Oak wood has always been associated with the concept of strength, power, and health. The oak tree itself is a majestic picture. Its wood is dense, hard, heavy and highly durable. It is also characterized by resistance to moisture, rotting and various fungi.

The wood is porous with a beautiful texture. The color is brown or yellowish brown. The sapwood part of oak wood has a light yellow color. Over time, the color of its wood darkens, which, however, gives it a more noble appearance.

Oak is a long-living tree; age of more than a century is not the limit for it. The height of the oak reaches 30 meters, and the diameter is from 1.2 to 1.8 m. Oaks growing in forests are characterized by the presence of a straight trunk without knots up to 15 meters in height.

Wood density: about 700 kg/m3. Hardness: 3.7 - 3.9 Brinell.

The influence of growing conditions on the properties of wood

If we compare the properties of wood from trees grown in different natural conditions, then you can notice significant differences. The worse the soil on which the oak grows, the better its wood. That is why oak wood from the northern regions is more valued.

Thus, oak, which grows in oak forests on sandy soils, has a thick, dark-colored bark, and its wood is painted a light straw color. The hardness of the wood of such oaks is high, but it lacks elasticity.

If an oak grows near water, for example, on the banks of a river or stream, or among alder swamps, then it is called lead, water, iron or als oak. It differs from its counterparts in its straight trunk and dense crown. The bark is leathery and spotted. Its color is light gray with a bluish tint. The wood has a pink tint, the layers are large. The elasticity is very good, but when dried it tends to crack. Unusually heavy.

Intermediate varieties of trees that grow in places located between oak forests and alder bogs have average elasticity values ​​in their qualities, and lower hardness values ​​than pine forests and alders. The bark of such oaks is thick, its color is brownish-gray. Often in the butt part of these trees there are hollows, and the apical part of the trunks is dry.

Where is oak wood used?

Summer oak wood is widely used in construction, and its moisture resistance properties allow it to be used in underwater structures or the hulls of wooden floating craft. It is also good for making souvenir crafts.

Winter wood is used in carpentry, furniture and parquet production. Oak firewood - no the best option, since coal cools quickly. And to maintain combustion you need good draft. And it’s a pity to use such valuable wood as fuel, unless waste from other industries can be used for firewood.

Features of working with oak wood

Oak wood should be dried under natural conditions. It is not recommended to try to speed up this process as this may result in cracking.

Stained oak wood takes on a dark purple hue

In order for the wood to acquire decorative look, staining is used - for oak this is done by keeping it in water for several years. After this exposure, the color of the wood becomes dark purple and silky. The hardness of long soaking only increases, although it becomes more fragile.

When working with oak wood, you should remember that it does not like alcohol varnishes, and it is useless to use polish due to its high porosity.

Oak wood does not like oils - they form unsightly stains on its surface. This wood does not need painting because it has a beautiful natural texture and color. To finish, it is enough to cover the surface of the product with a transparent varnish, preferably one that dries quickly.

For construction purposes, it is better to use wood with a large width of annual rings. This wood is highly resistant to wear. For making furniture, souvenir crafts, wooden sculptures and turned products, lighter and softer wood with narrow annual rings is better suited.

Quercus robur
Taxon: Beech family ( Fagaceae)
Other names: English oak, summer oak, English oak
English: Oak, English oak, Truffle Oak, Pedunculate Oak

Botanical description

A large, beautiful, powerful deciduous tree, reaching 40-50 m in height and 2 m in diameter, sometimes 1000 years old or more. During the warm season, oak evaporates more than 100 tons of water, 225 times its own weight. There are about 20 species of oak in our country. The most common of them is pedunculate oak. The root is powerful, widely branched; crown - well developed, spreading. The bark of young shoots is smooth, slightly pubescent, olive-brown, while that of old shoots is gray-brown, with cracks. The leaves are oblong, obovate, narrowed downward, pinnately lobed, alternate, simple, short-petiolate, glabrous, dark green, shiny with prominent veins. In spring, the oak blossoms late, one of the last among deciduous trees.
There are two known forms of common oak - early and late. Early oak leaves bloom in April and fall off for the winter, while late oak leaves bloom two to three weeks later and remain on young plants for the winter.
Oak blossoms in April - May, when it still has very small leaves. The flowers are unisexual, monoecious, very small and inconspicuous. Male or staminate flowers are collected in peculiar inflorescences - long and thin yellowish-greenish hanging earrings, reminiscent of hazel earrings. These earrings hang in whole bunches from the branches and are almost identical in color to young small leaves. The female or pistillate flowers of oak are sessile, very tiny - no larger than the head of a pin. Each of them looks like a barely noticeable greenish grain with a crimson-red tip. These flowers are located singly or in groups of 2-3 at the ends of special thin stems. Acorns grow from the female flowers in the fall. After flowering, first a small cup-shaped wrapper grows - a plus, and then the fruit itself - an acorn. Acorns ripen at the end of September - beginning of October. Acorns do not tolerate drying out; the loss of even a small part of the water leads to their death.

