Wood: properties of wood of various species. The meaning of wood, composition, properties, features and structure

• " onclick="window.open(this.href," win2 return false >Print
• Email

Details Category: Wood and Timber

Structure of wood and timber

Parts of a growing tree.

The tree consists of crown, trunk and roots . Each of these parts performs specific functions and has different industrial applications (see figure).

There are two concepts: “ tree" and "d wood».
Tree is a perennial plant, A wood - plant tissue, consisting of cells with lignified walls, conducting water and salts dissolved in it.

Wood is used as a consumable

traction material for the manufacture of various products.

Wood as a natural structural material is obtained from tree trunks by sawing them into pieces.

Trunk The tree has a thicker part at the base and a thinner part at the top. The surface of the trunk is covered bark . The bark is like clothing for a tree and consists of outer cork layer and inner bast layer(see picture).

Cork layer bark is dead. Bast layer serves as a conductor of juices that nourish the tree. The main interior of a tree trunk is made up of wood. In its turn, trunk wood consists of many layers, which are visible in the section as tree rings . The age of the tree is determined by the number of growth rings. 2 rings - dark and light make up 1 year of tree life. To find out the age of a tree, you need to count all the rings (dark and light), divide this number by 2 and add another 3 or 4 years (the annual rings of which have not yet formed and are visible only under a microscope.

The loose and soft center of the tree is called core and in cross section it looks like a dark spot with a diameter of 2-5 mm and consists of loose tissue that quickly rots. This circumstance made it possible to classify it as a wood defect.

From the core to the bark in the form of light shiny lines extend medullary rays . They have different colors and serve to conduct water, air and nutrients inside the tree. The medullary rays create pattern (texture) wood

Cambium - a thin layer of living cells located between the bark and wood. Only from cambium new cells are formed and the tree grows in thickness every year. « Cambium»- from Latin “exchange” (of nutrients).

To study the structure of wood there are three main sections ox (see figure).

Cut 2 , passing perpendicular to the core of the trunk is called end . It is perpendicular to the growth rings and fibers.

Section 3 passing through the core of the trunk is called radial . It is parallel to the annual layers and fibers.

Tangential cut 1 runs parallel to the core of the trunk and is removed from it at some distance. These cuts reveal various properties and patterns of wood.

All boards received at sawmill , have tangential cuts, with the exception of two boards cut from the middle of the log, so in practice tangential cuts are sometimes called board cuts. A very important cut when determining wood is the end cut. It shows all the main parts of the tree trunk at once: pith, wood and bark. To determine the type of wood in practice, it is enough to study macrostructure a small piece of wood that is sawn from a board into a block or ridge. Focusing on the growth rings, tangential and radial sections are made. All sections are carefully sanded first with coarse-grained and then with fine-grained sandpaper. It is also necessary to have a magnifying glass with fifty-fold magnification, a jar of clean water and a brush on hand.

In the middle of the trunk of many trees is clearly visible core . It consists of loose tissues formed in the first years of a tree's life. The core penetrates the tree trunk to the very top, every branch. In deciduous trees, the core diameter is often larger than in coniferous trees. The elderberry has a very large core. By removing the core, you can quite easily get a wooden tube. From time immemorial, such pipes were used by folk musicians to make various wind instruments: zhaleks, pipes and pipes. Most trees have a round core at the end cut, but there are species with a different core shape. The alder core at the end resembles the shape of a triangle, ash - a square, poplar - a pentagon, and the oak core resembles a five-pointed star. At the end around the core there are concentric rings located annual, or annual, layers wood On a radial section, the annual layers are visible in the form of parallel stripes, and on a tangential section - in the form of winding lines.

Every year the tree puts on a new layer of wood like a shirt, and due to this the trunk and branches become thicker. Between the wood and the bark there is a thin layer of living cells called cambium . Most of the cells go to the construction of a new annual layer of wood and a very small part - to the formation of bark. Bark consists of two layers - cork and bast. The cork layer located on the outside protects the wood of the trunk from severe frosts, hot sun rays and mechanical damage. The bast layer of bark conducts water with organic substances produced in the leaves down the trunk. Downward sap flow occurs in the oak fibers. Tree bark is very diverse in color (white, gray, brown, green, black, red) and texture (smooth, lamellar, fissured, etc.) Its uses are varied. Willow and oak bark contains a lot tannins, used in medicine, as well as in dyeing and leather dressing. Stoppers for dishes are cut from the bark of the cork oak tree, and the waste is used as filler for marine lifebelts. The well-developed bast layer of linden is used for weaving various household items.

In spring and early summer, when there is a lot of moisture in the soil, the wood of the annual layer grows very quickly, but closer to autumn its growth slows down and, finally, in winter it stops completely. This is reflected in the appearance and mechanical properties of the wood of the annual layer: that grown in early spring is usually lighter and looser, and in late autumn it is dark and dense. If the weather is favorable, a wide growth ring grows, but in harsh cold summers such narrow rings are formed that they can sometimes be barely distinguished by the naked eye. In some trees, the growth rings are clearly visible, while in others they are barely noticeable. But, as a rule, young trees have wider annual rings than older ones. Even the same tree trunk in different areas has different widths of growth rings. In the butt part of the tree, the annual layers are narrower than in the middle or apical part. The width of the annual layers depends on the location of the tree. For example, the annual layers of pine growing in the northern regions are narrower than the annual layers of southern pine. Not only the appearance of the wood, but also its mechanical properties depend on the width of the growth rings. The best coniferous wood is considered to be the one with narrower growth layers. Pine with narrow annual layers and brownish-red wood is called by masters ore and is highly valued. Pine wood with wide annual layers is called myandova. Its strength is much lower than that of ore.

The opposite phenomenon is observed in the wood of trees such as oak and ash. Their wood is more durable and has wide annual layers. And in trees such as linden, aspen, birch, maple and others, the width of the growth rings does not affect the mechanical properties of their wood.

