domingo, 4 de maio de 2008

Wood and me

I suppose I should spend some time on talking about wood as it as been part of my families history and mine, for the last 28 years of my career has been spent learning something about its nature and its spirit, mind you I am not one to go hugging trees, their beauty is certain and the idea of them having thoughts is well, absurd.
Please if anyone is to be hugged its your friend, neighbour, mother, father, brothers, sisters, bank manager(if you have difficulty getting a loan) but trees no. There is however one attribution that I think does work for family and trees and that is possitive thoughts and patience, it is also very much what you need if you wish to have a career using timber for making objects or just growing timber.
I have had quite a few assistants in the past and see them virtually all struggling to make wood into furniture, I have to tell them that the only result from a battle with the timber is that they will lose. Like a lot of things in life one day you discover that its not such hard work as you once thought, bending to the needs of the timber and designing accordingly, making ajustments in response to pieces of timber that is front of you, even some exceptions like the steaming of wood to bend it for chair or violin making still requires the cabinet maker to select the correct grain and pay attention to its density and brittleness, not all timber can be steamed bent and still be strong.

The illustration of various timbers is perhaps easier for me if I give the link here to John Boddy Timber, they are a family run business that I have used for more than 20 years and have great faith in the fact that they are truelly concerned at buying timber that is replenished and will not buy from dealers that rob the world of lumber.

Wood is hard, fibrous, lignified structural tissue produced as secondary xylem in the stems of woody plants, notably trees but also shrubs. This tissue conducts water to the leaves and other growing tissues and has a support function, enabling plants to reach large sizes. Wood may also refer to other plant materials and tissues with comparable properties.

Wood is a heterogeneous, hygroscopic, cellular and anisotropic material. Wood is composed of fibers of cellulose (40% – 50%) and hemicellulose (15% – 25%) impregnated with lignin (15% – 30%).[1]

Wood has been an important construction material since humans began building shelters, houses and boats. Nearly all boats were made out of wood till the late 1800s, and wood remains in common use today in boat construction. New domestic housing in many parts of the world today is commonly of timber-framed construction. In buildings made of other materials, wood will still be found as a supporting material, especially in roof construction and interior doors and their frames and exterior cladding. Wood to be used for construction work is commonly known as lumber in North America. Elsewhere, lumber will usually refer to felled trees, and the word for sawn planks ready for use is timber.

Broadly, there are two methods by which timber can be dried: (i) natural drying or air drying, and (ii) artificial drying.

Air drying

Air drying is the drying of timber by exposing it to the air. The technique of air drying consists mainly of making a stack of sawn timber (with the layers of boards separated by stickers) on raised foundations, in a clean, cool, dry and shady place. Rate of drying largely depends on climatic conditions, and on the air movement (exposure to the wind). For successful air drying, a continuous and uniform flow of air throughout the pile of the timber needs to be arranged . The rate of loss of moisture can be controlled by coating the planks with any substance that is relatively impermeable to moisture; ordinary mineral oil is usually quite effective. Coating the ends of logs with oil or thick paint, improves their quality upon drying. Wrapping planks or logs in materials which will allow some movement of moisture, generally works very well provided the wood is first treated against fungal infection by coating in petrol/gasoline or oil. Mineral oil will generally not soak in more than 1-2 mm below the surface and is easily removed by planing when the timber is suitably dry.

Kiln drying

The process of kiln drying consists basically of introducing heat. This may be directly, using natural gas and/or electricity or indirectly, through steam-heated heat exchangers, although solar energy is also possible. In the process, deliberate control of temperature, relative humidity and air circulation is provided to give conditions at various stages (moisture contents or times) of drying the timber to achieve effective drying. For this purpose, the timber is stacked in chambers, called wood drying kilns, which are fitted with equipment for manipulation and control of the temperature and the relative humidity of the drying air and its circulation rate through the timber stack

Kiln drying provides a means of overcoming the limitations imposed by erratic weather conditions. In kiln drying as in air drying, unsaturated air is used as the drying medium. Almost all commercial timbers of the world are dried in industrial kilns.