Spreading

Oak grows in the forest and steppe zone Europe. In ancient times, almost half of the forests in Europe were oak forests, but now oak forests account for about 3% of all forests in Europe. Often dominates the lineup mixed forests. On Far East, Crimea, and the Caucasus, other types of oak grow (downy oak, sessile oak).
Common oak is widespread in the middle and southern zones of the European part of Russia to the Urals. Oak does not tolerate cold and humid climates, while in the south it develops better.
Common oak forms frequent stands or grows in a mixture with other species throughout almost the entire territory of Ukraine (in the steppe - mainly along river valleys).
Oaks are divided into summer, winter and evergreen. Of the 3 types of oak growing in Ukraine, the most common and important for industry is common oak (pedunculate or summer oak) Quercus robur L.

Collection and preparation of medicinal oak raw materials

Oak bark, which is harvested, is mainly used as a medicinal raw material. in early spring, without cork and wood. To collect bark, only young trees cut down at logging sites and sanitary fellings can be used. Dry it under shelters in the open air or in well-ventilated areas. IN good weather can be dried in the sun. Dry bark breaks when bent, while under-dried bark bends. It is necessary to ensure that the bark does not get wet when drying, since this will cause it to lose a significant part of the tannins contained in it. According to the pharmacopoeia, for uncrushed oak bark raw materials, the numerical indicators should be: tannins no less than 8%, moisture no more than 15%, total ash no more than 8%; pieces of bark that have darkened with inside, no more than 5%, organic impurities no more than 1%, mineral impurities no more than 1%. The shelf life of raw materials is 5 years. Dry bark has no odor, but when infused in water and especially in hot water a characteristic smell characteristic of fresh bark appears. The taste is very astringent.

Biologically active substances of oak

First of all, oak raw materials are considered as a source of tannins. The bark contains 10-20% tannins; they are also included in the chemical composition of leaves and fruits (5-8%). Tannins are a mixture of structurally similar phenolic compounds. From this group, the composition of oak bark tannins includes both a group of condensed and a group of hydrolyzed tannins.
In addition to tannins, oak bark contains organic acids (gallic, eladic), carbohydrates, starch, pentosans (13-14%), flavonoids quarcetin, and protein substances. The bark also contains: microelements (mg/g): K - 1.40, Ca - 23.00, Mn - 0.60, Fe - 0.20; trace elements (µg/g): Mg - 142.60, Cu - 12.30, Zn - 10.20, Cr - 0.80, Al - 116.08, Ba - 537.12, V - 0.08, Se - 0.04, Ni - 1.84, Sr - 212.00, Pb - 3.04, B - 74.80. Ca, Ba, Se, Sr are concentrated.
The composition of oak fruits - acorns - includes starch, tannins and proteins, sugars, fatty oil (up to 5%). Thanks to this composition, acorns along with chicory are part of a mixture that is used as a coffee substitute and has fairly high nutritional properties.
Contains oak leaves in its chemical composition tannins, quercetin, quercitrin, pentosans.
The galls formed on oak leaves contain a large number of tannins.

