In this article we will discuss about:- 1. Introduction to Wood 2. Wood Structure 3. Properties 4. Moisture Content 5. Destroyers and Preservatives 6. Abnormal Woods 7. Seasoning Defects 8. Other Multiple Uses.
Introduction to Wood:
Wood is a cellular material of biological origin. It is a hard, fibrous tissue found in many plants. It is an organic material, a natural composite of cellulose, hemicelluloses and lignin. Wood is produced as secondary xylem in the stems of trees and woody plants. Wood is biodegradable and renewable natural resource.
Wood is hygroscopic i.e. it has the ability to absorb the moisture from air. Wood is anisotropic which means that its structure and properties vary in different directions and wood is combustible. In living trees, it transfers water and nutrients to leaves and other growing tissues and wood provides support to the tree. Wood is considered as the most important raw material.
Heartwood, Sapwood and Bark:
A stem cross-section shows a dark-coloured center portion surrounded by a lighter- coloured outer region. In mature trees, the xylem has both living and dead cells. Older wood in the middle of the trunk dies and becomes harder, darker and drier and is called heartwood. Heartwood is formed in the central part of the tree stem due to reduced water and oxygen availability which leads to death of parenchyma cells.
It is followed by formation and deposition of extractives in the cells. This is the best wood in the tree. Heart wood is resistant to decay and insect and disease resistant. Heartwood is heavier, stronger and usually begins to form around age 14-18 years. It is difficult to penetrate with liquids and preservatives.
The wood formed immediately inside the bark of a tree is called sapwood. It is lighter in colour and a lot wetter than heartwood. Sapwood contains all of the live cells in the xylem. Rays provide water and nutrient transport from phloem. Nutrients are stored in specialized cells called parenchyma cells which also provide strength to the tree stem.
Sapwood contains living cells that transport water from the roots to the branches and leaves at the top of the tree. Sapwood is permeable and readily accepts chemical treatments and preservatives. Bark is used for protecting the tree.
Bark is the outer layer and is composed of a dead outer phloem of dry corky material and a thin inner phloem of living cells. Its primary functions are protection and nutrient conduction. The thickness and appearance of bark vary substantially depending on the species and age of the tree.
Softwoods and Hardwoods:
The terms softwood and hardwood do not directly describe the hardness or softness of wood. Instead, they refer to the leaf form or mode of seed production on trees from which the wood is cut. The terms are especially confusing because some true hardwoods have softer or lighter wood than common softwoods.
Softwoods are also very strong for their weight in comparison to the hardwoods. Coniferous trees in temperate regions provide soft wood. They have needle or scale-like leaves and maintain these leaves throughout the year (evergreens). They can grow quickly with straight trunks. Softwoods are most commonly used for construction lumbers, heavy timbers, poles and piles where strength is important. E.g. pines, spruces, firs, larch, cedars, hemlock, redwood, yew, cypress.
Hardwoods are trees such as – oak, teak, sal and many others which have broad deciduous leaves. They grow slowly and sometimes have twisted trunks. They grow in tropical regions of the world. Hardwoods are often used for fine furniture and heavy timbers.
Planes or Surfaces of Wood:
Visible characteristics, shrinkage and mechanical properties of wood are defined in terms of the three planes in which wood can be cut. Characteristics of these surfaces can also be useful in the identification of wood.
i. Cross-Section or Transverse:
Cross-section is the surface exposed when wood is cut across the width of a log or board. Cross-section reveals the annual rings. It is the key surface for identification of wood. It absorbs preservatives more easily than radial or tangential surfaces because the cross section is porous. Cross-section is also termed as end grain.
The radial surface is exposed when a log is cut longitudinally from its center to the bark (along the radius). In the hardwood industry, lumber cut this way is known as quarter sawn lumber. This surface has high strength and has high resistance to wear.
The tangential surface is exposed when a log is cut parallel to the bark and tangent to the log diameter. The lumber cut this way is known as flat or plain sawn lumber. The method of this conversion is known as through & through sawing or slabbing.
