The Origin of Wood Drying and Related Technical Analysis

The Origin of Wood Drying

I. Necessity and Main Methods of Wood Drying

To prevent wood products from deformation, warping, and cracking, enhance wood strength, inhibit fungal decay and insect infestation, and facilitate processing and storage, wood must undergo drying treatment before use. The main methods are as follows:

1. Natural Drying Method: Stack wood pieces spaced apart in a well-ventilated shed to avoid direct sunlight and rain. Moisture in the wood evaporates through natural air convection to achieve moisture balance. This method is simple, low-cost but time-consuming, and wood is prone to insect infestation and decay.
2. Artificial Drying Methods: Including soaking method, steaming method, hot kang method, etc.

• Soaking Method: Immerse wet wood in flowing water for 2-5 months to dissolve the liquid components in the wood, followed by air-drying or steam-drying. This method reduces wood deformation and saves time compared to natural drying but slightly decreases wood strength.
• Steaming Method: First steam wet wood with hot steam, then naturally dry it. This method has a fast drying speed and can kill insects and sterilize, but it reduces wood elasticity and strength and impairs its luster.
• Hot Kang Method: Dry wood using a heated kang (a traditional Chinese heated brick bed). This method achieves a high degree of dryness but has a limited drying capacity and low efficiency. For more knowledge about wood drying, follow the “Wood Industry” official WeChat account.

II. Causes of Cracking in Solid Wood Furniture

Firstly, it should be understood that slight cracking is a common issue in solid wood furniture—cracking is an inherent tendency of all wood.

• Inadequate Drying Treatment: Solid wood panels should undergo drying treatment before furniture production to reduce the risk of cracking. However, some unscrupulous manufacturers skip strict drying to cut costs, leading to cracking in finished furniture. Higher wood moisture content increases the likelihood of cracking; inquire about this when purchasing furniture. Wood moisture content and drying stress determine product quality, so it is advisable to choose manufacturers with reliable product quality.
• Environmental Factors: External conditions can also cause cracking. For example, using heating systems in cold winter can expose furniture to prolonged heat, or direct sunlight can lead to cracking and deformation of wooden furniture.

III. Main Purposes of Wood Drying

1. Prevent Decay, Deterioration, and Insect Infestation: When wood moisture content is reduced below 20% or stored in water, it can avoid decay, discoloration, and insect damage. For instance, Pinus massoniana (Masson pine), widely distributed in southern China, has moderate density and strength, making it suitable for construction, vehicles, furniture, etc. This wood is prone to decay, discoloration, and insect infestation, but drying it to a moisture content below 20% can effectively preserve its inherent quality.
2. Prevent Deformation and Cracking: Drying wood to a moisture content compatible with the service environment prevents shrinkage and swelling, thereby avoiding deformation and cracking. For example:
   1. In arid northwestern China, the equilibrium moisture content (EMC) of wood is approximately 10%, so lumber should be dried to 7-9%.
   2. In humid coastal areas of southeastern China, the final moisture content after drying should be 12-13%.
   3. For wood products used in northeastern China or exported to North America, considering indoor heating conditions, the final moisture content should be 6-8%.
3. Enhance Mechanical Strength and Improve Physical Properties: When moisture content is below the fiber saturation point, wood mechanical strength increases as moisture content decreases. Additionally, moderately reducing moisture content improves physical properties, enhances bonding quality, and fully showcases wood grain, luster, and insulation performance.
4. Reduce Weight: Kiln-dried wood can reduce weight by 30-50%. For example, processing logs into lumber near forested areas, drying it to a transportation moisture content (20%), and then transporting it can save significant transportation capacity and costs. It also prevents fungal and insect damage during transportation, ensuring wood quality.

Wood drying is a crucial technical measure for rational utilization and conservation of wood, as well as a key process in wood processing. It is widely used in industries such as wood furniture, interior decoration, construction doors and windows, vehicles, shipbuilding, textile instruments, military industry, machinery manufacturing, cultural and sports goods, and toys—almost all sectors using wood require wood drying. Wood drying plays a vital role in the rational and economical use of China’s limited forest resources, maintaining ecological balance, and promoting national economic development and modernization.

IV. Functions of Wood Conditioning Rooms

Qualified wood needs to maintain a specific moisture content level before being processed into semi-finished products or receiving protective coatings. This process is called wood moisture equilibrium or wood conditioning. Facilities used for this purpose are known as conditioning rooms, curing rooms, equilibrium kilns, or curing kilns.

Whether it is a wood equilibrium room, conditioning room, or drying room, strict requirements are imposed on the indoor environment, especially temperature and humidity control. These rooms require constant temperature, constant humidity, or both, with varying temperature and humidity specifications for each stage, all of which can be manually controlled. Constant temperature and humidity system engineering falls under the category of indoor climate environment systems.