Use of oak in medicine

Galenic preparations of oak bark have anti-inflammatory and antimicrobial properties. The tannins of the plant determine the main tanning effect. When oak galenic preparations are applied to a wound or mucous membrane, interaction with proteins is observed, and a protective film is formed that protects tissues from local irritation. This slows down the inflammation process and reduces pain. Tannins denature the protoplasmic proteins of pathogenic microorganisms, which leads to a delay in their development or death.
Today, data have been accumulated on the spectrum of resorptive effects of tannins, including antispasmodic, hypotensive, antiviral and a number of other effects.
The composition of tannins includes a mixture of polyphenols, which, when interacting with oxidative radicals, form semiquinoid radicals and radical ions, in the presence of which the intensity of peroxidation decreases, so the antioxidant activity of tannins can be noted.
Anti-carcinogenic and anti-radiation activity has been established for tannins.
Based on the method of use, oak bark preparations can be divided into two groups: external and internal use.
Externally oak preparations are used for:
diseases of the oral cavity (gingivitis, stomatitis, amphodontosis);
inflammation of the tonsils;
;
bleeding gums;
skin diseases (ulcers, eczema, bedsores);
washing purulent and decaying wounds;
treatment of burns.
Internal oak preparations are used for:
treatment of enteritis, colitis, dysentery, cholera;
complex therapy of stomach diseases;
bleeding of the gastrointestinal tract;
complex therapy of kidney diseases and Bladder;
poisoning with alkaloids and salts of heavy metals, as an antidote.

It is worth noting that the data on the toxicological properties of tannins characterize them as practically non-toxic compounds.
Oak bark is included in various collections of medicinal plants and as part of complex medicines.
Oak bark is included in the preparations:
Dragee "Tonsilgon N", manufactured by Bionorica AG, is used for acute chronic diseases of the upper respiratory tract (tonsillitis, pharyngitis, laryngitis), prevention of complications in respiratory viral infections and as an addition to antibiotic therapy for bacterial infections;
Gel "Vitaproct" used for the treatment of acute and chronic;
The drug "Polygemostat" used in surgical practice as a hemostatic drug.

Use of oak in other industries

Common oak is used as a source of wood and raw materials for the tanning industry, as a phytoncidal, food, melliferous, fodder, ornamental and phytomeliorative plant.
For the tanning industry, oak bark aged 15-20 years is considered the best. Since bark is a good tanning agent, it is used directly as a tanning material, and tanning extracts are produced from the tree.
Oak wood has beautiful colour and texture. It is dense, strong, elastic, preserves well in the air, in the ground and under water, slowly cracks and deforms, pricks easily, and is resistant to rot and household fungus.
Oak wood is used in shipbuilding, the furniture industry, for the production of parquet, mine and hydraulic structures, for the manufacture of rims, runners, plywood, turning and carved products, parts of horse-drawn carriages (gobels, wheels). Particularly valued is “bog oak” - tree trunks that have lain at the bottom of lakes or for many years. Such wood becomes unusually strong and has an almost black color.
Oak wood does not have a special odor; barrels for wine, beer, alcohol, vinegar, and oil are made from it.
Oak wood is an excellent fuel.
Common oak is a spring honey plant. Bees collect a lot of highly nutritious pollen from it, and in some years they collect nectar from female flowers. But honeydew (exudation of plant juices) and honeydew (plant juice processed by insects) often appear on oak trees. In places where oak occupies large areas, bees collect a lot of honeydew and honeydew, from which they produce honey that is unsuitable for winter consumption. To avoid mass death of bees during wintering, such honey is pumped out.
Oak leaves contain the pigment quercetin, which, depending on the concentration, dyes wool and products made from it yellow, green, brown and black.
Oak acorns are a highly nutritious food for wild animals and domestic pigs. However, there are known cases of poisoning of other domestic animals by acorns (especially green ones). Acorn flour is also suitable for human food.
Oak brooms in a Russian bath are not inferior to birch brooms, and even surpass them.
used in landscaping as an ornamental and phytoncidal plant to create suburban groves, alleys, single plantings in parks and forest parks. Decorative forms of the common oak are known - with a pyramidal crown, in which the foliage falls 15 - 20 days later than the usual one.

1. Density is given at 15% moisture content and represents the average value for each rock. The density of these wood species varies widely depending on the geographic area where it grew and also depending on where the sample was cut from the log. The values ​​given in columns 2 and 3, calculated from density, also have an average value for each breed.