In many trees, the annual layers at the end are more or less regular circles, but there are species in which the annual layers form wavy closed lines at the end. Such species include juniper: the waviness of the annual rings is a pattern for it. There are trees whose annual layers have become wavy due to abnormal growth conditions. The waviness of the annual layers in the butt part of maple and elm increases the decorativeness of the wood texture.

If you carefully examine the end section of deciduous trees, you can distinguish countless light or dark dots - these are vessels. In oak, ash and elm, large vessels are located in the area of ​​early wood in two or three rows, forming clearly visible dark rings in each annual layer. Therefore, these trees are usually called ring-vascular . As a rule, ring-vascular trees have heavy and durable wood. Birch, aspen and linden have very small vessels, barely visible to the naked eye. Within the annual layer, the vessels are evenly distributed. Such breeds are called disseminated vascular . In ring-vascular species, the wood is of medium hardness and hard, while in diffuse-vascular species it can be different. For example, in maple, apple and birch it is hard, while in linden, aspen and alder it is soft.

Water with mineral salts is supplied from the root through the vessels up to the buds and leaves, and upward sap flow. By cutting wood vessels in early spring, harvesters collect birch sap - pasoku. In this way, sugar maple sap is prepared, which is used to produce sugar. There are trees with bitter sap, such as aspen.

Simultaneously with the growth of a new annual layer inside the trunk, the earlier annual layers located closer to the core gradually die off. In some trees, the dead wood inside the trunk turns a different color, usually darker than the rest of the wood. The dead wood inside the trunk is called core , and the rocks in which it is formed are sound . The layer of living wood located around the core is called sapwood . Sapwood is more moisture-rich and less durable than seasoned heartwood. The core wood cracks less and is more resistant to attack by various fungi. Therefore, heartwood has always been valued more than sapwood. Sapwood, saturated with moisture, cracks severely when it dries, tearing the core at the same time. When harvesting a small amount of wood, some craftsmen prefer to trim off a layer of sapwood from the ridge immediately before drying. Without sapwood, heartwood dries more evenly.

TO sound breeds relate: pine, cedar, larch, juniper, oak, ash, apple tree and others. In another group of trees, the wood in the central part of the trunk almost completely dies, but does not differ from the sapwood in color. This kind of wood is called ripe , and the breed mature wood . Mature wood contains less moisture than living wood, because upward sap flow occurs only in the layer of living wood. TO mature wood species relate spruce and aspen .

The third group includes trees whose wood in the center does not die and is no different from sapwood. The wood of the entire trunk consists entirely of sapwood living tissues through which upward sap flow occurs. Such tree species are called sapwood . TO sapwood breeds include birch, linden, maple, pear and others.

Perhaps you have noticed that in a birch woodpile you sometimes come across a log with a brown spot in the middle, very similar to the core? You now know that birch is a kernel-free species. Where did it get its core from? The fact is that this core is not real, but false. False core in carpentry spoils the appearance, its wood has reduced strength. It is not so difficult to distinguish a false nucleus from a real one. If for a real core the border between it and the sapwood goes strictly along the annual layer, then for a false one it can cross the annual layers. The false core itself sometimes acquires a wide variety of colors and bizarre shapes, reminiscent of either a star or the corolla of an exotic flower. False core is formed only in deciduous trees, such as birch, maple and alder , but conifers do not have it.

On end on the surface of a tree trunk, in some tree species, light shiny stripes are clearly visible, running fan-shaped from the core to the bark - this medullary rays . They conduct water horizontally in the trunk and also store nutrients. The core rays are denser than the surrounding wood, and after wetting with water they become clearly visible. On a radial section, the rays are visible in the form of shiny stripes, dashes and spots, on a tangential section - in the form of dashes and lentils. In all coniferous trees, as well as in deciduous trees - birch, aspen, pear and others - the core rays are so narrow that they are almost invisible to the naked eye. In oak and beech, on the contrary, the rays are wide and clearly visible in all sections. In alder and hazel (hazel), some of the rays seem wide, but if you look at one of them through a magnifying glass, it is not difficult to discover that it is not a wide ray at all, but a bunch of very long thin rays gathered together. Such rays are usually called false wide beams .

On the wood of birch, rowan, maple and alder you can often see brown spots scattered chaotically - these are the so-called core repetitions . These are overgrown insect passages. In longitudinal sections, the repeat cores are visible in the form of streaks and shapeless spots of brown or brown color, sharply different from the color of the surrounding wood.

If the end cut of coniferous wood is moistened with clean water, then some of them will have light spots located in the later part of the growth rings. This resin passages . On radial and tangential sections they are visible as light lines. Pine, spruce, larch and cedar have resin passages, but are absent in juniper and fir. In pine, resin ducts are large and numerous, in larch they are small, in cedar they are large but sparse.

You have probably noticed more than once on the trunks of coniferous trees that have damage, influxes of transparent resin - resin . Sap - valuable raw materials that find various uses in industry and in everyday life. To collect resin, harvesters deliberately cut the resin ducts of coniferous trees.

The wood of some widespread deciduous trees of the middle zone lacks the brightness of color and striking texture pattern that is found in exotic trees imported from southern countries. It matches the Central Russian nature - its colors are muted, its texture pattern is simple and restrained. But the more you look at the wood of our trees, the more subtle color shades you begin to discern in it.

At a quick glance at the wood of birch, aspen and linden, it may seem that all these trees have the same white wood. But, looking closely, it is not difficult to discover that birch wood has a slight pinkish tint, aspen wood has a yellowish-green tint, and linden wood has a yellowish-orange tint. And of course, not only for its excellent mechanical properties, linden became a favorite and traditional material among Russian carvers. The warm and soft color of its wood gives figurines and other carvings an extraordinary liveliness. Most conifers texture pattern expressed very clearly. This is explained by the contrasting color of the late and early parts of the wood in each annual layer. Thanks to large vessels located along the annual layers and clearly visible to the naked eye, deciduous trees - oak and ash - have a beautiful textured pattern.

Each tree species has its own smell . Some have a strong and persistent odor, while others have a weak, barely perceptible odor. In pine and some other woody plants, the smell of the heartwood is very persistent and can last for many years. Oak, cherry and cedar wood have very persistent and unique odors.

