Wood

Hard, fibrous material found from trees. Has been used for thousands of years as construction material and fuel. Naturally occurring- made up of cellulose material, which is embedded in the lignin. Wood is produced as secondary xylem in stems of woody plants. This functions as support and in mediating the transfer of nutrients and water to other growing tissues in the plant. Can be used as fuel mostly in rural areas of the world and hardwood is preffed over softwood for the making of fuel. Three main, interwoven components: 1. Cellulose 2. Hemicellulose 3. Lignin Wood's chemical composition is extremely important because it contains more lignin than cellulose and in turn has much harder cell walls. The lignin makes for a stronger linkage between cellulose strands which make up individual cell walls. This makes for a much more solid form of plant life which holds it shape when dead (unlike plant leaves). Most uses of wood, besides those involving combustion can be traced back to this unique property.



contains lots of lignen

Wood products make up 47 percent of all industrial raw material manufactured in the United States, yet consume only 4 percent of the energy needed to manufacture these industrial materials. As a construction material, wood is 4 times more efficient as an insulator than an equivalent thickness of cinder block, 6 times more efficient than brick, 15 times more efficient than concrete, 306 times more efficient than steel, and 1,770 times more efficient than aluminum.

Glued engineered wood products are leading building products into the future. They utilize more of the available resource with minimum waste. They can also carry greater loads over longer spans than solid wood of equivalent size.

44 percent (or about 45 million tons per year) of all paper in the United States is recovered for recycling each year. The industry intends to increase this to 50 percent by the year 2002.Wood fiber is used in the production of photographic film, rayon products, and even interior moldings of __car__ doors. It is also being used as an important industrial fuel. Wood by-products are often used in paints and adhesives.

The American forest products industry provides jobs for one and a half million Americans and contributes more than $200 billion to the national __economy__.

Four-fifths of the wood harvested in developing countries, and 55% of all wood harvested in the world, is consumed as fuel. Wood is the principal energy source for cooking and heating for almost half of the world's population.

Global wood consumption has tripled this century, roughly paralleling population growth.

In 1997, forest landowners planted nearly 1.6 billion seedlings. That is more than 5 trees for each American. This does not include the natural regeneration of managed forests.

The forest products industry is one of the top ten largest United States manufacturing industries.

Today the United States has far more trees than in the 1920's and grows over one-third more wood than is used and lost to natural causes each year.

Since 1940 we have been growing more wood in America than harvested.

The United States forest, wood and paper industry will continue to be the global leader in providing safe and essential products based on forest resources.

Wood provides a source of clean __renewable energy__ and is the principal energy source for over half the world's population. The earth contains about one trillion tons of wood, which grows at a rate of 10 billion tons per year.

As an abundant, carbon-neiutral renewable resource, woody materials have been of intense interest as a source of renewable energy.

In 1991, approximately 3.5 billion cubic meters of wood were harvested. Dominant uses were for furniture and building construction.



There are two main classifications of wood: deciduous and coniferous. Deciduous trees do loose their leaves and are generally categorized as hard woods. Coniferous trees are those such as pines with needles and keep a green color all year long. These types of trees are more prominently soft wood.

When counting a tree's age there are two different "rings" that grow from the center or "pith" of the tree. The larger, lighter colored rings are spring growth, and the darker, smaller rings are summer growth. Interestingly, trees in the rainforest have no rings because they are continually growing year round.

History
A 2011 discovery in the Canadian [|province] of [|New Brunswick] uncovered the earliest known plants to have grown wood, approximately 395 to 400 [|million years ago]. [|[3]] People have used wood for millennia for many purposes, primarily as a [|fuel] or as a construction material for making houses, tools, [|weapons], furniture, packaging, artworks, and paper. Wood can be dated by [|carbon dating] and in some species by [|dendrochronology] to make inferences about when a wooden object was created. The year-to-year variation in tree-ring widths and isotopic abundances gives [|clues] to the prevailing climate at that time.

Growth rings
Wood, in the strict sense, is yielded by trees, which increase in [|diameter] by the formation, between the existing wood and the inner [|bark], of new woody layers which envelop the entire stem, living branches, and roots. This process is known as secondary growth; it is the result of cell division in the [|vascular cambium], a lateral meristem, and subsequent expansion of the new cells. Where there are clear seasons, growth can occur in a discrete annual or seasonal pattern, leading to [|growth rings] ; these can usually be most clearly seen on the end of a log, but are also visible on the other surfaces. If these seasons are annual these growth rings are referred to as annual rings. Where there is no seasonal difference growth rings are likely to be indistinct or absent.

If there are differences within a growth ring, then the part of a growth ring nearest the center of the tree, and formed early in the growing season when growth is rapid, is usually composed of wider elements. It is usually lighter in color than that near the outer portion of the ring, and is known as earlywood or springwood. The outer portion formed later in the season is then known as the latewood or summerwood. [|[5]] However, there are major differences, depending on the kind of wood

