About wood
Wood properties
The appearance of the wood is determined by color, glitter, texture and macrostructure.
Color
The wood receives the color from tannins, being in it, resinous and coloring matters, which are contained in the caves of cells. The wood of species, which are grown in different climatic conditions, has different color: from white (aspen, fir, lime tree) up to black (Ebony). The wood of species, which are grown in hot and southern regions, has the brighter color in comparison with the wood of species from temperate zone. Within the climatic zone each wood species has its special color, which may serve as an additional feature for its recognition. So, the hornbeam wood has light grey color, oak and ash tree – brownish, nut wood – brown. Under effect of light and air the wood of many species loses its brightness, obtaining the grayish color outdoors.
Glitter
The glitter is the ability pointedly to reflect the light flux. The glitter of the wood depends on its density, quantity, dimensions and location of wood rays. The wood of beech, maple, elm, plane tree, black locust, oak differs with special glitter. The wood of aspen, lime tree, poplar, possessing very narrow wood rays and rather thin walls of cells in strengthening tissues, has the dull surface.
Texture
The texture is the picture, which is made on the profiles of wood by cutting its fibers, annual rings and wood rays. The texture depends on the peculiarities for anatomical structure of some wood species and direction of cut. It is determined by the width of annual rings, difference in color of early and late wood, available wood rays, large vessels, incorrect location of fibers (wavy or tangled). The pine woods provide with wonderful texture on the tangential saw cut due to sharp difference in color of early and late wood. The hardwood with expressive annual rings and developed wood rays (oak, maple, English elm, elm, plane tree) has very beautiful texture on radial and tangential saw cuts. The wood with the incorrect location of fibers has especially beautiful picture. The texture determines the decorative value of the wood that is especially important by manufacture of artistic parquet, different handicrafts, musical instruments, etc.
Humidity
The absolute humidity of the wood is the ratio of moisture mass, being in this volume of the wood, and the mass of absolutely dry wood, expressed in percents. The relative humidity of the wood is the ratio of moisture mass, being in the wood, and the mass of the wood, being in wet conditions, expressed in percents. The total amount of moisture in the wood consists of free and bound moisture. The moisture, being in the caves of cells and intercellular spaces, is called free, or capillary, and in cellular walls – bound or hygroscopic.
There are the following degrees of wood humidity:
Wet wood is the wood, being in water for a long time, humidity is over 100%;
Green wood – humidity 50-100%;
Air-dried wood is the wood, which has been stored outdoors for a long time, humidity 15-20%, depending on climatic conditions and season of the year;
Indoor dried wood – humidity 8-10%;
Absolutely dried wood – humidity 0%.
The moisture is removed until it will evenly be distributed in the wood and its humidity will correspond to the temperature and relative humidity of air. The same processes occur by sorption (imbibitions) of moisture.
The equilibrium humidity is the average value between steady humidity of wood by sorption (imbibitions) and desorption (evaporation), which corresponds to certain combination of temperature and humidity of ambient air. The equilibrium humidity may be determined by the diagram from L.S. Sergovskiy. On the diagram vertically you can see the relative humidity of air, horizontally – its temperature. At crossing of these indices you can see the slanting line, which shows the humidity of the wood in percents. The humidity of parquet according to GOST shall be (9±3%).
Shrinkage
Shrinkage is the reduction in linear dimensions and volume of the wood by removing the bound moisture from it. The shrinkage is not the same due to different directions. The shrinkage, which occurs by removing the bound moisture from it, is called full one. In order to have the full shrinkage the humidity of the wood shall be reduced from saturation point up to 0. In average the full linear shrinkage in the tangential direction is 6-10%, in radial one – 3-5 and along fibers – 0.1-0.3%.
Internal stress:
The stresses, which arise without participation of external forces, are called internal. The reason for formation of stress by wood drying is the unevenness in moisture distribution. The internal stresses are sometimes kept in dried material and produce the changes in the dimensions and shapes of details by mechanical wood processing. In order to reduce the stress, one should use high-technology automatic drying chambers. The residual stress, preserved after processing, may be removed through additional processing (mechanical one or with vapor moistening). If the wood is dried or moistened, the shape of plank’s cross section is changed. Such change is called casting.
Wood cracking and power sections:
а – external crack in the log;
b – the same in beams;
с – internal cracks;
d – power sections.
Kinds of casting:
a, с – change in shape of bars’ cross section with different location of layers on the butt end;
b – the same for planks (medullar and lateral);
d – bow;
e – twist.
Density of wood
The density of the wood is the ratio of wood mass and its volume. The density is expressed in kg/cm3. The density of the wood depends on its humidity. All indices of physical mechanical properties of the wood are determined by humidity of 12%. There is the close connection between the durability and density. Heavier wood is, as a rule, durable. The density is determined by the quantity of wood substance per unit of volume. The density varies within very wide ranges. According to the density by humidity of 12% the wood may be divided into 3 groups: species with small density (510 and less), with average density (550-740) and with high density (750 and more). The density of wood has a large practical importance. The wood with high density (box tree, horn tree, beech, maple, pear tree) is of special demand at production due to its durability and free cutting.