This is the cheapest, easiest and most common way of producing lumber today and it results in a characteristic U or V-shaped grain pattern in the softwoods with distinct early wood and latewood brands and the coarse grained hardwoods.
It is the direction of the long axis of longitudinal wood cells.
i. Straight Grain:
If it is straight or parallel to the tree stem, it is called straight grain orientation.
ii. Spiral Grain:
When wood cells are spirally arranged about the stem axis, it is called spiral grain orientation. It is caused by abnormal cell division or anticlinal cell division (production of new initials by radial partitioning) in which new cambial cell formation occurs in one direction only. This is typically low in strength and stiffness and may tend to twist as it dries.
iii. Interlocked Grain:
Woods with interlocked grain are produced when grain spirals in one direction for several years and then reverses direction to spiral oppositely. It is genetically controlled and wood is difficult to split, may shrink longitudinally upon drying and can warp unpredictably. It is sometimes desirable from an appearance standpoint as alternating grain direction cause light to reflect in varying patterns across radially cut wood, giving a ‘ribbon stripe’ figure.
iv. Fine Grain:
Wood is fine-grained if its annual rings are narrow. In wood industry, a fine-grained wood is capable of high polish.
v. Coarse Grain:
Wood is coarse-grained if its annual rings are wide and not capable of high polish.
Wood is composed mostly of hollow, elongated, spindle-shaped cells that are arranged parallel to each other along the trunk of the tree. The characteristics of these fibrous cells and their arrangement affect strength properties, appearance, resistance to penetration by water and chemicals, resistance to decay and many other properties. The microscopic structure of wood resembles a bundle of straws glued together.
Each straw represents a cell with a cellulose wall and a hollow centre (lumen) and lots of fine perforations through the wall. The cell walls are impregnated with lignin (phenolic compound), a natural polymer that glues the cells together and gives the wood its strength. Most cells in wood are oriented longitudinally (parallel to the tree trunk), with some cells radially aligned.
The primary structural building block of wood is the tracheid or fibre cell. Cells vary from 16 to 42 µm in diameter and from 870 to 4000 µm long. Thus, a cubic centimeter of wood could contain more than 1.5 million wood cells. When packed together they form a strong composite. Each individual wood cell is even more structurally advanced because it is actually multilayered, reinforced, close-end tube.
Each individual cell has four cell wall layers (Primary, S1, S2 and S3). Each layer is composed of a combination of three chemical polymers: cellulose, hemicelluloses and lignin. The cellulose and hemicelluloses are linear poly-saccharides (i.e., hydrophilic multiple-sugars) and the lignin is an amorphous phenolic (i.e., a three-dimensional hydrophobic adhesive).
Cellulose forms a long unbranched chains and hemicelluloses forms short branched chains. Cellulose is a polysaccharide of repeated glucose molecules which may reach 4 µm in length. These cellulose molecules are arranged in an orderly manner into structures about 10- 25 nm wide called microfibrils. This microfibrils wind together like strands in a cable to form macrofibrils that measure about 0.5 µm in width and may reach 4 µm in length. This framework of cellulose macrofibrils is cross-linked with hemicellulose, pectin and lignin. Lignin encrusts and stiffens these polymers.
These strands are very strong as like a steel cable. Lignin gives the cell wall rigidity and it is the substance that cements the cells together. Because carbohydrate and phenolic components of wood are assembled in a layered tubular or cellular manner with a large cell cavity, specific gravity of wood can vary immensely. Wood excels as a viable building material because the layered tubular structure provides a large volume of voids, is has an advantageous strength-to-weight ratio.
Structure of Softwoods:
Tracheids or Fibres:
Tracheids are the principal longitudinal cell type in softwoods. Tracheids or fibres comprise 90-95 per cent of the volume of the wood. They have the length diameter ratio of 100:1 and contribute greatly to the strength of the wood. Softwood fibres range from about 3 to 8 mm in length. The diameter of tracheids varies from one species to the next and useful for identification purpose.