2. These values ​​indicate the amount of heat in kJ required to raise the temperature of 1 m of wood by 1°. The heat capacity of soft wood with 2% moisture content and at 90°C is approximately 1.72 kJ/kg°C. The lower the number in this column, the better.

3. The thermal conductivity (K) of wood is given at 2% moisture content and at 90°C, which corresponds to normal conditions in the sauna during use. The lower the digital value, the better.

4. Water absorption of wood, tangential and rational, is given as a percentage of its value at 20% moisture content for every 1% decrease in moisture content. The percentage of water absorption is determined by adding the water absorption in the tangential and radial directions (T+P) as follows: 0.25 - very small; 0.25-0.28 - small; 0.30-0.34 - moderately small; 0.35-0.39 - moderately large; 0.40 is big. A small change in humidity is preferable.

5. To compare the resistance of wood to splitting due to drying (compression), the corresponding properties of tensile forces acting perpendicular to the grain were expressed as a function of their percentage designation of tangential moisture movement. The obtained figures were classified into the following three categories: 1 - high resistance to splitting, 2 - medium, 3 - low. The lower the value in this column, the better.

6. Rot resistance has been classified into three categories as follows: 1 - rot resistant, 2 - moderately resistant, 3 - non-rot resistant.

The smell of wood depends on the resins it contains, essential oils, tannins and other substances. Conifers - pine and spruce - have a characteristic smell of turpentine. Oak smells of tannins, while backout and rosewood smell of vanilla. Juniper smells pleasant, so its branches are used to steam barrels. The smell of wood is of great importance when making containers. When freshly cut, wood has a stronger odor than after drying. The kernel smells stronger than the sapwood. Individual species can be identified by the smell of wood.

2.5. Macrostructure

Macrostructure. To characterize wood, it is sometimes sufficient to determine the following macrostructure indicators.

The width of the annual layers is determined by the number of layers per 1 cm of a segment measured in the radial direction on the end section. The width of the annual layers affects the properties of wood. For coniferous wood, an improvement in properties is noted if there are at least 3 and no more than 25 layers in 1 cm. In deciduous ring-vascular species (oak, ash), the increase in the width of the annual layers occurs due to the late zone and therefore the strength, density and hardness increase. For wood of deciduous scattered vascular species (birch, beech) there is no such clear dependence of properties on the width of the annual layers.

The content of late wood (in%) is determined on samples from coniferous and ring-vascular deciduous wood. The higher the latewood content, the greater its density, and therefore the higher its mechanical properties.

The degree of equilayering is determined by the difference in the number of annual layers in two adjacent sections 1 cm long. This indicator is used to characterize the resonant ability of spruce and fir wood.

When processing wood with cutting tools, hollow anatomical elements (vessels) are cut and irregularities are formed on the surface of the wood. In species such as oak, ash, and walnut, the amount of structural irregularities is significant. Since the wood of these species is used for finishing products, before polishing it is necessary to reduce the size of these irregularities. For this purpose, a special operation is performed, which is called pore filling.

2.6. Wood moisture content

The moisture content of wood is understood as the ratio of the amount of moisture removed to the mass of wood in an absolutely dry state. Wood moisture content is expressed in %.

Absolutely dry wood in small samples can be obtained by drying it in special cabinets. In nature and in production, wood always contains some amount of moisture. Moisture in wood permeates cell membranes and fills cell cavities and intercellular spaces. Moisture that permeates cell membranes is called bound or hygroscopic. Moisture that fills cell cavities and intercellular spaces is called free, or capillary. When wood dries, first free moisture evaporates from it, and then hygroscopic moisture. When moistening wood, moisture from the air permeates only the cell membranes until they are completely saturated. Further moistening of wood with filling of cell cavities and intercellular spaces occurs only with direct contact of wood with water (soaking, steaming, rafting, rain).