In the trees of the middle zone, linden, aspen, alder, willow, spruce, pine, cedar and others have soft, pliable wood. Hard wood from birch, oak, ash, maple, larch; such as boxwood, pistachio, zelkova and dogwood grow only in the southern regions of the Caucasus and Europe.

The harder the wood, the faster cutting tools become dull and break. If a carpenter cuts a building made of larch, then he has to sharpen the ax much more often than when working with spruce or pine, and sharpen the saw more often. When working with hard wood, the woodcarver faces the same difficulties. When sharpening tools, he takes into account the hardness of the wood and makes the sharpening angle less sharp. Working with hard wood takes more time than working with soft wood. But craftsmen have always been attracted by the opportunity to apply the finest cuts on hard wood, its beautiful deep color and increased strength. Folk craftsmen knew this well. Where special strength was required, individual parts were made of hard wood. During the haymaking season, a peasant cannot do without a wooden rake. The rake should be light, so the handle for them was made from pine, spruce or willow flyer. Strength was required from the block and teeth. They used mainly birch, pear and apple wood.

Look at old porch steps, floorboards, or railroad bridge decks that are littered with knots. It seems that the knots have come out of the boards. But this is not so: the knots remained in place, but the wood surrounding them was worn away. The knots owe such resistance to abrasion not only to their resinous content, but also to their special position in the board. After all, each knot faces outwards. And from the end, as is known, wood has increased strength and less abrasion. Therefore, since ancient times, road workers have laid out particularly strong wooden pavements using end blocks.

Wood has a property that other natural materials do not have. This cleavability , or fissionability . When splitting, wood is not cut, but splits along the grain. Therefore, you can even split a log with a wooden wedge. Straight-grained elastic coniferous wood of pine, cedar and larch splits well. Among deciduous trees, oak, aspen and linden are easily split. Oak splits well only in the radial direction. Spliability depends on the condition of the wood. Slightly moistened or freshly cut wood splits better than dry wood. But wood that is too moisturized, wet, splits with difficulty, as it becomes too viscous. If you have ever cut firewood, then you have probably noticed how easily and quickly frozen wood splits.

The splitting properties of wood are of practical importance. Splitting wood produces blanks for matches, rivets for cooper's utensils, in the wagon business - blanks for spokes and rims, in construction - roofing chips, shingles and plaster shingles. Peasant craftsmen wove baskets for mushrooms and laundry from thin strips of split pine, and in between they made funny figurines of deer and skates for children from wood chips.

If you bend a splinter of dry wood into an arc and then release it, it will instantly straighten. Wood is an elastic material. But her elasticity largely depends on the type of wood, structure and humidity. Heavy and dense wood with high hardness is always more elastic than light and soft wood. When choosing a branch for a fishing rod, you try to choose one that is not only straight, thin and long, but also elastic. It is unlikely that there will be an angler who would like to make a fishing rod from a branch of brittle elderberry or buckthorn, and not from a flexible and elastic branch of rowan or hazel. The American Indians preferred to make fishing rods from elastic cedar branches. It is difficult to imagine the history of mankind without an ancient weapon - the bow. But the invention of the bow would have been impossible if wood lacked elasticity. The bow required very strong and resilient wood, and was most often made from ash and oak.

Thanks to the same elasticity, wood is used where it is necessary to soften recoil. For this purpose, a massive wooden block was placed under the anvil, and the hammer handle was made from wood. More than one century has passed since the invention of firearms. Flintlock guns and rifles are a thing of the past, the weapon has become perfect, but the butt and some other parts are still made of wood. Where can you find a material that would so reliably dampen recoil when fired? It has long been noted that straight-grained wood is more elastic than twisted wood. Even the wood of the same tree has different elasticity in different parts. For example, mature core wood, located closer to the pith, is more resilient than young wood, located closer to the bark. But if the wood is wetted or steamed, its elasticity will sharply decrease. The bent strip of wood retains its shape after drying.

The wetter the tree, the taller it is plastic and lower elasticity. Plastic the opposite of elasticity. Plasticity is of great importance in the production of bent and wicker furniture, sports equipment, basket weaving, wagon and cooperage. Elm, ash, oak, maple, bird cherry, rowan, linden, willow, aspen and birch acquire high plasticity after boiling in water or steaming. For the production of bent furniture, blanks are used from maple, ash, elm and oak, and wicker - from willow and hazel. Harness arches are bent from birch, elm, bird cherry, maple and rowan. The arches made from these trees are very strong, but if you need them to be lighter, willow and aspen are used. Coniferous wood has low ductility, so it is almost never used for bent or wicker products. The exception is pine, thin chips of which are used for weaving boxes and baskets, as well as pine, spruce, cedar and larch roots, which are used for weaving root baskets.

Wood saturated with moisture swells, increasing in volume. In many wood products swelling - a negative phenomenon. For example, a swollen desk drawer is almost impossible to push or pull out. It is difficult to close the sashes of an open window after rain. To prevent wood from swelling, wooden products are most often coated with a protective layer of paint or varnish. Craftsmen constantly struggle with swelling of wood. But for cooper's utensils this property turned out to be positive. Indeed, when the rivets - the planks from which cooperage utensils are assembled - swell, the gaps between them disappear - the utensils become waterproof.

Previously, when ships came in for repairs in winter, their wooden lining was traditionally caulked flax or hemp oakum. First of all, a lot of valuable raw materials were wasted, and in severe frosts, tow became fragile and it was very difficult to work with it. This is where the so-called wood wool - very thin shavings - came to the rescue. Wood wool does not mind frost; it easily fills all the cracks in the sheathing. And when the ship is launched into the water, the wood wool swells and tightly clogs the smallest cracks in the hull.

Wood species are determined by their following characteristic features: texture, smell, hardness, color .

Trees with foliage are called deciduous , and those having needles - coniferous .