Parenchyma cells are generally short, thin-walled cells which are connected together in strands and serve primarily for storage and distribution of carbohydrates.
Resin canals are tubular passageways lined with living parenchyma cells which exude resin or pitch. In pine, resin canals are easily seen with the naked eye on the end grain as wells as the side grain.
Individual cells in softwoods are connected together by means of pits that allow movement of liquids between cells. Pits are important in softwoods since preservatives move from one cell to the next through them.
Structure of Hardwoods:
The hardwoods have more cell types and the variation in the size and arrangement of these cells is greater than in the softwoods. As a result, the hardwoods are more varied in appearance and may have unique characteristics making certain species more desirable for selected end uses than others.
Hardwoods have specialized structures called vessels for conducting sap upward. Vessels are relatively large diameter, thin-walled, round cells with open ends that are connected end to end to form microscopic tubes that are ideal for sap conduction. In most hardwoods, the ends of the individual cells are entirely open; in others, they are separated by a grating.
On the cross-section surface, vessels appear as holes and are termed as pores. The size, shape and arrangement of pores vary considerably between species. It is the vessels which constitute pores in hardwoods. The presence of large diameter unobstructed vessels makes some woods easy to treat.
Fibres are relatively smaller diameter, elongated cells with closed ends. They usually have small cavities and relatively thick walls. Fibres are primarily responsible for the strength characteristics of hardwood. Thin places or pits in the walls of the wood fibres and vessels allow sap to pass from one cavity to another.
In the hardwoods, parenchyma cells vary widely in size and arrangement. In some trees, the rays (mostly parenchyma cells) are conspicuous to the naked eye and aid greatly in the identification of species. Wood rays are strips of short horizontal cells that extend in a radial direction. Their function is food storage and lateral conduction. Most of the rays in flat-grain surfaces are two to five cells wide, but their width and height vary in different species of hardwoods.
Hardwoods contain other miscellaneous features which are often important in identification, as well as the end use of the product. For example, white oak (Quercus alba) contains tyloses. Tyloses are literally plugs in the vessels or pores which restrict sap movement. Thus white oak is used to make tight cooperages (alcohol barrels) and does not accept preservative treatment well.
Individual cells in hardwoods are connected together by means of pits that allow movement of liquids between cells.
Groups of Hardwoods:
The distribution or large diameter cells (pores or vessels) within growth increments of hardwoods allows grouping of these hardwoods into three categories based on their cross sectional appearance viz.:
i. Ring porous
ii. Semi-ring porous
iii. Diffuse porous
i. Ring Porous Wood:
Group of hardwoods in which the pores (vessels) are formed comparatively large in size during favourable condition at the beginning of each growth increment. Pores decrease in size abruptly during unfavourable condition (outer portion of the ring). The pores can be easily seen with the naked eye. E.g. oak, ash, elm.
ii. Semi-Ring Porous Wood:
Semi-ring porous woods have pores that are initially large and then gradually decrease in diameter throughout the growth increment. E.g. walnut, butternut
iii. Diffuse Porous Wood:
The pores are uniform in size across the entire growth increment and are generally too small to be seen without the use of a hand lens. E.g. maples, sweetgum, yellow poplar
Chemical Composition of Wood:
Cellulose comprises about 40 to 45 per cent of the oven-dry weight of wood. Long strands of cellulose molecules arranged more or less parallel to each other within the thick walls of structural cells (fibres and tracheids) contribute a high tensile strength to wood.
Hemicellulose constitutes from 20 to 35 per cent of the oven-dry weight of the wood. The exact function of the hemicelluloses is not clear. Some possibility exists that they serve as a temporary matrix before lignifications.
Lignin constitutes from 15 to 35 per cent of the oven-dry weight of the wood. Lignin is a complex polymer and it reinforces the cellulose portion of the cell wall, thereby contributing to the rigidity of wood.
The inorganic materials, or ash generally constitute less than one per cent of the oven-dry weight of the wood. The most common constituents are calcium, potassium, magnesium, carbonates, phosphates, silicates and sulfates.
