernible. In the spring, when water is abundant, the growth of the tree is greater and the individual annual rings are wider this is springwood or early wood. As the year progresses and water becomes less abundant, the individual rings are much narrower and denser this is summerwood or late wood. Therefore every year has two growth rings one of springwood and one of summerwood. In tropical areas where the supply of water is constant, rings become less distinct. These annual rings will portray the availability of water to that particular tree in that particular tree. Because there are variations of the annual precipitation world wide, these rings can provide valuable data when compared to one another when determining when these trees were growing. Dendrochronology came about when the tree ring growth patterns from vigas in kivas were analyzed. These ceiling beams were from a structure whose construction date was known and the connections made by the botanists and scientists gave us a window into the growth rings of trees and their growing dates to cover all known examples of extant wooden objects. The narrowest growth rings occur between 1830 and 1870, the widest rings occur after 1900 and continue today. This has been carried back to petrified wood, the growth rings set in stone indicating global weather conditions for tens of thousands of years.
Cellulose is the very fiber of wood and lignin is the glue that holds it all together. The cellulose (C6H10O5) forms in long fibers that are added in thin layers on the outside of the tree just under the bark at the cambium layer. Together with lignin, a polymer related to cellulose that solidifies and binds the fibers together. This is the only living part of the tree. That layer is only living for a short while. The wood on the inside of the tree is to support the tree and to provide moisture to the growing ends of the tree at the terminal buds on the ends of the branches. Eventually the wood that supports the tree becomes merely the support of the rest of the tree, is no longer involved in sap transportation and is in effect dead, the heart wood. The sap wood, the wood surrounding the heart wood is actively involved in the movement of moisture and nutrients from the roots where the water is taken up to the green leaves (needles) where the food for the tree is produced as an interaction between the chemical chlorophyll and sunlight. In the process, trees use carbon dioxide and in return produce oxygen. Trees and other vascular plants, the largest biomass on earth produce most of the oxygen that the earth has. In other words we cannot live without trees.
All trees consist of three parts. The roots are where the water and nutrients are absorbed and where the tree is attached to the earth. The trunk, which connects the roots to the crown, provides the support and structure of the tree and the wood that we use. The crown of the tree where the thousands of leaves and needles of the tree produces the food on which the tree grows and thrives. The nutrients and water flow up specialized cells with one-way valves called the xylem. These cells allow the water to flow up by a vacuum created by the evaporation and transpiration of the water and the one way valves prevent the water and dissolved mineral nutrients from flowing back down, a sort of check valve. The water also has a positive electrical charge causing cohesion that helps the water travel up the tree. The food and fuel for the tree to grow is transmitted back down the tree from the leaves or needles in specialized cells called phloem where the tree stores in the roots and medullary rays within the tree itself
As a woodworker, one wonders why these things are important. You must be familiar with the characteristics of wood in order to understand how it is to be utilized the wood in the optimum manner. For instance, fence poles should be placed in the ground upside down from how it grows so the natural capillary action of the wood does not suck up ground water. The outside of the tree to the outside of the project, produces the most durable and lasting results. Understanding wood on a molecular level allows you to know how wood will react to all situations. You are familiar with the physical properties of different species of wood because you work with them everyday. Know how the chemical constituents of wood contribute to the physical properties of the wood. Be aware of the nature of wood on its most intimate level and you will be able to work that wood to better results.
Wood is a natural material that is full of moisture. As the tree is growing, water is an essential part of wood and this water is in the form of bound water, water within the cells of the wood and also contains free water within the fibers of the cell wall. Wood can also contain free water, water absorbed after the wood has dried and this water is in the fibers of the wood not within the cells themselves. After wood has been cut it must be seasoned to removed the bound water. An atmosphere of temperature and humidity needs to be reduced from the high moisture content of green wood to a state of where the moisture is in equilibrium with the surrounding atmosphere. The relative humidity and temperature will determine the Equilibrium Moisture Content of wood. Wood does not start to dry until it reaches 30% moisture content, it is at this point that the free water has left and only bound water remains. Wood is usually air-dried, that is the wood is exposed to the environment and allowed to adjust naturally to the surrounding humidity. Wood can also be artificially seasoned or dried using a kiln to dry the wood to a particular E.M.C. The idea is to get the wood to a point where it can be worked with relative ease. Sometimes it is desirable to use green wood for certain applications. Large timbers are easier to work when they are green, most wood is cut when it is green. Some furniture such as chairs can be constructed from green wood to the advantage of the chair maker. If the upright parts are made of green wood and the stretchers are made of dry wood, when they are assembled the mortises in the green uprights will shrink around the dry wood tenons of the stretcher and lock the chair together. Because of the way that wood dries and shrinks the round green wood will become oval shaped as it dries. Square green wood becomes a diamond shape as it dries. Wood dries most tangential, slightly less radially and only appreciably longitudinally. As wood dries the bound water becomes free water and then evaporates and adjusts to the surrounding environment. This is called seasoning. Most wood seasons or air-dries at the rate of one year drying per inch of thickness of the wood. A 4 inch thick piece of wood will air dry in 4 years under nominal conditions. Artificial seasoning can reduce the drying time to days instead of years.