Deciduous breeds are birch, aspen, oak, alder, linden and etc., coniferous species - pine, spruce, cedar, fir, larch etc. A tree is called larch because, like deciduous trees, it sheds its needles in the winter.

Wood is a complex composite material created by nature. When considering the structure of wood, it is customary to distinguish between the macrostructure, visible to the naked eye, and the microstructure, visible using optical and electron microscopy.

Macrostructure of wood - the structure of wood visible to the naked eye. Three main sections of the trunk are considered: transverse - end and two longitudinal - radial, passing through the axis of the trunk, and tangential, passing tangentially to the annual rings (Fig. 3.1).

A cross-section of the wood of the trunk shows concentric annual rings located around the core. Each annual ring has two layers: early (spring) and late (summer) wood. The early wood is light in color and consists of large thin-walled cells.

Rice. 3.1. Tree trunk structure:

Late wood is darker in color and consists of small cells with thick walls; therefore, it is less porous and has greater strength than spring.

As the tree grows, the cell walls of the wood of the inner part of the trunk, adjacent to the core, gradually change their composition, become woody and are impregnated with resin in coniferous species, and with tannins in deciduous species. The movement of moisture in the wood of this part of the trunk stops, and it becomes stronger, harder and less susceptible to rotting. This part of the trunk is called core or mature wood in different species.

Microstructure of wood. Studying the structure of wood under a microscope, you can see that the bulk of the wood consists of mechanical tissue cells, spindle-shaped and elongated along the trunk.

Felled wood consists of dead cells, i.e., only cell membranes (Fig. 3.2). Cell walls are composed of several layers of very thin fibers called microfibrils, which are compactly arranged and directed in a spiral in each layer at a different angle to the cell axis (like individual strands in a rope). This provides high strength to the wood.

Chemical composition of wood. Microfibrils consist of long, chain-like cellulose macromolecules (from the Latin cellula - cell). These chains are built from a large number (several hundred) glucose units (therefore cellulose can be called a polysaccharide):

Cellulose macromolecules, due to the presence of highly polar -on groups, are rigidly bound to each other, which explains the absence of a region of highly elastic state in wood, which occurs when heated in most linear polymers (for example, polyethylene). These same hydroxyl groups explain the hygroscopicity of wood and the accompanying swelling and shrinkage (see section 3.4). The mechanism of hygroscopicity consists in the formation of an electrostatic bond between polar groups of cellulose and water dipoles: depending on their type, location, size, as well as on the purpose of wood products. The same defect in some types of products makes wood unsuitable, while in others it lowers its grade or is not of significant importance. Therefore, the standards for specific types of forest products contain instructions on acceptable defects.

Wood defects can be divided into several groups: defects in the shape of the trunk, defects in the structure of the wood, knots, cracks, chemical stains and fungal infections and warping. The main types of defects are discussed below.

Defects in the shape of the trunk are easily determined on a growing tree, so the trunks of such trees can be rejected at the cutting site. This group of defects includes camber, rootiness and curvature of the trunk (Fig. 3.3).

Tapering is a significant reduction in diameter along the length of the trunk. A normal runoff is considered to be a decrease in diameter by 1 cm per 1 m of trunk length. This defect reduces the yield of edged lumber. In addition, the material contains a lot of cut fibers, which reduces its strength.

Buttiness is a sharp increase in the diameter of the butt (lower) part of the trunk. The backbone can be round or ribbed. In any case, it increases the amount of waste and artificially causes cross-cutting in the finished product.

Trunk curvature is the curvature of a tree trunk in one or more places. Strong curvature makes the wood unsuitable for construction purposes.

Defects in the structure of wood are deviations from the normal arrangement of fibers in the tree trunk: inclination of fibers, twisting, tilting, double core, etc. (Fig. 3.4).

Rice. 3.3. Trunk shape defects:

The inclination of the fibers (oblique) is the non-parallelism of the wood fibers to the longitudinal axis of the lumber. This phenomenon (especially at large angles of inclination of the fibers) causes a sharp decrease in the strength of the wood and makes it difficult to process.

Rice. 3.4. Wood structural defects:
a - fiber inclination; b - curliness; c - heel; g - double core

Cross-grained lumber has an increased tendency to warp when humidity changes.

Curling is an extreme manifestation of cross-layering, when the wood fibers are arranged in the form of waves or curls.

Curling in some species (walnut, Karelian birch) gives a beautiful texture to the wood; Such rocks are used in finishing work.

Klin is a change in the structure of wood when the annual rings have different thicknesses and densities on different sides of the core. The heel disrupts the uniformity of the wood.

Knots are the most common and inevitable wood defect, representing the base of branches embedded in the wood. They disrupt the homogeneity of the wood structure and cause fibers to bend (kink). Knots reduce the working cross-section of lumber, reducing their strength by 1.5...2 times (and more in thin boards and bars).

Based on the degree of fusion of knots with the wood of the trunk, knots are distinguished between fused, partially fused, and unfused (falling out) knots. Branched (fingered) knots are especially dangerous (Fig. 3.5).

Rice. 3.5. Different types of knots: a - fused healthy; 6 - drop-down; c - stitched; g - branched (palmated)

Healthy knots have hard and dense wood with no signs of rot. Often the knots rot until they turn into a loose powdery mass - these are the so-called tobacco knots.

For the manufacture of load-bearing wooden structures, wood that has only healthy fused knots is used. The number and placement of knots determine the grade of the material.

Cracks can appear both on a growing tree and when felled wood and lumber dry out. They violate the integrity of timber, reduce the yield of high-grade products, reduce strength and even make them unsuitable for construction purposes. In addition, cracks contribute to wood rotting.

The following types of cracks are distinguished: metic, frost and crack, which form on a growing tree, and shrinkage cracks, which form on felled wood (Fig. 3.6).

Metik - internal cracks running along the trunk from the center to the periphery; There can be several cracks, either located in the same plane or crosswise.

Rice. 3.6. Types of cracks: a, b - metic simple and complex; c, d - frost hole open and closed; d, e - ring and partial peel

A frost hole is an external open longitudinal crack, tapering towards the center. Such cracks occur when moisture in the trunk freezes during severe frosts.