Most woods usually contain some type of extractives which are located in the heartwood and are water soluble. They are often responsible for the general darkening of the heartwood, for the resistance of some heartwood to decay and insect attack, for specific odour and for good dimensional stability.
Extractives make woods from some tree species more durable than others. If the extractives have a toxic or repellent effect, heartwood is more resistant than the sapwood. The more important organic extractives include the terpenes, resin acids, polyphenols, tannins and tropolenes.
Colour lends beauty to the wood and is of great value in the determination of its quality. It also aids in the identification of wood. The heartwood of tree species varies in colour viz. black colour of persimmon, dark brown of walnut, light brown of white oak, reddish brown of red oak, yellowish white of tulip and poplars, brownish red of redwood and cedars.
They are all reliable marks of distinction and colour. Nearly formed wood in the outer few rings has little colour compared to old wood. The sapwood is generally light in colour. The different tints of colours are due to pigments or wood being denser.
Odour depends on chemical compounds or extractives and it is not due to the part of wood substance itself. When new surface is exposed in the wood during cutting, it gives characteristic odour and exposure to weather reduces and often changes the odour. Heartwood is more odouriferous than sapwood.
Many kinds of wood are distinguished by strong and peculiar odours. This is especially true in the case of camphor tree, cedar, pine, oak, mahogany, teak etc. Decomposition of wood is usually accompanied by pronounced odours.
iii. Density and Specific Gravity:
Density is the single most important factor to indicate strength in defect-free wood and other characteristics such as ease of machining and hardness. Density is the weight of wood per unit volume. Moisture content will greatly affect the density. Generally as the density of wood increases, its strength also increases.
Specific gravity is another important factor for comparing the relative weights of different woods. Specific gravity is the ratio of the oven-dry weight of a given volume of wood to that of weight of an equal volume of water at a standard temperature.
The permeability of wood is the extent to which it allows fluid flow through a porous medium under the influence of a pressure gradient. There is a good correlation between wood permeability and treatability. The treatability describes the ease with which preservatives can be forced into wood under pressure and treatability varies with the different wood species.
Moisture Content of Wood:
The Moisture Content (MC) of green or fresh wood as found in the living tree or logs are highly variable which depends on species, location, season of the year, heartwood and sapwood content of wood. Amount of water in wood (moisture content) affects wood treatability, durability and stability. The moisture content of wood is defined as the weight or amount of water in wood given as s percentage of oven-dry weight.
The following formula is used to calculate the moisture content in wood:
Wood cut from a green log often contains as much or more than its oven-dry weight in the form of sap or water. Water is contained in wood as either bound water or free water. Bound water is held within cell walls by bonding forces between water and cellulose molecules. Free water is contained in the wood cell cavities.
i. Fibre Saturation Point (FSP):
When the wood has dried to about 30 per cent moisture content, it is at the fibre saturation point. In this state, the cell cavities are emptied of free water, but the cell walls are still saturated with bound water. Wood shrinks as it dries below the FSP. At moisture content above FSP, Wood can be attacked by decay fungi. The water remaining in the cell walls after wood has dried to the FSP is called bound water.
ii. Equilibrium Moisture Content (EMC):
Wood is a hygroscopic material and the amount of water which the wood will lose depends on the relative humidity. Hence it responds to changes in atmospheric humidity. Wood loses or gains bound water until the amount it contains is in balance with that of surrounding atmospheric relative humidity. When this balance of moisture exchange is established, the amount of bound water eventually contained in wood is called equilibrium moisture content. EMC is always below 30 per cent.
iii. Shrinking and Swelling:
Wood shrinks and swells due to the loss or gain of bound water from the cell walls. The amount of movement depends on the amount of water gained or lost, the orientation of the wood cells and species. When wood dries below fibre saturation point, it begins to shrink.
Conversely, wood that is below FSP will swell as it takes on moisture and this will continue until FSP is reached. Changes in moisture content above FSP have no effect on shrinkage and swelling. Wood should be dried to its anticipated equilibrium moisture content to minimize problems due to changing moisture contents.