Knots
   A knot on a tree at the [|Garden of the Gods] public park in [|Colorado Springs], Colorado (October 2006) A knot is a particular type of imperfection in a piece of wood; it will affect the technical properties of the wood, usually for the worse, but may be exploited for visual effect. In a longitudinally sawn plank, a knot will appear as a roughly circular "solid" (usually darker) piece of wood around which the [|grain] of the rest of the wood "flows" (parts and rejoins). Within a knot, the direction of the wood (grain direction) is up to 90 degrees different from the grain direction of the regular wood. In the tree a knot is either the base of a side [|branch] or a dormant bud. A knot (when the base of a side branch) is conical in shape (hence the roughly circular cross-section) with the inner tip at the point in stem diameter at which the plant's vascular cambium was located when the branch formed as a bud. During the development of a tree, the lower limbs often die, but may remain attached for a time, sometimes years. Subsequent layers of growth of the attaching stem are no longer intimately joined with the dead limb, but are grown around it. Hence, dead branches produce knots which are not attached, and likely to drop out after the tree has been sawn into boards. In grading lumber and structural timber, knots are classified according to their form, size, soundness, and the firmness with which they are held in place. This firmness is affected by, among other factors, the length of time for which the branch was dead while the attaching stem continued to grow.    Wood Knot Knots materially affect cracking and warping, ease in working, and cleavability of timber. They are defects which weaken timber and lower its value for structural purposes where strength is an important consideration. The weakening effect is much more serious when timber is subjected to forces perpendicular to the grain and/or [|tension] than where under load along the grain and/or [|compression]. The extent to which knots affect the strength of a [|beam] depends upon their position, size, number, and condition. A knot on the upper side is compressed, while one on the lower side is subjected to tension. If there is a season check in the knot, as is often the case, it will offer little resistance to this tensile stress. Small knots, however, may be located along the neutral plane of a beam and increase the strength by preventing longitudinal [|shearing]. Knots in a board or plank are least injurious when they extend through it at right angles to its broadest surface. Knots which occur near the ends of a beam do not weaken it. Sound knots which occur in the central portion one-fourth the height of the beam from either edge are not serious defects. [|[6]] Knots do not necessarily influence the stiffness of structural timber, this will depend on the size and location. Stiffness and elastic strength are more dependent upon the sound wood than upon localized defects. The breaking strength is very susceptible to defects. Sound knots do not weaken wood when subject to compression parallel to the grain. In some decorative applications, wood with knots may be desirable to add visual interest. In applications where wood is [|painted], such as skirting boards, fascia boards, door frames and furniture, resins present in the timber may continue to 'bleed' through to the surface of a knot for months or even years after manufacture and show as a yellow or brownish stain. A knot [|primer] paint or solution, correctly applied during preparation, may do much to reduce this problem but it is difficult to control completely, especially when using mass-produced kiln-dried timber stocks.

Color
In species which show a distinct difference between heartwood and sapwood the natural color of heartwood is usually darker than that of the sapwood, and very frequently the contrast is conspicuous (see section of yew log above). This is produced by deposits in the heartwood of chemical substances, so that a dramatic color difference does not mean a dramatic difference in the mechanical properties of heartwood and sapwood, although there may be a dramatic chemical difference. Some experiments on very resinous [|Longleaf Pine] specimens indicate an increase in strength, due to the [|resin] which increases the strength when dry. Such resin-saturated heartwood is called "fat lighter". Structures built of fat lighter are almost impervious to rot and [|termites] ; however they are very flammable. [|Stumps] of old longleaf pines are often dug, split into small pieces and sold as kindling for fires. Stumps thus dug may actually remain a century or more since being cut. [|Spruce] impregnated with crude resin and dried is also greatly increased in strength thereby.    The wood of [|Coast Redwood] is distinctively red in color Since the latewood of a growth ring is usually darker in color than the earlywood, this fact may be used in judging the density, and therefore the hardness and strength of the material. This is particularly the case with coniferous woods. In ring-porous woods the vessels of the early wood not infrequently appear on a finished surface as darker than the denser latewood, though on cross sections of heartwood the reverse is commonly true. Except in the manner just stated the color of wood is no indication of strength. Abnormal discoloration of wood often denotes a diseased condition, indicating unsoundness. The black check in western [|hemlock] is the result of insect attacks. The reddish-brown streaks so common in hickory and certain other woods are mostly the result of injury by birds. The discoloration is merely an indication of an injury, and in all probability does not of itself affect the properties of the wood. Certain [|rot-producing fungi] impart to wood characteristic colors which thus become symptomatic of weakness; however an attractive effect known as [|spalting] produced by this process is often considered a desirable characteristic. Ordinary sap-staining is due to fungal growth, but does not necessarily produce a weakening effect.

Water content
Water occurs in living wood in three conditions, namely: (1) in the [|cell walls], (2) in the [|protoplasmic] contents of the [|cells] , and (3) as free water in the cell cavities and spaces. In heartwood it occurs only in the first and last forms. Wood that is thoroughly air-dried retains 8–16% of the water in the cell walls, and none, or practically none, in the other forms. Even oven-dried wood retains a small percentage of moisture, but for all except chemical purposes, may be considered absolutely dry. The general effect of the water content upon the wood substance is to render it softer and more pliable. A similar effect of common observation is in the softening action of water on paper or [|cloth]. Within certain limits, the greater the water content, the greater its softening effect. Drying produces a decided increase in the strength of wood, particularly in small specimens. An extreme example is the case of a completely dry [|spruce] block 5 cm in section, which will sustain a permanent load four times as great as a green (undried) block of the same size will. The greatest strength increase due to drying is in the ultimate crushing strength, and strength at [|elastic limit] in endwise compression; these are followed by the modulus of rupture, and stress at elastic limit in cross-bending, while the [|modulus of elasticity] is least affected.

= Hard and soft wood =

There is a strong relationship between the properties of wood and the properties of the particular tree that yielded it. The density of wood varies with species. The density of a wood correlates with its strength (mechanical properties). For example, mahogany is a medium-dense hardwood that is excellent for fine furniture crafting, whereas balsa is light, making it useful for model building. One of the densest woods is black ironwood.

It is common to classify wood as either softwood or hardwood. The wood from conifers (e.g. pine) is called softwood, and the wood from dicotyledons (usually broad-leaved trees, e.g. oak) is called hardwood. These names are a bit misleading, as hardwoods are not necessarily hard, and softwoods are not necessarily soft. The well-known balsa (a hardwood) is actually softer than any commercial softwood. Conversely, some softwoods (e.g. yew) are harder than many hardwoods.