There is one biggest worry in making mahogany or solid wood furniture, that is, the wood is dry! Generally speaking, wood drying is to evaporate the water contained in the wood by a certain method, so that it is balanced with the use environment. To stabilize its features!
Therefore, controlling the moisture in the solid wood is the most important work of the solid wood manufacturer. As the saying goes, "Know yourself and know each other, there is no war." If you want to dry the wood well, you must first understand the knowledge of the "water" in the wood! Today, we will share with you the things about how much "water" the wood has... ...
Classification of water
There are three types of moisture in wood: free water, sorbed water and combined water.
Free water refers to the water present in the cell cavity and cell gap, also known as capillary water; sorbed water refers to the water present between the microfibrils of the cell wall, also known as attached water, sorbed water or bound water; combined water refers to the existence Moisture in the chemical composition of wood.
Free water and absorbing water are the main moisture in the wood. Free water is closely related to wood density, combustion, drying and infiltration; sorption of water affects the properties of wood; combined water has little to do with wood properties.
Fiber saturation point (water content)
The moisture content of the wood cell wall (sucking water) is saturated, and the water content of the cell cavity without free water is called the fiber saturation point or fiber saturation.
Fiber saturation points vary from species to tree, typically 23% to 31%, and typically 30% is the average of the fiber saturation points for each tree species. The fiber saturation point of poplar wood is 23%.
The change in fiber saturation point is a turning point in the change in wood properties. Above the fiber saturation point, the strength of the wood is unchanged;
Below this point, the strength of the wood increases due to the decrease in water content, and conversely, decreases as the moisture content increases until it reaches the fiber saturation point.
Similarly, when the moisture content of wood is above the fiber saturation point, the wood does not shrink or expand. When the water content decreases below the fiber saturation point, the wood shrinks with the decrease of water content, and when it reaches zero, the shrinkage reaches the maximum;
Conversely, it expands as the moisture content increases until the fiber saturation point moisture content is reached.
Raw material moisture content
The raw material is the tree that has just been felled, and the water content refers to the moisture content of the wood that has just been felled. The moisture content of raw materials has a certain significance for the transportation and storage of wood.
The water in the raw material is mainly free water and sorbed water, and its water content varies with the season, but there is no regularity in each tree species. Generally, there are more springs in the winter and sap flow, which can reach 80%-100% of the weight of the whole tree.
Balanced moisture content
Wood that has been stored in the atmosphere for a long time is called dry gas. When the water vapor pressure in the air is greater than the water vapor pressure on the surface of the wood, the wood absorbs water from the outside; on the contrary, the moisture in the wood is lost.
If the water vapor pressure in the air is the same as the water vapor pressure on the surface of the wood, the wood neither absorbs water nor loses moisture. At this time, the moisture content of the wood is called the equilibrium moisture content. The equilibrium moisture content of the wood varies with the state of the surrounding air.
If the actual moisture content of the wood is less than the equilibrium moisture content, the wood will exhibit moisture absorption; otherwise, the wood will evaporate.
Extended reading: How to identify ancient furniture through the moisture content of wood
Moisture movement
The movement of moisture in wood can be divided into two cases: water content below the fiber saturation point and above the fiber saturation point:
Moisture movement in wood when moisture content is below the fiber saturation point
In this state, the interior of the wood does not contain free water, and all cells are filled with air. Due to the evaporation of moisture on the surface of the wood, a moisture content gradient is formed inside the corresponding water vapor partial pressure gradient.
Under this gradient, water vapor begins to diffuse from the inside to the outside through the cell cavity and through the pores in the pits and pits.
The movement of moisture in the wood, when the water content is lower than the fiber saturation point, in addition to the above-described movement in the vapor state, the movement of the water along the microcapillary system in the cell wall due to the action of the capillary.
Due to the evaporation of moisture on the surface of the wood, the tension of the capillary at the surface portion is increased, and the water layer is thinned. The difference in tension causes the sorbed water to move from a portion having a high water content to a portion having a low water content.
When the water content is lower than the fiber saturation point, the movement of moisture in the wood will have a third way of conduction, that is, the vapor state and the liquid state constantly alternate with each other, that is, along the microcapillary and the cell cavity in the adjacent cell wall. Between the large capillaries, water vapor or liquid water alternates with each other.
The above-mentioned way of moving moisture is applicable to all types of wood. However, the relative efficiency of various water-conducting pathways varies greatly depending on tree species, density, and structural characteristics of wood, changes in water content, and temperature.
Moisture movement in wood when the water content is higher than the fiber saturation point
In this state, water is mainly in a liquid state in the lumen of the wood cell, and water vapor is also present in the cell cavity in a saturated state.
Since the water vapor pressure of each part is consistent, in this case, theoretically, there is no steam state of moisture moving inside the wood, only the liquid water caused by the difference in capillary tension - free water along the cell cavity and the pit mobile.
If all the cell compartments in the wood are filled with water, it is almost impossible for moisture to move from one cell cavity to another.
When a part of the outer surface of the wood has a water content below the moisture content of the fiber saturation point, the inside of the wood will move from the inside to the outside.
At the beginning, only a few rows of cells on the surface of the wood evaporate the water outward, and the thickness of the water film in the cell gradually becomes smaller, so that the curvature of the crescent in the capillary increases sharply, and the capillary surface and the inside of the wood form a capillary tension difference. To cause free water to move from the inner rows of cells to the evaporation surface.
The continuous progress of this process causes the evaporation surface to gradually move toward the interior of the wood, which constitutes the mechanism for the diffusion and movement of the above three modes of moisture transmission, and the wood is gradually dried.
Wood is a natural material, and there are many uncontrollable factors. If you want to use it well, it is really a matter of learning.
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