A tree grows by a system of transporting the water and minerals in the soil up the trunk to the leaves and there are many changes that occur in the movement of this water with its solution of minerals.

The basic principle is called osmosis.

When water evaporates from the mesophyll cells of a leaf and diffuse out of the stomata (transpiration), the cells involved develop a lower water potential than the adjacent cells. Because the adjacent cells then have a correspondingly higher water potential, replacement water moves into the first cells by osmosis. This continues across rows of mesophyll cells until a small vein is reached. Each small vein is connected to a larger vein, and the larger veins are connected to the main xylem in the stem, which in turn is connected to the xylem in the roots that receive water, via osmosis, from the soil. As transpiration takes place it creates a "pull" or tension on water columns, drawing water from one molecule to another all the way through the entire span of xylem cells. The cohesion required to move water to the top of a 300 foot redwood tree is considerable.

Water is primarily "pulled" upward due to the cohesion of water molecules within the xylem tracheids and vessels. Like a steel wire, the chain of water molecules is literally pulled through the plant's vascular system, from the roots to the leaves. As water molecules move out through the stomata into the atmosphere, they are replaced by new molecules entering the roots from the soil. Since the water in xylem ducts is under tension, there is a measurable inward pull (due to adhesion) on the walls of the ducts. It has been estimated that only about one percent of all water molecules transported upward are used by a tree; the other 99 percent are needed to get that one percent up there. Water molecules must literally grow with the plant in order to form continous chains within the xylem tubes.
  1. Free water: The bulk of water contained in the cell lumina is only held by capillary forces: it is not bound chemically and is termed free water. Free water is not in the same thermodynamic state as liquid water: energy is required to overcome the capillary forces. Furthermore, free water may contain chemicals, altering the drying characteristics.
  2. Bound or hygroscopic water: Bound water is bound to the wood via hydrogen bonds. The attraction of wood for water arises from the presence of free hydroxyl (OH) groups in the cellulose, hemicelluloses and lignin molecules in the cell wall. The hydroxyl groups are negatively charged electrically. Water is a polar liquid. The free hydroxyl groups in cellulose attract and hold water by hydrogen bonding.
Tangential boards (crown, plain or flat sawn) are used extensively for beams and joists. They are stronger when placed correctly edge up with the load in the tangential axis. These type of boards suffer from 'cupping' if not carefully converted, seasoned, and stored properly. Annual growth rings form an angle less than 45 degrees.

Radial boards (radial, figured or quarter sawn) are typically cut on 'the quarter' and produce a pattern of the medullary rays especially in quartered oak. Such timber is expensive due to the multiple cuts required to convert this board. The radial face of the board is slightly stronger and stiffer than the tangentially face but the cross section and condition of the timber has more effect on strength. Annual growth rings form an angle greater than 45 degrees.

Crown sawn is obtained by sawing tangentially to the annual rings. It is also referred to as 'Plain Sawn' or 'through and through'.

Rift sawn is the cut which falls between crown and true quarter sawn. It is straight grained and in oak, does not reveal any 'silver ribbon' grain features. Quality floor boards are prepared from rift sawn timber because it wears well and shrinks less. Annual growth rings form an angle between 30 and 60 degrees.

Quarter sawn boards are radial cut from the centre of the tree. It produces the distinctive silver ribbon effect (in oak) across the whole board. Annual growth rings form an angle greater than 45 degrees. True quartered boards producing the best features will have the angle on or very much closer to 90 degrees.

'Figured' - is the cut between 'rift' and 'true quartered'. It has varying degrees of 'silver ribbon' (in oak) showing through but not the full figured effect found in true quarter sawn boards.

Different species have their best features enhanced by choosing the best cut appropriate to their species.

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