The term “green wood” is a fundamental concept in wood science that refers to the state of lumber or timber immediately after a tree is harvested. Contrary to what the name might suggest, “green” does not relate to the color of the wood. It is a description of the wood’s moisture content, indicating it is freshly cut, unseasoned, and saturated with water. Understanding this initial state is important for nearly every application.
Defining Green Wood
Green wood is defined as freshly felled timber that has not yet undergone a significant drying process. This wood retains the natural water that was present in the living tree, known as its sap and cellular moisture. The moisture content of green wood can vary dramatically between species, often ranging from 30% to over 200% when measured as a percentage of the wood’s oven-dry weight. This high moisture level makes the wood heavier and softer than seasoned lumber. It is considered unseasoned because it has not been allowed to reach equilibrium with the surrounding air.
The Importance of Moisture Content
The amount of water contained within wood dictates the wood’s future stability and behavior. Water exists in two primary forms: free water held within the cell cavities, and bound water held within the cell walls themselves. The loss of free water, which happens first, does not cause any dimensional change. The critical threshold in the drying process is known as the Fiber Saturation Point (FSP). This is the point at which all the free water has evaporated, but the cell walls remain completely saturated with bound water. For most wood species, the FSP occurs at approximately 30% moisture content. Shrinkage, which causes warping and cracking, only begins once the wood dries below the FSP. This threshold is a fundamental concept in wood engineering.
Practical Implications for Use
Using green wood in construction or woodworking without proper seasoning can lead to numerous problems as the material dries out in service. The subsequent shrinkage and uneven drying cause the lumber to warp, bow, cup, and twist, severely affecting the structural integrity and aesthetics of a project. As the wood shrinks, gaps can open up in joints and seams, and stresses can lead to surface checking and splitting along the grain. This movement also compromises the holding power of fasteners, potentially causing nails to pop out or screws to loosen over time.
For firewood, the high moisture content of green wood makes it a very inefficient and potentially unsafe fuel source. A significant portion of the heat energy produced by the fire must be used to boil off the trapped water before the wood itself can combust. This process results in a fire that burns cooler and produces substantially more smoke and water vapor. The cooler smoke from burning green wood is a major cause of creosote buildup inside chimneys and flues. Creosote is a highly flammable, tar-like residue that adheres to the flue walls, creating a serious fire hazard. To ensure a clean, hot, and safe burn, firewood moisture content should be reduced to below 20%.
Transitioning from Green to Usable
The process of reducing the moisture content of green wood to a stable, usable level is called seasoning or drying. The two primary methods employed for this transition are air drying and kiln drying.
Air Drying
Air drying is the traditional, slower method where wood is carefully stacked outdoors with spacers, called stickers, to allow natural air circulation to remove moisture. This process is highly dependent on ambient conditions and typically takes many months or even years. Air drying reduces the moisture content to the local Equilibrium Moisture Content (EMC), often in the 12% to 20% range.
Kiln Drying
Kiln drying is a much faster, controlled industrial process that uses heated chambers to regulate temperature and humidity. This method can reduce moisture content in days or weeks and can achieve a much lower final moisture level, typically 6% to 9% for interior-grade lumber. The target for all drying is to reach an EMC, which is the moisture content at which the wood is in balance with the environment where it will be used. Matching the wood’s EMC to its intended service environment is the final step in ensuring its dimensional stability and long-term performance.