The total amount of moisture in wood consists of free and bound moisture. The maximum amount of free moisture depends on how large the volume of voids in the wood is that can be filled with water. The condition of wood in which the cell membranes contain the maximum amount of bound moisture, and the cell cavities contain only air, is called the hygroscopic limit. Thus, the humidity corresponding to the hygroscopic limit at room temperature (20°C) is 30% and practically does not depend on the breed. When hygroscopic humidity changes, the dimensions and properties of wood change dramatically. The following stages of wood moisture are distinguished: wet - exposed to water for a long time, humidity above 100%; freshly cut - humidity 50-100%; air-dry - for a long time stored in air, humidity 15-20% (depending on climatic conditions and time of year); room-dry - humidity 8-12% and absolutely dry - humidity 0%. The moisture content in the trunk of a growing tree varies along the height and radius of the trunk, and also depending on the time of year. The moisture content of pine sapwood is three times higher than the moisture content of the core. In deciduous trees, the change in humidity along the diameter is more uniform. Along the height of the trunk, the moisture content of the sapwood in conifers increases up the trunk, but the moisture content of the core does not change. In deciduous trees, the moisture content of the sapwood does not change, but the moisture content of the core decreases up the trunk. Young trees have higher humidity and its fluctuations throughout the year are greater than older trees. The greatest amount of moisture is contained in the winter (November-February), the minimum - in the summer months (July-August). The moisture content in the trunks changes throughout the day: in the morning and evening the humidity of the trees is higher than during the day.

To determine the moisture content of wood, gravimetric and electrical methods are used. With the weight method, samples of prismatic wood measuring 20x20x30 mm are cut, cleaned of sawdust and burrs, after which they are immediately weighed with an error of no more than 0.01 g. Then they are placed in a drying cabinet and kept at a temperature of 103 + 2 ° C. The temperature should not be raised above 105°C to avoid the release of resin (from conifers) and decomposition of the wood. The first weighing of the sample is carried out depending on the type of wood 6 hours after the start of drying (samples of oak and ash wood after 10 hours), the second and subsequent ones - every 2 hours. Dry the sample to a constant weight, i.e. until upon further weighing, its mass will cease to change.

Wood moisture content W, determined by weight, is calculated as a percentage using the formula

W=[(m1-m2)/m2]x100,

where m1 is the mass of the wood sample before drying, g; m2 is the mass of the same sample in an absolutely dry state, g. The advantage of the weight method is a fairly accurate determination of wood moisture content for any amount of moisture. Its disadvantage is the duration of sample drying (from 12 to 24 hours).

With the electrical method, wood moisture content is determined using an electric moisture meter. The operation of this device is based on measuring the electrical conductivity of wood depending on changes in its humidity. The working part of the most common electric moisture meter is needles with electrical wires connected to them. The needles of an electric moisture meter (sensor) are inserted into the wood to a depth of 8 mm and an electric current is passed through them, while the actual moisture content of the wood is immediately shown on the dial of the device. The advantage of the electrical method is the speed of determination and the ability to check the moisture content of wood of any size. Disadvantages - determination of humidity only at the point of contact of wood with the sensor; low accuracy. In the measurement range up to 30% humidity, the error is 1-1.5%, above 30 ±10%.

PHYSICAL PROPERTIES OF WOOD

These include: appearance, smell, macrostructure indicators, humidity and associated changes (shrinkage, swelling, cracking, warping), density, electrical, sound and thermal conductivity.