The structure of wood is composed of fibrils lined up to form the cell walls and the space within, the cell holds water. These cells have specialized structures of one-way check valves that allows the water to capillary up the tree for storage. These are the pores of the wood and are divided into two basic types: open ring porous and diffused ring porous. All softwoods and some hard woods are diffused ring porous, where the rings are not obviously in one plane but are throughout the wood. Hard woods such as maple, cherry and beech are diffused ring porous. Open ring porous woods have these pores lined up along a distinctive plane and these are woods such as oak, ash and hickory. The tree grows on the outside at the cambium just under the bark. It forms layers around the tree forming the annual rings. The sapwood is where the tree is actively storing and transporting fuel and water. As the tree ages the sapwood turns to heartwood which is not longer active for storing and merely provides the support for the tree. The minerals deposited in the heartwood start to oxidize, therefore the heartwood is usually much darker than the sapwood. There are some exceptions such as ash and Englemann spruce, which show no visible difference between heart and sapwood. Another universal structure are the medullary rays that radiate out from the center of the tree and go directly across the grain of the wood. All wood contains these but they are most visible on white oak, sycamore and beech when cut on the quarter. These structures are specialized cells for food storage within the wood. The thickness of the cell walls determines the strength of the wood. Because of the long cylinder shape of the cells they do not tend to shrink much from end to end along the grain. The space within the cells collapse as the water leaves the wood after it is harvested and the wood will shrink across the grain. When the cells have dried out, they tend to seal off and when they are worked the cells are opened back up again. This can effect how the wood adjusts to the environment, so wood should be worked on all surfaces to make sure the wood will be stable. This also effects how the wood will take a finish.
Wood after it is harvested and as it is seasoned can develop problems that are considered defects. As the wood seasons it can warp, bow, cup, crook, twist, crack, check, collapse, honeycomb, diamond and caseharden. These are caused by the natural tendency of the wood and how it is seasoned or dried. One interesting twist is the result of the Coriolis effect. As trees grow on the spinning Earth the trees will also twist as a result of this effect. It twists one way in the Northern Hemisphere and the opposite twist in the Southern Hemisphere. Other growing conditions can cause the wood to have a different twist, but this is the general rule. The magnetic pull of the Earth will also effect the tree as it is growing, in the northern hemisphere the center of the tree is more towards North and the wood on that side of the tree will have different properties. The wood from the south side of ash trees growing in the Northern Hemisphere is springier than the wood on the north side of the tree. Sugar bush, any tree that can be tapped for sugar such as birch and maple are tapped on the south side of the tree, there is more sap on the south side of trees.
Curved or cupped grain wood when it shrinks as it dries and seasons tends to straighten out, the curves in the annual rings always flatten. Wood shrinks more tangentially rather than radially. Square pieces will develop a diamond shape, circles will become ovals and slab cut boards from the edge of the tree will cup. Wood moves, it is homogenous, it adjusts to the environment and there is no stopping it. The challenge is to know what wood will do in advance, anticipate its movement, engineer and design for changes and don’t be surprised when it moves.
Trees can also have other defects within the wood. Spalding is a discoloration in the form of lines or areas that are stained. Caused by mineral deposits, insect damage or fungal infection, the wood is usually weakened by spalding. Some consider it a defect others consider it a premium, the effect can be striking. The butt swell of a tree will produce a different kind of grain in the wood. Sometimes it is staining like spalding, sometimes it is dense interlocking grain. Burls are the trees reaction to some kind of damage and the tree attempts to heal by sending bud growth to the damaged area. Crotches are the places on trees were two or more major branches form and are cut into lumber and veneer. These crotches and burls are unstable in solid form and are more suitable for veneers where the tension and compression factors of the wood are minimized. The wood becomes interlocked with tight circular eyes that produces beautiful swirling grain. Pollarding is a gardening procedure that was used to cut off all of the long branches just above the top of the trunk. This causes the wood to become interlocked as all of the growing energy is not going into large limbs but is contained and concentrated within the trunk. This type of wood is very dense. It can be done to any tree but certain trees such as the oaks are trees produce the best pollard wood. See Historical Cabinet Woods.
Wood is a natural material and its consistency will vary greatly even among the same species. Even boards cut from the same log will have a different look and working characteristics. A square piece of wood will look different if it is viewed as a vertical surface or on the horizontal. The sides will look different than the front surface that is in direct view. The visible nature and color of a piece of wood can change depending upon the angle from which it is viewed. Round shapes of wood have less dramatic changes as the lighting and shading changes gradually. But wood is natural, it will look and act like it is suppose to, if what is desired is something that is uniform, consistent, unchanging, even and perfectly matching, choose something besides wood or a good coat of paint.