Typical shrinkage values of wood on three surfaces are:
Maximum longitudinal shrinkage = 0.1 – 0.3%
Maximum radial shrinkage = 2.1 – 7.9%
Maximum tangential shrinkage = 4.7 – 12.7%
iv. Wood Seasoning:
The process of drying of timber is called wood seasoning. The rate of drying of wood is dependent on temperature, relative humidity and air circulation. Wood may be seasoned in two ways – air drying and kiln drying.
Air drying is simple method of drying the wood in open condition and it takes longer time up to 6-12 months. Kiln drying is the process of drying wood by keeping them in closed chamber in controlled temperature, relative humidity and air circulation.
These large drying ovens are called kilns and it takes 4-5 weeks for drying wood in kilns. The kilns are categorized into progressive type, compartment type and solar kilns. Proper stacking of timber helps in uniform drying and reduces seasoning defects. Horizontal and vertical stacking methods are followed for drying different types of wood.
Wood Destroyers and Preservatives:
Wood Destroying and Staining Fungi:
i. Brown Rot:
Fungi are able to break down the cellulose of wood, leaving a brown residue of lignin and identified by dark brown colour of wood.
ii. Dry Rot:
Brown rot when it is dry is called dry rot. Dry rot fungi decay relatively dry wood and they have water conducting strands that are able to carry water from damp soil to wood.
iii. White Rot:
It breaks down both lignin and cellulose, have a bleaching effect, which may make the damaged wood appear.
iv. Soft Rot:
Soft rot fungi usually attack green wood (high MC) causing gradual softening from the surface inward that resembles brown rot.
v. Pocket Rot:
It describes the decay in wood that is characterized by small cavities of severe decay, scattered throughout the wood.
vi. Sap Staining Fungi:
Fungi penetrate and discolour sapwood and stain cannot be removed by brushing or planning.
vii. Mold Fungi:
Fungi cause green, yellow, brown or black fuzzy or powdery surface growths on wood during warm & humid weather.
Termites use wood for food and shelter and are the most destructive of all wood insects. Three major groups of termites are destroying the wood viz. subterranean or ground inhabiting termite, dry wood termite, dampwood termite.
Carpenter ant may be black or red and they use wood for shelter, not for food. They make galleries in wood and mounds of saw dust indicate the presence of carpenter ants.
The major wood destroying beetles include powder post or lyctus beetle, anobiid beetle, long horn beetle or roundhead borers, flathead borers or metallic wood-boring beetle.
iv. Marine Borers:
The major marine borers are shipworm, pholad mollusks, and crustacean borers.
Wood preservatives fall into three categories – Creosote and creosote solutions (creosote and oily byproduct of bituminous coal), Oilborne Preservatives (Pentachlorophenol or Penta), Waterborne Preservatives (Various Metallic Salts, Inorganic arsenicals and compounds i.e. combinations of copper, chromium, arsenic and fluoride)
Creosote and Creosote Solutions:
i. Coal-tar-creosote (Creosote) is a black or brownish oil made by distilling coal tar that is obtained after high temperature carbonization of coal. It has high toxicity to wood destroying organism. Relative insolubility in water and low volatility.
ii. Coal tar or petroleum oil has been mixed with coal-tar-creosote in various proportions to prepare Creosote solutions.
iii. Wood-tar-creosote, Oil-tar-creosote, Water-gas-tar-creosote are also used to some extent.
Other Oilborne Preservatives:
Copper Naphthenate, Chlorothalonil (CTL), Chlorpyrifos (CPF), Oxine Copper (Copper-8-quinolinolate), Zinc Naphthenate, Bis (tri-n-butyltin) Oxide (TBTO), 3-Iodo-2- Propynyl Butyl Carbamate (IPBC), Alkyl Ammonium Compound (AAC) or Didecyldim- ethylammonium chloride (DDAC), Propiconazole, Tebuconazole (TEB)
Other Waterborne Preservatives:
Acid Copper Chromate (ACC), Ammoniacal Copper Zinc Arsenate (ACZA), Chromated Copper Arsenate (CCA), Ammoniacal Copper Quat (ACQ), Copper bis (dimethyldithio carbamate) or CDDC, Ammoniacal Copper Citrate (CC), Copper Azole-Type A (CBA-A), Inorganic Boron (Borax/Boric Acid)
Application of Preservatives:
i. Pressure Process:
It includes full-cell process and empty-cell process. The basic principle of pressure process involves the placement of wood material in an airtight, steel cylinder or retort and immersing it in a preservative under pressure to force the preservative into the wood.