Appearance of wood

The appearance of wood is characterized the following properties: color, gloss, texture and macrostructure.
1.Wood color.
Under color wood understand certain things visual sensation, which depends mainly on the spectral composition of the light flux reflected by it. Color is one of the most important characteristics appearance wood It is taken into account when choosing species for interior decoration, furniture manufacturing, musical instruments, artistic crafts, etc.
The wood of oak, beech, white acacia, and velvet tree has the greatest shine among domestic species; from foreign ones - satinwood and mahogany (mahogany) wood.
Wood color shades have a wide range. It must be remembered that the color of wood can vary not only depending on the species, but within one species there can be several dozen variants of tonal relationships. This factor is influenced climatic conditions, in which the tree and other natural factors grew. Identifying and using a color palette is a crucial moment in the design search. The color of wood is given by coloring tannins found in its fiber. Wood species with warm shades predominate (yellow, ocher, red, red-brown, brown), but there are green, blue, purple and black wood species, which are considered exotic in our country.
Color shades various breeds can be classified into main groups, where one color of wood will be predominant:
yellow - birch, spruce, linden, aspen, hornbeam, maple, fir, ash (whitish yellow with light shades of pink and red), barberry (lemon yellow), mulberry (golden yellow), hawthorn, Karelian birch, lemon tree , acacia (sapwood), bird cherry (reddish-brownish yellow), ailanthus (pinkish yellow);
brown - cedar, poplar, elm kernel (light brown), beech, larch, alder, pear, plum (reddish-pinkish-brown), chestnut, rowan (brown-brown), acacia (yellow-brown), Anatolian walnut ( greenish-brown);
brown - cherry (yellowish-brown), apple (yellowish-pinkish-light brown), apricot, walnut (light (dark) brown);
red - yew, maclura, paduk, mahogany;
red-violet - amaranth;
pink - cherry laurel (yellowish-pink), pear, alder, plane tree (dark pink);
orange - buckthorn;
purple - lilac, privet (kernel);
black - stained oak, ebony, Macasar;
greenish - persimmon, pistachio.
2. Wood shine- this is the ability to reflect light flux from a surface in a certain direction. Different breeds have different shine; To a large extent, this property is manifested in beech, maple, plane tree, and white acacia. Poplar, linden, aspen, and teak have a matte (satin) sheen; silky - willow, elm, ash, bird cherry; golden - cherry; silver - Siberian cedar; moire - birch, gray maple, cherry laurel.
The shine of the wood depends not only on the presence and size of the core rays, but also on the nature of their placement along the cuts: the larger the core rays (for example, in oak) and the denser the wood, i.e., the more crowded the core rays are (for example, in maple ), the greater the shine of the wood. The distribution of gloss over the surface is uneven and depends on the type of cut: in the radial plane it is stronger, in the transverse plane it is weaker.
The light and shadow tints in some species are clearly visible only in a longitudinal section of the trunk, in others - in all sections. They significantly affect the decorative qualities of wood, enhancing or weakening its expressive sound, so the shine of wood is taken into account when compiling mosaic sets.
Distinctive features and use of wood species.
3. Wood texture.
Wood texture is the natural pattern of wood fibers on the treated surface, due to the peculiarities of its structure. The texture depends on the anatomical structure of individual wood species and the direction of the cut. It is determined by the width of the annual layers, the difference in the color of early and late wood, the presence of medullary rays, large vessels, and irregular arrangement of fibers (wavy or tangled). Coniferous species in a tangential section have a beautiful structure due to the sharp difference in the color of early and late wood. Deciduous species with pronounced annual layers and developed medullary rays (oak, beech, maple, elm, elm, plane tree) have a very beautiful structure of radial and tangential sections. There is a particularly beautiful pattern on sections of wood with a directional and confused (twisted) arrangement of fibers (burls, growths), as well as with traces of dormant buds (eyes). Softwoods and softwoods have simpler and less varied patterns than hardwoods. The decorative value of wood is determined by its texture, which is enhanced and revealed with transparent varnishes.

Wood texture
Table 1.

The smell of wood.

The smell of wood depends on the amount of essential oils, resins and tannins. Wood from a tree that has just been cut down or has been mechanically processed has a strong odor; conifers have a stronger odor than hardwoods.
The characteristic smell of turpentine is found in conifers - pine and spruce. Oak smells of tannins, while backout and rosewood smell of vanilla. You can determine the type of wood by the smell of wood.

Macrostructure

The macrostructure is characterized by the width of the annual layers, determined by the number of layers per 1 cm of a segment measured in the radial direction on a transverse section. Coniferous wood has higher physical and mechanical properties if there are at least 3 and no more than 25 layers in 1 cm. In deciduous ring-vascular species (oak, ash), the increase in the width of the annual layers occurs due to the late zone and therefore the strength, density and hardness increase. The wood of deciduous scattered vascular species (birch, beech) does not have a clear dependence of its properties on the width of the annual layers. Based on samples of coniferous wood and ring-vascular deciduous species, the latewood content is determined as a percentage. The higher the latewood content, the greater its density and, therefore, the better the mechanical properties.

Humidity.