Shooting is a complete or partial separation of the central part of the trunk from the peripheral part as a result of shrinkage of the former. Such cracks are located along the annual rings.

Shrinkage cracks are very common in wood of all species; they arise as a result of stresses caused by uneven shrinkage when wood is rapidly dried in air. These cracks are directed from the periphery to the center along the wood fibers.

Fungal lesions and chemical stains are caused by the simplest living organisms - fungi that develop from spores and use wood as a nutrient medium, or microorganisms. For the development of mushrooms, air oxygen, certain humidity and positive temperature are required. There are fungi that infect trees growing in the forest, freshly cut wood, and fungi that develop on wooden structures.

Wood-staining fungi can develop on growing trees. They feed on the contents of cells without affecting their walls. Therefore, the strength of such wood changes slightly, but colored spots and stripes appear on the wood.

A change in the color of wood without changing its mechanical properties can occur due to the biochemical oxidation of tannins provoked by microorganisms.

Wood-decaying fungi are much more dangerous. They feed on the material of cell walls - cellulose, decomposing it with the help of enzymes to glucose.

This is only possible if the wood has sufficient moisture. Glucose in the body of the mushroom is used in the process of its life and, ultimately, turns into carbon dioxide and water:

Rotting is essentially the same as burning, but at a very low speed.

A large number of wood-decaying fungi are known. Among them, the most common are the so-called house mushrooms. When affected by such fungi, the wood becomes rotten and light, and a coating of mold appears on its surface in the form of soft pads. House fungus can destroy wood very quickly (within a few months).

The rotting process stops when the wood moisture content decreases to 18...20% (dry wood does not rot), the temperature drops below 0 ° C or the supply of oxygen is excluded.

Insect damage (wormholes) are passages and holes made in wood by insects (bark beetles, borers) that live in it and feed on it. Borer beetles can develop in dry wood and even furniture.

Rice. 3.7. Longitudinal warping

Surface wormholes do not affect the mechanical properties of wood, since they go into the slab when sawing. Deep wormholes damage the integrity of the wood and reduce its strength.

Warping is a violation of the shape of lumber when its humidity changes during drying and storage or under the influence of internal stresses when longitudinally sawing large elements into smaller ones. Warping can be transverse, longitudinal (simple and complex) and helical (winged) (Fig. 3.7).

None of the building materials available on the market today have such unique qualities as natural wood. It is very convenient and easy to process, so that you can make anything, even a spoon, even an airplane fuselage. The wood has excellent strength, it is light and has a pleasant smell. Working with wood is a real pleasure if you understand the types of wood and the characteristics of lumber.

Types of wood cuts

If you look closely at any block of wood, you can see on it a textured pattern formed by growth rings. Its appearance depends on the direction in which the tree trunk was cut. It is customary to saw it in three directions: along and across the grain, as well as at an angle of 45 degrees. If the cut is made at an angle, then it is called tangential. It forms a texture similar to cone-shaped lines.

If the cut is made along the fibers, then it is called radial. The parallel lines formed by the fibers are clearly visible on it. A cross section shows us the annual rings of a tree trunk in all its glory. The pattern is important for the external beauty of wood products, therefore, before making a wooden blank, you need to clearly imagine yourself in which direction we want the patterns.

Internal structure of wood

In order to understand the structure of a tree trunk, it is necessary to make a complete cross-section. The top layer is called the bark. It is of no interest, so it is removed. The next thin layer is the so-called growth zone. It is difficult to see, but if the tree is young, then after removing the bark you can see green fibers that are damp to the touch. They are also called cambium. After it, the wood itself begins with pronounced annual rings. Professionals call it sapwood. In the center of the trunk there is a darker core or one that merges with the sapwood. It depends on the type of wood, which can be sapwood or heartwood.

Sound tree species are represented by all conifers (cedar, pine, spruce, larch, yew) and some common deciduous species such as oak, poplar, ash. The vast majority of deciduous trees are sapwood: birch, alder, hornbeam, maple.

The density of wood cells affects the strength and other physical qualities of wood, but the creation of artistic compositions and the possibility of using this or that raw material in work is influenced by the pattern of growth rings and heart-shaped vessels. These are macrostructural elements, and they also include knots, growths, undeveloped shoots that deflect growth rings and form various curls.

Wood with a pronounced macrostructure is the most interesting for processing, therefore, without exception, all conifers are used for crafts

Physical characteristics of wood

Like any building material, wood has a number of physical properties:

• Density is measured in g/cm 3 and depends on the type of wood and its moisture content. The higher this indicator, the stronger and heavier the material, it is more durable and less susceptible to rotting. The most dense wood is considered to be oak, ash, maple and larch, and the least dense is aspen, spruce and fir.
• The moisture content of wood indicates the degree of its quality and durability. Room-dry has 8 - 12% moisture, air-dry from 12 to 18%, and atmospheric-dry 18 - 23%. If the humidity is even higher, then such wood is called damp.
• Sound conductivity and thermal conductivity are important qualities. High-quality dry wood perfectly retains heat and sound in the transverse direction. Thermal conductivity along the fibers is reduced, but sound travels perfectly along the trunk. This is even an indicator of quality and dryness.
• Resistance to corrosion, which is higher in coniferous wood, due to the presence of resin in it.
• Texture, color, smell and shine allow us to determine the type of wood and determine its decorative value.

All of the listed physical properties are very important for the use of a particular tree species.

Mechanical characteristics of wood

The mechanical properties of different types of wood are more important. After all, they influence the strength and durability of buildings or wood products. Mechanical strength is the ability to resist various static and dynamic influences from the outside. The strength of a material depends on the direction of the load. In this regard, it is customary to distinguish between shear or shear strength, bending strength and compression strength. Any wood has greater strength along the grain than across it.

It is worth noting that damp wood becomes less durable. The same is observed in light and loose breeds.