ii. Non-Pressure Process:
It includes brushing, spraying, pouring, dipping, cold soaking, steeping, hot and cold bath (thermal processing), double diffusion, vacuum process. In the vacuum process, wood products are enclosed in an airtight container from which air is removed with a vacuum pump and container is then filled with the preservative. The partial removal of air from the wood, by the vacuum, followed by addition of the preservative creates a slight pressure that drives the preservative into the wood.
In both hardwoods and softwoods, depending on age and location in the tree, some woods are substandard and low quality. This wood is called abnormal wood. The strength and durability of this wood are significantly lower than that of normal wood. These abnormal woods are formed as a result of natural defects.
i. Juvenile Wood:
Juvenile wood is that material formed near the center or pith of the tree and is prevalent in both softwoods and hardwoods. It is the wood produced during the first 5-15 years of growth. Juvenile wood is characterized by wide growth rings with shorter and thin-walled cells and fewer late wood cells, thus resulting in a lower density and reduced strength values. The juvenile wood will create warping defects during drying. The change from juvenile wood to normal wood is gradual, thus making identification of juvenile wood difficult.
A knot is the basal portion of a branch or limb which has been surrounded by subsequent growth of the tree. The grain of wood deviates around knot and considered as a weak point in the wood. Two types of knots are prevalent: tight knot and loose knot.
Incorporation of living branches into the stem is termed as tight knot or inter grown knot. This knot is integral parts of the surrounding wood. Loose knots occur when stem growth encases the dead branch base. Branch stub is progressively covered by seasonal growth following pruning.
iii. Spiral and Interlocked Grain:
When longitudinal cells in wood are not arranged parallel to the main axis of the tree stem and slightly spiral about the stem, it is called spiral grain. Lumber sawn from these logs, will have a cross or diagonal grain pattern. These wood products are typically low in strength and stiffness and tend to twist upon drying.
When spiral grain reverses direction to spiral oppositely following some years, it is called interlocked grain or reverse spiral grain. These wood products will show warping defects upon drying.
iv. Reaction Wood:
Wood formed in the leaning tree or branches in order to correct growth irregularity in a stem is termed as reaction wood. It is an attempt by the tree to straighten itself out. Reaction wood comprises compression wood and reaction wood.
In softwoods, reaction wood is called compression wood and it is formed on the lower side of leaning trees. The part of the growth ring with reaction wood is usually wider than the rest of the ring and has high proportion of latewood. As a result, the tree develops an eccentrically shaped stem and the pith is not centered.
It is often darker in colour and presents serious problems in wood manufacturing since it is much lower in strength than normal wood of the same density. Also, it tends to shrink excessively in the longitudinal direction causing cross grain checking.
In hardwoods, reaction wood is called tension wood and forms predominantly toward the upper side of the leaning tree. It may form irregularly around the entire stem and is often difficult to detect. When machined, tension wood may show a fuzzy, wooly or fibrous appearance in cut surface.
Solid wood products have lower quality due to low strength. It has the tendency to collapse upon drying. It can produce good paper properties if pulping conditions are modified and also good for ‘dissolving pulps’ and cellulose source for making cellophane, rayon and nitrocellulose.
Wood Seasoning Defects:
Wood is anisotropic as it dries below the fibre saturation point i.e. shrinkage is not equal in all directions. Longitudinal shrinkage is negligible, except in reaction wood. Tangential shrinkage is 1.5 to 3 times higher than radial shrinkage.