1. Improve Dimensional Stability of Wood Products and Prevent Cracking/Deformation: Wood exposed to air for a long time undergoes swelling and shrinkage. Uneven swelling and shrinkage often cause cracking and deformation, affecting usability and resulting in waste. Using wet or inadequately dried wood to manufacture products (such as doors, windows, flooring, furniture) may seem satisfactory initially, but over time, as the wood dries, problems like door frame skewing, floor warping, joint loosening, or panel cracking may occur, leading to substantial losses. Drying wood to the required moisture content before use ensures the structural stability of wood products, making them aesthetically pleasing and durable.
2. Reduce Grade Loss, Conserve Wood Resources, and Prevent Mold, Decay, and Insect Infestation: Failure to dry sawn lumber promptly after log cutting or improper drying processes can cause defects such as cracking, deformation, and discoloration. Additionally, wood with a moisture content between 20% and 100% is prone to mold growth, leading to decay and insect infestation. Only by drying wood to below 20% moisture content or storing it in water can these issues be avoided. Each year, China suffers severe grade losses due to wood deformation, cracking, decay, and insect infestation, rendering wood unfit for manufacturing products.
3. Enhance Mechanical Strength and Improve Processing Performance: When wood moisture content is below the fiber saturation point, strength increases as moisture content decreases. For example, when pine moisture content drops from 30% to 18%, its modulus of rupture increases from 50MPa to 110MPa. Furthermore, dried wood improves cutting conditions, enhances the strength of wood structural components, bonding strength, and surface finishing quality of wood products. Wood’s thermal and electrical conductivity change with moisture content; reducing moisture content decreases conductivity, improving thermal insulation and electrical insulation properties.
4. Reduce Weight and Improve Transportation Efficiency: Newly processed sawn lumber can reduce weight by 30-50% or more after drying. Therefore, concentrating log processing and drying sawn lumber to the transportation moisture content (20%) near forested areas or import ports before transporting to user regions for final drying to the required moisture content not only reduces transportation costs but also minimizes cracking and deformation.

V. Wood Drying Methods,How to Kiln Dry Wood

1. Conventional Drying: Uses atmospheric pressure moist air as the drying medium, steam or hot water as the heat source, and indirectly heats air. Wood is dried through air convection.
2. High-Temperature Drying: Similar to conventional drying but uses a higher temperature drying medium. It offers fast drying speed, good stability, but darkens wood color and hardens the surface.

1. Dehumidification Drying: Uses atmospheric pressure moist air as the medium (similar to conventional drying). It produces high-quality dried wood, is pollution-free, and energy-efficient.
2. Solar Drying: Utilizes solar radiation to heat air, which then circulates between collectors and wood stacks for drying. Solar energy is low-cost but intermittent.
3. Vacuum Drying: Uses moist air as the medium and operates under atmospheric pressure. It features fast drying speed, short cycle time, and high drying quality.
4. High-Frequency and Microwave Drying: Both methods use wet wood as a dielectric. Under the action of a magnetic field, water molecules in the wood rotate at high speed, generating heat through friction to achieve drying.
5. Flue Gas Drying: Typically used in the initial stage of conventional furnace gas drying. It employs small-scale drying chambers built with traditional methods. This method has low investment and costs but causes severe environmental pollution due to smoke and dust, and drying quality is difficult to guarantee.

VI. Wood Carbonization (Thermal Modification)

Wood carbonization, also known as high-temperature thermal modification, is an environmentally friendly physical wood protection technology that involves short-term pyrolysis of wood at 130-200°C using heat transfer media such as steam, inert gas, oil, or water. High-temperature treatment induces physical and chemical changes in wood components, improving dimensional stability, durability, and color. The extent of property changes depends on wood species and process parameters.

Over the past 10 years, industrial application of wood carbonization technology has been realized in European countries such as the Netherlands, France, Germany, and Finland. Additionally, Japan, Canada, and Taiwan region of China have conducted extensive research on carbonized wood and achieved significant results.

Principles of Wood Carbonization

Wood carbonization primarily acts on hemicellulose, the amorphous regions of cellulose, and hydrogen bonds connecting cellulose and hemicellulose.

For example, heating wood to approximately 175°C in an oxygen-free environment causes partial degradation of hemicellulose, reducing hydrophilic structures such as hydroxyl groups and enhancing moisture stability. The increase in crystallinity improves dimensional stability and strength; enhanced lateral connections in the lignin network also contribute to stability and strength. Formic acid, acetic acid, and phenolic compounds produced during the process delay decay and extend service life.

Specific process parameters (temperature and humidity curves) vary by manufacturer and require repeated testing and optimization. Precise control of temperature, humidity, and time is critical: exceeding the glass transition temperature of cellulose and hemicellulose for an extended period leads to excessive degradation and reduced strength; similarly, excessive reduction of hydrogen bonds weakens overall strength.

Effects of Carbonization on Wood Properties

1. Reduced Density: Mainly due to partial loss of hemicellulose. Most studies indicate that 5-8% hemicellulose degradation barely sacrifices structural strength.
2. Increased Strength: Proper temperature-controlled carbonization enhances wood strength, but excessive or overheating carbonization results in strength loss.
3. Reduced Moisture Content: All wood absorbs moisture from the atmosphere; equilibrium moisture content (EMC) is related to relative humidity and internal structure. During carbonization, the loss of hydrophilic groups (e.g., hydroxyl groups) reduces wood’s moisture absorption capacity.

Thus, carbonized wood not only has a lower EMC at a given relative humidity but also a smaller increment in EMC with increasing relative humidity, making it less prone to volume expansion and shrinkage.

Carbonization improves dimensional stability and relieves internal wood stress because chemical changes in hemicellulose (especially polysaccharide glycosides) convert it into weakly hygroscopic monomers; additionally, hydroxyl groups within cellulose molecular chains form hydrogen bonds.

4. Darkened Color

Color darkening of carbonized wood is mainly caused by chemical changes and degradation of hemicellulose, lignin, and certain extractives.

5. Enhanced Decay Resistance: Carbonization improves decay resistance by altering wood components, cutting off nutrient sources for fungi, and reducing moisture content to inhibit fungal growth.