Weight of wood
The weight of wood depends on its density and the amount of moisture it contains. There are specific gravity of wood matter and bulk density of wood.
The specific weight of wood does not depend on the type of wood. It expresses the weight of compacted wood material in a unit volume, without moisture and air, and is 1,5.
In practice, the volumetric weight of wood mass is used, that is, the weight 1 cm³ of wood mass expressed in grams. Based on the volume weight, the weight of the wood and its technical properties are estimated. With increasing humidity, the volumetric weight of wood also increases. The higher the density, the denser and less porous the wood.
Wood moisture
Types of water in wood
The water in the tree is divided into:
- capillary (free) — fills cell cavities;
- hygroscopic — found in cell walls;
- chemical — enters the chemical composition of the substances that make up wood.
The amount of water in wood, expressed in weight percent, is called wood moisture. There are absolute and relative humidity.
Relative and absolute humidity
If the weight of wood in its original state is marked with the letter A, the weight of absolutely dry wood with the letter A₁, the relative humidity in percent with the letter B, and the absolute humidity in percent with the letter B₁, the relative humidity can be determined according to the following formula:
Form for relative humidity of wood.
Absolute humidity is determined according to the following formula:
Absolute wood moisture form.
Determining the humidity of wood
A prism is cut from the middle of the board and measured on a scale with an accuracy of 0,01 g. The resulting value represents the size of A. The prism, whose weight should not be less than 20 g, is then dried at a temperature of 105° until it reaches a constant weight A₁.
Constant weight is considered to be achieved if the difference between two consecutive measurements is not greater than 0,3% of dry weight. By entering the values A and A₁, obtained by measurement, in the specified forms, the relative or absolute humidity of the wood is determined.
If the original weight of the prism cut from the middle of the board was 240 g, and the weight of the dried wood was 160 g, the absolute humidity of the tested sample is:
Example of calculating the absolute humidity of the tested sample.
The humidity obtained in this way is considered the humidity of the whole lot of wood.
Fiber saturation point
When drying wood, free water evaporates first. The moment when all free water evaporates is called the hygroscopic limit or fiber saturation point. During this drying period, the dimensions of the wood being dried do not change.
The humidity corresponding to the hygroscopic limit for different types of wood is listed in the following table:
| Type of wood | Humidity |
|---|---|
| Common pine | 29% |
| Weymouth pine | 25% |
| Spruce | 29% |
| Larch | 30% |
| Spruce | 30% |
| Beech | 31% |
| Linden | 29% |
| Ash | 23% |
| Chestnut | 25% |
The influence of humidity on processing and durability of wood
Wood with increased humidity is a good conductor of heat, it is processed worse by machines, it is bad at gluing, painting, varnishing and polishing. Paint and varnish quickly disintegrate on the surface of wet painted wood. Damp wood causes nails and screws to rust.
The dimensions of carpentry construction products made of raw wood — doors, windows, plank floors, parquet and other products — shrink during drying. Because of this, cracks appear and the stiffness of the connection between the elements is lost.
Therefore, the quality of wood in construction, its durability and resistance against rotting are determined first of all by its humidity, and then by the type of wood and the conditions of exploitation. Under normal conditions of exploitation, dry wood can serve in buildings for dozens of years.
Weighting of wood during drying
During drying, the wood changes its dimensions: in the longitudinal direction by 0,10%, in the radial direction by 3–6%, and in the tangential direction by 6–12%. This change is called weighting.
Weighting begins when the humidity reaches the fiber saturation point, from 23% to 31%. Anatomical elements that make up the wood in the drying process shrink unevenly, so the weighting of the wood is different in different directions.
Wood with a high density, such as oak, weighs more than wood with a lower density, such as linden. In conifer species, the size of weighting also depends on the participation of late wood. With an increase in the percentage of late wood in pine, the weight increases, because the late wood of coniferous species weighs significantly more when drying than the early wood.
Weighting of early and late wood of coniferous species
Weighting of different types of wood
Weathering and cracks
The uneven change in dimensions in the process of weighting due to drying, as well as the application of improper drying regimes, causes internal and external stresses in the wood. They lead to weathering and the appearance of external and sometimes internal cracks.
Tangentially cut boards are more prone to weathering than radially cut boards. The closer they are to the periphery, the greater the wind shear.
Image 3. Winding the boards according to their position in the log.
External cracks are caused by uneven drying of the outer and inner layers of wood. Due to the large difference between their humidity, tensile stresses appear on the surface of the wood, which lead to external cracks.
In order to avoid the appearance of external cracks, the drying process should be carried out slowly and evenly. Then the change in dimensions takes place gradually, so the forces that cause splashing are small.
Wood dries faster from the front, which is why the fronts of boards, billets and round timber crack earlier than other surfaces. In order to avoid cracking, foreheads should be protected from the permanent effects of the sun’s rays by coating them with protective agents or placing them in the shade.
Tree swelling
Wood swelling is the reverse process of drying and weighting. Dried wood is able to absorb moisture and increase its dimensions. This feature is used to moisten dry barrels, wooden pipes, tanks and other objects, causing them to swell.