This differential shrinkage sets up strains which cause ruptures or fractures in the wood tissues and warping defects, significantly devaluing the wood product. Rupture of wood tissues includes checks, cracks, splits, shake, collapse, honey comb etc. Warping defects include bow, crook, cup, twist, kink, diamond, etc.
i. Surface Checks:
They are failures or very slight separations (shallow cracks) that usually occur in the wood rays on the flat sawn faces of boards and confined to the tangential surface. They result from the separation of the thinner-walled early wood cells. They occur because of drying stresses exceed the tensile strength of the wood perpendicular to the grain and they are caused by tension stresses that develop in the outer part of boards as they dry.
ii. End Checks:
They are failures or very slight separations usually occur in the wood rays but on the end-grain surfaces. End checks occur because moisture moves much faster in the longitudinal direction than in either transverse direction. Therefore, the ends of boards dry faster than the middle and stress develop at the ends. They occur in the early stages of drying and can be minimized by using high relative humidity or by end coating.
Cracks are typically much deeper separations in a wood protruding deeply into the interior portion of wood.
Split is the deepest of separations in a wood, potentially going so far to completely separate a wood into two sections. It is the separation of the wood parallel to the fibre direction, due to the tearing apart of the wood cells.
v. Box-Heart Split:
A split originating in the wood surrounding the pith during drying. It is caused by stresses set up because of the differences in tangential and radial shrinkage of the wood near pith.
vi. Ring Shake or Ring Failure:
It is the separation of the wood parallel to the grain along the growth rings. It can occur as a failure in the end grain in the initial stages of drying and extend in depth and length as drying progresses.
It is a distortion, flattening or crushing of wood cells. Collapse usually shows up as grooves or corrugations.
Internal splitting in wood that develops in drying. It is an internal crack caused by a tensile failure across the grain of the wood and usually occurs in the wood rays. This defect occurs because of the internal tension stresses that develop in the core of the board during drying. It occurs when the core is still at relatively high moisture content and when drying temperatures are too high for long period.
ix. Checked Knots:
Checked knots are considered as defects and the checks appear on the end grain of knots in the wood rays. They are the result of differences in shrinkage parallel to and across the annual rings within knots.
x. Loose Knots:
Due to differential drying, encased knots can become loose during drying since their wood is usually denser and shrinks more than the surrounding tissue. This dried dead knot is smaller than the knothole and frequently falls out during handling or machining.
It results from too rapid drying, where the surface dries below the FSP first, but cannot shrink, putting tension on the surface. Nearly uniform moisture content but residual stresses, tension in the interior and compression in the outer layers of cells, causes casehardening.
xii. Compression Failure:
It is the deformation of the wood fibres resulting from excessive compression along the grain either in direct end compression or in bending. In surfaced, lumber, compression failures appear as fine wrinkles across the face of the wood.
xiii. Cross Break:
A cross break is the failure of the wood cells across the grain and severely reduces the wood strength. Such breaks may be due to internal stress resulting from unequal longitudinal shrinkage.
It is the longitudinal curvature, flat-wise from a straight line. It is a deviation from edge-to-edge on the flat surface of board.
It is the longitudinal curvature, edge-wise from a straight line. It is a deviation from edge-to-edge along the long surface of board.
It is the curving of the face of a plank so that it assumes a trough-like shape. It is a deviation from edge-to-edge on the end of board.
It is the distortion of board sot that the two end surfaces do not lie on the same plane. One corner of a piece of wood twists out of the plane of the other three.
It is an abrupt deviation from the flatness or straightness due to localized grain distortion around knot or deformation caused by misplaced stickers in kiln.
It is a form of warp found in squares or thick lumber end. The cross-section assumes a diamond shape during drying caused by difference between radial and tangential shrinkage in squares in which growth rings run diagonally from corner to corner.
It is also called as bark inclusion for bark incursion. Any area of a piece of lumber that includes the normally occurring edge of the tree it was cut from.