Plasticity is the property that allows you to create bent parts from wood. More plastic rocks retain the shape obtained under a certain long-term exposure. Humidity and temperature greatly increase this indicator, so to make curved parts, wood is exposed to hot water or steam. Beech, elm, oak, and ash boast high plasticity. This cannot be said about conifers, since the structure of their fibers is too linear.

The hardness of wood is the ability to resist various penetrations of foreign bodies into it. There are hard tree species, such as: beech, maple, larch, oak, ash, elm (the hardest are boxwood and acacia) and soft ones, such as: linden, alder, spruce, pine. The level of wear resistance of wood directly depends on its hardness.

Characteristics of various types of wood

One type of wood or another is used for different purposes. All of them are divided into coniferous and deciduous. The former have a sharp resinous odor and a pronounced macrostructure. The most common coniferous species are: cedar, pine, fir, spruce and larch.

• Pine is the most common building material. Its color varies from pale yellow to reddish yellow. The wood is quite light and durable. The main thing is that it is very convenient for processing. It contains a lot of resin, so it rots poorly and is not particularly afraid of precipitation. Because of its softness, it easily accepts various dyes and varnishes. Warping during drying almost does not occur in pine. The disadvantage is the impossibility of high-quality finishing and painting. However, it is successfully used for the production of furniture and plywood.
• Spruce can be placed in second place after pine in terms of use. There is not much resin in it, so it is more susceptible to rotting and exposure to precipitation. Spruce wood is strong and light, but at the same time it has a large number of knots, which significantly reduces its consumer qualities. The advantages include the white color of the wood and low resin content. It holds various fasteners well. In construction, not the most important parts are made from it.
• Cedar, or correctly Siberian pine, is in no way inferior to spruce in its construction qualities, and is far superior to it in resistance to rotting. Despite the softness of cedar wood, it has good density and strength, and is easy to process.
• Fir is no different from spruce: it can be easily processed and does not tolerate caustic chemicals. It contains quite a bit of resin, which is why the wood rots too quickly without the use of special treatment.
• Larch is valued for its hardness and strength. Its density is such that the trunk of this tree sinks in water. But larch wood practically does not rot.

Hardwoods are usually divided into soft and hard. Their wood is odorless. It is only found on fresh saw cuts. Hardwood species include oak, ash and birch, and softwood trees include aspen and alder.

• Oak has very high strength and resistance to rotting. Its wood has a beautiful color and texture. It does not crack or warp, which is why furniture, luxury items and art are made from oak. Tannins have powerful antiseptic properties. The most durable and beautiful oak wood is obtained when it is kept in running cold water for 1.5 years. Her color turns black. Expensive furniture is made from such stained wood. This is an ideal material for furniture production, but it is very inconvenient to process due to its density and strength.
• Birch wood has medium density and hardness. It is strong and quite viscous, does not have a very pronounced texture, but is homogeneous. The disadvantages of this material are susceptibility to severe cracking and warping, too much shrinkage, low resistance to rotting, and quite frequent damage to diseases such as wormholes. However, it can be easily processed with hand tools, glued into plywood, easily polished and painted, and makes it possible to produce very fine relief carvings.
• Aspen has a fairly soft wood, with so few knots that it lends itself well to any processing. However, its porous structure does not allow making small parts.
• Linden is highly valued in the manufacture of various carved parts for furniture production. It does not warp and does not crack at all when dried. Linden wood has a fairly strong structure that is very resistant to rotting.
• Maple has a strong, dense and low-drying wood. It hardly warps, but rots quickly and is highly susceptible to wormholes. This wood is well processed, glued, finished and painted. It is used in carving work and the manufacture of solid wood parts.
• The mahogany tree, which grows in evergreen tropical forests, has red-colored wood. This is not just one species, but many with similar properties. Mahogany wood is very soft and lends itself well to processing, is easy to polish, and also absorbs varnish. Some pieces of furniture are made from such wood. Its high cost does not allow making the entire product out of it.

Lumber and its varieties

Mostly dry wood is sold at the lumberyard and in the store. Raw is rarely sold. If you decide to build something or make a piece of furniture, then you will come across the names of types of lumber, the meaning of which is worth understanding:

• The ridge is essentially solid tree trunks without bark or pieces of them of sufficiently long length. It is important that their diameter exceeds 25 cm.
• Podvyaznik is the same ridge, but with a diameter of less than 25 cm.
• A pole is a solid trunk without bark with a diameter of less than 9 cm.
• A plate is half a ridge that is sawn along the grain.
• Quarter - half of a plate sawn along the grain.
• A log, or in other words a beam with a wane, is a log hewn on both sides that can be placed on one of two planes.
• I call a beam a log that is hewn on four sides with a cross-section of at least 100x100 mm. If it is smaller, then the product is called a bar.
• The board can be very different depending on the method of its processing and size: unedged, edged, slab, planed on four sides, tongue-and-groove, folded.

All lumber that is commonly used in construction has its own specific name. They differ in the thickness of the product, as well as the ratio of width to this thickness. For boards, this ratio should never be more than 2. The maximum permissible board thickness is 100 mm. The length of any hardwood material does not exceed 5 m, and that of coniferous trees does not exceed 6.5 m.

After reading this article, you have become familiar with the basic concepts and characteristics of wood. Therefore, with such knowledge, you can safely purchase lumber, using terms no worse than sellers. Today, almost no major construction or renovation can be done without the use of wood to one degree or another, so such knowledge will be very useful.

The content of the article

WOOD, a relatively hard and durable fibrous material, the main part of the trunks, branches and roots of trees and shrubs hidden by bark. It consists of countless tube-shaped cells with shells mainly of cellulose, firmly cemented with calcium and magnesium pectates into an almost homogeneous mass. In its natural form it is used as a building material and fuel, and in crushed and chemically processed form - as a raw material for the production of paper, fiber boards, and artificial fiber. Wood was one of the main factors in the development of civilization and even today remains one of the most important types of raw materials for humans, without which many industries could not do.

Sources.

