Understanding oil in wood is essential for anyone working with natural timber, from hobbyist woodturners to professional furniture makers. This substance, often referred to as wood oil, is a fundamental component of the material's structure, influencing its appearance, stability, and how it interacts with the environment. The presence of these oils dictates much of the wood's behavior, affecting everything from its feel in the hand to its susceptibility to decay.
What is Wood Oil?
Wood oil is not a single chemical compound but rather a complex mixture of hydrophobic substances naturally produced by the tree. These compounds are primarily lipids, including triglycerides, waxes, and sterols, which are synthesized within the living cells of the tree. Unlike drying oils used for finishing, such as linseed or tung oil, the oils inherent to the wood are bound within the cell walls and lumen, serving critical biological functions long before the tree is ever felled.
Biological Function
Within the tree, these oils act as a protective barrier and a stored energy source. They provide water repellency, preventing the wood fibers from becoming oversaturated during periods of high humidity or flooding. This hydrophobicity is crucial for the tree's survival, allowing it to maintain structural integrity and transport nutrients efficiently. Additionally, the oils can deter insects and fungi, making the wood less palatable and more resilient to decay while the tree is alive.
The Impact on Woodworking
The amount and type of oil present vary significantly between species, growth conditions, and even the age of the tree. This variance is why some woods, like teak or cedar, are naturally oily and resistant to moisture, while others, like maple or birch, are considered "lean" and require careful handling. For woodworkers, this oil content is a primary factor in determining the suitability of a timber for a project, influencing adhesion, finishing, and long-term durability.
Workability: Oily woods can be slippery to machine, requiring adjusted feed rates and sharper tools to achieve clean cuts.
Adhesion: The natural oils create a barrier that prevents glue from bonding effectively, often necessitating surface preparation or specific adhesive types.
Finishing: These woods may resist water-based stains, requiring oil-based products or intensive surface sanding to achieve an even finish.
Managing Oil in the Drying Process Freshly cut "green" wood contains a high percentage of moisture and oil. As it dries, whether through air or kiln drying, the rapid loss of moisture can cause the wood to shrink and warp. The oils help moderate this process by slowing down the movement of water from the interior to the surface. However, if the drying is too aggressive, the oils can become concentrated, leading to surface checking or honeying, where the wood develops a sticky, oily residue. Finishing and Maintenance
Freshly cut "green" wood contains a high percentage of moisture and oil. As it dries, whether through air or kiln drying, the rapid loss of moisture can cause the wood to shrink and warp. The oils help moderate this process by slowing down the movement of water from the interior to the surface. However, if the drying is too aggressive, the oils can become concentrated, leading to surface checking or honeying, where the wood develops a sticky, oily residue.
When it comes to finishing, the wood's natural oil content plays a pivotal role in how it accepts a final treatment. A surface with high endogenous oil content may not require additional oil, though most craftsmen prefer to apply a finish to enhance the grain and provide a durable sacrificial layer. Conversely, dry woods readily absorb finishes, allowing for a deeper penetration that results in a rich, durable patina. Understanding the balance between the wood's oil and the applied finish is key to achieving a surface that is both beautiful and resilient to daily use.
You can often identify high-oil woods through simple sensory tests. Rubbing the wood with your thumbnail or a cloth will often leave a visible mark or a slight sheen on the surface. The aroma is also a strong indicator; species like cedar and rosewood release a distinct, pleasant scent when sanded or planed. This characteristic aroma is a direct result of the volatile organic compounds within the oil, which also contribute to the wood's resistance to pests and decay.