Other Multiple Uses of Wood:
Wood has long been used as an artistic medium. It has been used to make sculptures and carvings for millennia. Certain types of musical instruments, such as those of the violin family, guitar, clarinet and recorder, xylophone and marimba are made mostly or entirely of wood.
The tree species widely used for manufacture of musical instruments, sports goods and agricultural implements are presented below:
a. Musical Instruments:
The choice of wood may make a significant difference to the tone and resonant qualities of the instrument, and tone-woods have widely differing properties, ranging from the hard and dense African blackwood (used for the bodies of clarinets) to the light but resonant European spruce (Picea abies) (traditionally used for the soundboards of violins).
The most valuable tone-woods, such as the ripple sycamore (Acer pseudoplatanus), which is used for the backs of violins, combine acoustic properties with decorative color and grain which enhance the appearance of the finished instrument.
Indian woods used in musical instruments are:
Maple (Acer spp.), Teak (Tectona grandis) Rosewood (Dalbergia latifolia), Eboney (Diospyrus spp) Sundari (Heritieria minor)
Toon (Toona ciliata), Deodar (Cedrus deodara), Teak (Tectona grandis), Sissoo (Dalbergia sissoo)
iii. Vina and Tambora:
Jackfruit (Atrocarpus heterophyllus), Gamari (Gmelina arborea), Bijasal (Pterocarpus marsupium)
White dhup (Canarium euphyllum),
Teak (Tectona grandis)
vi. Pianao Cases:
Mahogany (Swietenia spp.), Walnut (Juglans regia), Satin wood (Cholroxylon swietienia), Paduk (Pterocarpus dalbergioides)
Ash (Fraxinus spp.), Siris (Albizia spp.), Sissoo (Dalbergia sissoo)
b. Sports Goods:
Many types of sports equipment are made of wood, or were constructed of wood in the past. For example, cricket bats are typically made of white willow. The baseball bats which are legal for use in major league baseball are frequently made of ash wood or hickory, and in recent years have been constructed from maple even though that wood is somewhat more fragile.
Many other types of sports and recreation equipment, such as skis, ice hockey sticks, lacrosse sticks and archery bows, were commonly made of wood in the past, but have since been replaced with more modern materials such as aluminum, fibreglass, carbon fibre, titanium, and composite materials.
Indian wood suitable for sports goods are:
i. Cricket Bats, Stumps and Bails:
Cricket bat Willow (Salix alba var. caerulea), Mulberry (Morus alba), Persian lilac (Melia azedarach) Gutel (Trewia nudiflora), Chinese tallow tree (Sapium sebiferum), Sandan (Ougeinia oojenensis).
ii. Hockey Sticks:
Mulberry (Morus alba), Celtis (Celtis australis).
iii. Bows and Arrows:
For Bow – Yew (Taxus baccata), Parrotia (Parrotia jacmontiana), Dhaman (Grewia tilaefolia), Bijasal (Ptercarpus marsupium), Khair (Acacia catechu). For Arrow – Sissoo (Dalbergia sissoo), Poon (Callophyllum spp), Bijasal (Pterocarpus marsupium), Reeds and bamboos
Sissoo (Dalbergia sissoo), Mulberry (Morus alba), Axelwood (Anogeissus latifolia).
v. Fishing Rods:
Sagopalm (Caryota urens), Haplophragma adenophyllum, Balck chuglum (Terminalia manii), Chooi (Sageraea elliptica), Eboney (Diospyrus spp).
c. Agriculture Implements and Handles:
Most of the strong quality small woods are used in different agricultural tools and implements still in rural India and important species used for tool handles are: Bijasal (Ptercarpus marsupium), Khair (Acacia catechu), Babul (Acacia nilotica), Amaltas (Cassia fistula), Hopea (Hopea odorata), Mesua (Mesua ferrea), Oak (Quercus spp), Jamun (Syzygium cumini), Irul (Xylia xylocarpa), Ber (Zizyphus maurtiana), Ash (Fraxinus spp), Siris (Albizia spp.), Sissoo (Dalbergia sissoo), Sandan (Ougeinia oojenensis).