Wood drying

Fresh wood has a lot of water inside.

We must dry it out before we use it.

Some people dry wood in the air. Others use a big oven called a kiln.

Drying wood helps it stay strong.

If wood dries too fast, it can get hurt. It might bend or change shape. This is because the outside shrinks first.

Dry wood is also better for fires. Wet wood wastes energy when it burns. It is better to use dry wood for heat.

Wood can take in water from the air. It can also give water back to the air. This helps the wood stay just right.

Fresh wood contains a lot of water.

This water can be over 50% of its weight. We must dry wood before we use it. This is called seasoning.

There are two main ways to dry wood. Some people use air-drying. This is a very old method. Others use a kiln. A kiln is a large oven.

Kiln-dried wood is often more valuable. In fact, it can cost much more than wet wood.

We dry wood for different reasons. If we use wood for building, we must dry it carefully. If it dries too fast, it can suffer from case hardening. This happens when the outside shell shrinks too quickly. The inside stays wet and tries to shrink later. This can make the wood warp or bend.

Dry wood is also better for fires. Wet wood wastes energy. It uses heat just to turn the water into vapor.

Wood also talks to the air. It can take in moisture or give it off. It tries to reach a balance. This balance is called the equilibrium moisture content.

Wood drying is a very important step in using timber. This process is also called seasoning. We dry wood to lower its moisture content. If we use wood for building, we must be careful. Wood will absorb or release water to match its surroundings. This state is called equilibrium. If wood changes its moisture too fast, it can get damaged.

This is why drying must be controlled.

There are two main ways to dry wood. Air-drying is a traditional method where wood sits in the open.

Another way is using a kiln. A kiln is a large, purpose-built oven.

Drying wood in a kiln makes it known as kiln-dried timber. Drying wood adds a lot of value to it. In Australia, green hardwood might cost $350 per cubic metre. Once it is dried, it can cost $2,000 or more.

Sometimes, drying happens too quickly. This causes a problem called case hardening. The outside shell of the wood dries and shrinks first. This shell becomes hard and resists further shrinking. However, the core of the wood is still wet. When the core finally starts to dry, it tries to shrink too. The hard shell fights this movement. This creates stress inside the wood. This stress can make the wood warp when it is sawn.

Wood contains water in different ways. Fresh logs can be over 50% water by weight. Some water is called free water. This water sits in the cell lumina and is held by capillary forces. The first water to leave during drying is this free water. Other water is called bound water. This water is attached to the wood through hydrogen bonds. The point where all free water is gone is the fibre saturation point. This usually happens at 25 to 30% moisture content.

Wood also reacts to the air around it. It is a hygroscopic substance, which means it can take in or give off moisture. It moves water as vapour to reach a balance with the air. This balance is called the equilibrium moisture content, or EMC.

The amount of water left depends on the humidity and temperature. For example, wood in a dry, heated office might have an EMC of only 6 to 7%. In some parts of Australia, the EMC can be as high as 18%.

Wood drying, often called seasoning, is the process of reducing the moisture content in timber. This step is vital for both construction and fuel. When wood is used for building, it must reach equilibrium with its environment. Equilibrium means the wood absorbs or releases moisture to match the surrounding air. If this happens too quickly, the wood can suffer permanent damage. For firewood, drying is done to improve burning efficiency.

Burning wet wood wastes energy because heat is used to evaporate water. A 50% wet log burned at high temperatures can waste about 5% of its energy just heating water vapor.

To understand drying, we must look at how water exists within wood. Fresh logs are often over 50% water by weight. Water is held in three forms: free water, bound water, and vapor. Free water sits in the cell lumina, which are the hollow spaces in wood cells. It is held there by capillary forces rather than chemical bonds. Bound water is different because it is attached to the wood via hydrogen bonds. This happens because the hydroxyl groups in the cellulose and lignin are negatively charged. Since water is a polar liquid, it is attracted to these groups.

As wood dries, it moves through specific stages. The first stage is the removal of free water from the cell lumina. This continues until the wood reaches the fibre saturation point (FSP). The FSP is the moisture content where all free water is gone, but cell walls are still saturated with bound water. In most woods, this occurs at 25% to 30% moisture content. Below this point, many properties change. The wood's strength generally increases as bound water is lost. Its electrical resistivity also increases very rapidly once it dries below the FSP.

If drying is not managed carefully, a defect called case hardening can occur. This happens when the surface of the lumber dries much faster than the center. The outer shell shrinks and becomes a hard, set layer. This shell then resists the shrinkage of the wetter core. This creates internal stresses, specifically compression on the shell and tension in the core. These unrelieved stresses can cause the wood to warp significantly when it is eventually sawn.

Wood is a hygroscopic substance, meaning it constantly exchanges moisture with the air. It moves water in the form of vapor to reach the equilibrium moisture content (EMC).

The EMC is determined by the temperature and relative humidity of the surrounding environment. If the air's vapor pressure is lower than the pressure inside the wood, moisture leaves the wood. This process continues until the pressures are equal. In dry, air-conditioned offices, the EMC might be as low as 6% to 7%. However, in parts of Australia, the EMC can reach 15% to 18%.

Different types of wood react to drying in different ways. Wood is categorized into softwoods and hardwoods based on botanical origin. Softwoods, like pine, are generally lighter and easier to process. Hardwoods are denser and have more complex structures. Because hardwoods are less permeable, they are much more difficult to dry. While there are a hundred times more hardwood species than softwood species, softwoods provide the main commercial supply. This is because they can be processed and dried much faster.

Dimensional changes like shrinkage and swelling are also part of the process. Shrinkage occurs as moisture decreases, while swelling happens as moisture increases. This movement is not equal in all directions. The greatest change occurs tangentially, which is in a direction parallel to the growth rings. Radial shrinkage is much smaller, and longitudinal shrinkage along the grain is often negligible. In some species, tangential shrinkage can be five times greater than radial shrinkage. Drying wood to an EMC close to its future environment helps minimize these movements.

Properly drying timber is a way to add significant economic value to wood products.

In Australia, green sawn hardwood might sell for $350 per cubic meter. Once that wood has been dried, its value can increase to $2,000 per cubic meter or more. This makes seasoning a critical part of the modern timber industry.