The moisture content of (absolute) wood is the ratio of the mass of moisture contained in a given volume of wood to the mass of absolutely dry wood, expressed as a percentage.
Moisture in wood permeates cell membranes (bound or hygroscopic) and fills cell cavities and intercellular spaces (free or capillary).
When wood dries, first free moisture evaporates from it, and then hygroscopic moisture. When moistening wood, moisture from the air permeates only the cell membranes until they are completely saturated. Further moistening of wood with filling of cell cavities and intercellular spaces occurs only with direct contact of wood with water (soaking, steaming). It follows from this that once dried wood, without being in direct contact with water, cannot have a moisture content above the hygroscopic limit - state of wood in which the cell membranes contain maximum amount bound moisture, and in the cavities of the cells there is only air.
The complete saturation of wood with water is called the hygroscopic limit. This stage of humidity, depending on the type of wood, is 25-35%.
Wood obtained after drying at a temperature of 105 o C with the complete release of all hygroscopic moisture is called absolutely dry wood.
In practice, wood is distinguished: room-dry (with a humidity of 8-12%), air-dry artificially dried (12-18%), atmospheric-dry wood (18-23%) and damp (humidity exceeds 23%).
Wood from a tree that has just been cut down or has been in water for a long time is called wet, its moisture content is up to 200%. There is also operational humidity, which corresponds to the equilibrium moisture content of wood under specific conditions.

Average humidity in freshly cut condition, %
Table 2.

Shrinkage.

Shrinkage is a reduction in the linear dimensions and volume of wood during drying. Drying begins after the complete removal of free moisture and the beginning of the removal of bound moisture.
Shrinkage in different directions is not the same. On average, complete linear shrinkage in the tangential direction is 6...10%, in the radial direction - 3...5% and along the fibers - 0.1...0.3%.
The reduction in wood volume due to the evaporation of bound moisture is called volumetric shrinkage.
When sawing logs into boards, allowances are made for shrinkage so that after drying the lumber and workpieces have the specified dimensions.

Drying of wood (from water-saturated state to completely dry)
Table3.

Internal stresses

Tensions that arise without participation external forces, called internal. The reason for the formation of stress when drying wood is the uneven distribution of moisture.
If tensile stresses reach the limit of the wood's tensile strength across the grain, cracks may appear: at the beginning of the drying process on the surface of the log, and at the end - inside.
Internal stresses remain in the dried material and cause changes in the size and shape of parts during mechanical processing of wood. Residual stresses are removed by additional processing of lumber (steam humidification).

Warping.

When wood dries or gets wet, the cross-sectional shape of the board changes. This change in shape is called warping. Warping can be transverse and longitudinal. Transverse is expressed in a change in the cross-sectional shape of the board. This happens due to the difference in shrinkage in the radial and tangential directions. Core boards decrease in size towards the edges: boards whose outer part is located closer to the tangential direction shrink more than the inner ones, which have a radial direction. The closer the board is to the core, the greater its warping.
The boards can bend along their length, taking on an arched shape or the shape of a helical surface (winged). The first type of longitudinal warping occurs in boards containing core and sapwood (the shrinkage of core and sapwood along the length of the fibers is slightly different). Winging is observed in lumber with tangentially inclined fibers. To prevent warping, proper laying, drying and storage of wood is necessary.

Swelling.

Swelling is an increase in the linear dimensions and volume of wood with an increase in the content of bound moisture. Swelling is observed with increasing humidity to the hygroscopic limit; an increase in free moisture does not cause swelling. Just like shrinkage, the greatest swelling of wood is observed in the tangential direction across the grain, and the least - along the grain.

Water absorption.

Water absorption is the ability of wood, due to its porous structure, to absorb droplet-liquid moisture. Water absorption occurs when wood comes into direct contact with water. At the same time, the content of both bound and free moisture in the wood increases.

Wood Density

The density of wood depends on humidity and for comparison, density values ​​always lead to the same humidity - 12%.
There is a close relationship between the density and strength of wood. Heavier wood is generally more durable.
The density value varies within very wide limits. Based on density at a humidity of 12%, wood can be divided into three groups:
- low-density species (510 kg/m3 or less): pine, spruce, fir, cedar, poplar, linden, willow, alder, chestnut, walnut;
- medium density species (550...740 kg/m3): larch, yew, birch, beech, elm, pear, oak, elm, elm, maple, plane tree, rowan, apple, ash; - species with high density (750 kg/m3 and above): white acacia, iron birch, hornbeam, boxwood, saxaul, pistachio, dogwood.

Wood density (g/cm3)
Table 4.