Although ferns also have woody tissue, people obtain almost all their wood from the trees of the two main divisions of the kingdom of higher plants - gymnosperms and angiosperms. Gymnosperms are a very ancient form, represented exclusively by woody species, which include conifers (“softwoods”), namely pine, spruce, cedar, which supply the bulk of the wood used by mankind. The angiosperm department is distinguished by great diversity and is divided into two classes - monocotyledons and dicotyledons. Only a few monocots (bamboo, palms, yucca) produce woody tissue, which is of limited, mainly local, importance. As for dicotyledons, this class includes important deciduous (“hard”) species - oak, eucalyptus, maple, the wood of which is especially valuable for furniture, interior decoration, etc.

Structure.

Wood cells, like bark cells, arise from repeatedly dividing procambium and cambium cells, which form an almost continuous layer of educational tissue between the bark and wood. The cambium arises from cells separated from the growth cone of the stem or root. The latter originates in the cellular educational center of the embryo in the seed. There are two classes of cells in wood - parenchymal and prosenchymal. Parenchyma cells are usually thin-walled with simple (not bordered) pores. In sapwood they perform the function of physiologically active living tissue (provide storage of nutrients). Prosenchymal cells are thick-walled with bordered pores. They lose their protoplast as they grow and reach their final wall thickness, whereupon they become a fluid-conducting and support-providing medium.

Wood is characterized by annual rings, caused by changes in cell size and cell wall thickness due to changes in growth conditions. In temperate climate zones, the contrast of rings is associated with the difference between the “summer” wood of one year and the “spring” wood of the next. The age of the tree can be determined by the number of rings at ground level.

Chemical composition.

Wood contains a number of complex organic compounds. Full chemical analysis shows that it contains about 50% carbon, 6% hydrogen and 44% oxygen. The cell wall has a network structure of interconnected long-chain cellulose molecules, filled with other hydrocarbons (hemicelluloses), as well as lignin and various extractives. The cementing intercellular substance is mainly calcium and magnesium pectates, and resins, gums, fats, tannins, pigments and minerals accumulate in the cellular cavities, especially in deciduous wood. The composition of wood includes 45–60% cellulose, 15–35% lignin and 15–25% hemicelluloses. The amount of foreign, extractive substances largely depends on the species and is not the same in sapwood and heartwood. The mineral content (ash content) of wood is usually significantly less than 1%.

Physical properties.

The relative density of wood ranges from 0.1 (balsa) to ~1.3 (ironwood and some other tropical species). The relative density of most industrial wood is 0.2–0.75, density – 190–850 kg/m3. The relative density of wood matter is approximately 1.5. Consequently, only about 1/6 of the volume of light industrial wood is solid matter, while in heavier grades it accounts for about half the volume. The relative density can be different for one tree species, which is due to the variability of growing conditions. Thus, for long-leaf pine this value can range from 0.25 to 0.80 (average value 0.53).

Both standing wood and industrial wood strongly absorb water, which is due to its capillary structure. Free water fills the cellular cavities, and bound water is retained due to adsorption in the spaces between the fibers. When all free water has been removed during drying, so that the entire vascular system is filled with bound water, the wood reaches the point of fiber saturation, which for most species corresponds to a moisture content of about 28%. Further removal of water leads to shrinkage, since when adsorbed water is desorbed, the fibers shrink and the lumen of the vessels decreases.

Depending on the presence of moisture, wood shrinks or swells. Shrinkage from the saturation point of the fibers to the state after drying in an oven is maximum (4–14%) in the tangential direction (parallel to the growth rings), approximately half as much (2–8%) in the radial direction (across the growth rings) and is practically absent (0. 1–0.2%) along the fibers. Tangential, radial and volumetric shrinkage are approximately proportional to the change in the moisture content of the wood.

The mechanical properties of wood are closely related to its fiber-cellular structure. Its strength is maximum along and quite low across the fibers. The tensile strength (per unit mass) of wood when stretched along the grain is 40 times, and when compressed, 3–4 times greater than that of steel. The compressive strength along the fibers is approximately 6 times, and the shear strength is approximately 4 times greater than across the fibers. Since compressive and bending forces are typical in structures, wood is particularly suitable for use in building structures as columns and short beams. Almost all strength properties of wood vary proportionally to density and inversely proportional to moisture content below the saturation point of the fibers. The inclination of the fibers, i.e. deviation of their direction from the longitudinal axis reduces the strength of the wooden structural element. In the same way, it decreases if there are knots in boards and logs, embedded parts of branches that disrupt or completely interrupt the flow of fibers. However, in the absence of tensile and bending loads, small knots are acceptable. The strength of wood is also reduced due to damage by putrefactive microorganisms and insects.

Use of wood.

Application in construction.

Wood is used in construction in forms such as square lumber (timbers, planks), veneer, plywood, railroad ties, posts, piles, posts, shingles, and fiberboard. The most consumed lumber is rectangular. They are produced by sawing logs, then trimmed to standard widths and lengths, graded for quality, dried and supplied to consumers in raw, processed or molded form. Plywood is made by gluing an odd number of thin layers of wood (veneer) together so that the fibers of adjacent layers are mutually perpendicular. Plywood panels differ from conventional lumber in that (along with the absence of width restrictions) their strength is more uniform in different directions, they resist splitting better, and their dimensions change less under variable humidity conditions.

Fuel and wood pulp.

The use of wood as fuel throughout the world is still very important. In highly industrialized countries, fuel consumption of wood has continuously decreased over the past decades due to the transition to coal, gas, oil and electricity. This trend is likely to continue in the future as other types of fuel and heat sources become increasingly available with further developments in technology. The use of wood in the form of wood pulp, on the contrary, has been continuously increasing recently and is projected to continue to increase in the foreseeable future. Wood is converted into wood pulp by mechanical abrasion using water or by treatment with chemicals that break the lignin bond and release the fibers. The wood pulp is then converted into various types of paper, boxboard, and fiberboard. After special processing, it is used as cellulose raw material for the production of synthetic fabrics and plastics.

Technology improvements.

Thanks to new technological developments, wood has become more widely used in traditional areas and has found new applications. Such advances include improvements in drying technology, anti-rot and fire-retardant treatments, layered structures, prefabricated structures, and high-performance wood adhesives. Great strides have been made in the pulp and paper industry, as well as in the production of chemically processed wood-based materials such as synthetic fiber, cellophane, alcohol, yeast, fiberboard, polymer-impregnated wood, wood laminates and various molded products. Progress in the processing and use of wood has stimulated further development of forestry.

Man has been using wood since time immemorial. Fuel, building materials, furniture, musical instruments - products made from it accompany us throughout our lives. In addition, trees are natural calendars and living historical monuments.

There is a whole branch of science - dendrochronology, which allows you to find out the age of the product, as well as in what area the tree from which it was made was cut down. By studying sections of annual rings, you can learn about the nature and atmosphere of ancient times. Advantages and disadvantages, structure, wood as a building material, properties - all these issues deserve attention.

How everything works

The properties and characteristics of the material cannot be understood without first studying the structure and composition of wood. The concept itself depends on who uses it. For an ordinary person and a builder, this is exclusively the part of the tree under the bark that can be used in everyday life or production. For a botanist and wood, this is the whole complex, including all elements from roots to crown.

The crown is used insignificantly in industry, and the branches are used as raw material for cardboard. The trunk is of primary importance. A cross-section reveals the structure of a wood trunk. The topmost layer, the cortex, protects living cells from external influences. Between the bark and the body of the trunk there is a layer of living cells - cadmium. In the very center, the core runs through the entire trunk. The loose fabrics of which it is composed make it unsuitable for utilitarian needs.

The core of the tree consists mainly of dead cells, resin deposits, coloring and tannins. The core is surrounded by sapwood, the part of the tree that is responsible for conducting water to the leaves from the roots. Accordingly, it contains a lot of moisture, it allows more water to pass through and is more susceptible to rotting. Not all trees have a clearly defined core. In some of them there is no difference between the central and peripheral parts of the trunk. Such species are called sapwood.

Microscopic structure of wood

Using a microscope, you can study the structure more deeply. Wood consists largely of dead cells. Young plant cells consist of a shell and an internal part - cytoplasm and nucleus. The basis of the thin transparent membrane is cellulose or fiber. Over time, plant cells undergo metamorphosis and, depending on the intended function, turn into either bark (corking) or wood (lignification).

Lignin is constantly produced in cells. This is what causes lignification. There are two types of wood cells - prosenchymal and parenchymal. The first type makes up the bulk of the wood, depending on the species - from 85% to 99%. In turn, they are divided according to their functions. Conductive cells are responsible for the delivery of nutrients and moisture from the roots to the foliage, mechanical cells are responsible for the strength and stability of the tree. Parenchyma cells serve as a storeroom for the plant. They accumulate nutrients (fats, starches) and release them as needed during difficult periods.

Conifers

Depending on the type of trees, their structure also differs. Wood species are divided into coniferous and deciduous. The structure of coniferous trees is more simple. The main mass consists of tracheid cells. Features of conifers include the presence of cells that produce resin. In different species they can be either chaotically scattered or combined into a system of resin passages.

Hardwood

The rocks and their structure are more complex. Wood consists of vessels, libriform fibers and parenchyma cells. Since deciduous trees shed their leaves in the fall, they need a large supply of food in the winter. Hence the greater number of parenchyma cells responsible for the accumulation of nutrients than in coniferous species. This can be seen by the pronounced core.

Properties

Wood has a number of characteristic properties. The structural features are the reason for this. The strength of wood is quite high, and among building materials in this indicator it occupies an intermediate position. And given its low specific gravity, it is comparable in this regard to metal. The weak point of wood is that it is an anisotropic material. The ability to resist fracture depends on the direction of the force relative to the arrangement of the fibers. The best strength indicators are visible when the material is exposed along the fibers.

The rigidity of wood is low, the reason for this is its specific structure. Wood is a porous, flexible material. Beams are able to restore their shape after a short-term load. But residual deformations, due to prolonged exposure, remain forever. A wooden beam will not be able to regain its shape after long use.

The hardness of building materials is determined by the load required to press a steel ball of a certain size. For the hardest wood species it is only 1000 N. Moreover, low hardness is one of the main advantages of the material. Wood is easy to process and holds nails, screws, and self-tapping screws firmly.

Determined by the specific moisture content in the pores. In just now it reaches 100%. Depending on the purpose, freshly cut wood is dried to the required levels of 40 to 15%.

Advantages

Wood has a low thermal conductivity value. It can be successfully used as a heat-insulating material. Ease of processing allows the use of a wide range of tools. It is impossible to imagine any orchestra without musical instruments made of wood. The enchanting sounds of a violin are the result of such a property of wood as the ability to resonate. Wood bends easily, which opens up a large selection for the manufacture of various bent structures. Wooden products also have good sound-absorbing characteristics. A beautiful surface opens up space for imagination when designing rooms.

Flaws

The ability of wooden products to absorb loads depends on the direction of application of force. This is explained by the anisotropic structure of wood. In addition, the strength characteristics also depend on the proximity to the center of the trunk, humidity, the presence of knots and cracks. This forces you to spend a lot of time selecting suitable material for work.

Being an organic material, wood is defenseless against insects, mold, and fungi. For long-term operation, expensive chemical treatment is required. It is worth noting that wooden structures without pre-treatment are easy prey for fire.

Wood processing

In general, three types of wood processing can be distinguished:

• The most common is the mechanical method. The tree is sawed, planed, split.
• During chemical-mechanical processing, the material is subjected to intermediate preparation. mixed with a binder and heated. A chemical polymerization reaction occurs, and the output is materials such as plywood, particle boards, and fiberboard.
• During chemical treatment, wood is exposed to acids, alkalis, salts, and heat. Products of such processing include rosin, gum, tannins, and cellulose.

Trees are hundreds of millions of years older than humans. All civilizations that have ever existed are based on the use of wood. Books, furniture, musical instruments - all this is possible thanks to this unique natural material.

Return