Understanding Timber Defects: A Comprehensive Guide
Timber Defects
Wood quality can vary due to several factors that manifest as irregularities or imperfections. These imperfections, known as defects, affect the wood’s physical, mechanical, and chemical properties, ultimately determining its applications. These defects can be categorized based on their origin:
- Natural structure: Occurring during the tree’s life.
- Handling or external factors: Occurring after the tree is felled.
Structural Defects
These defects arise from wood tissue abnormalities during the tree’s life due to inherited characteristics, injuries, wind, and other factors. Here are some of the most important:
Knots
One of the most common defects, knots are remnants of branches embedded in the wood during growth. They are classified as live (green) or dead knots based on the branch’s condition at the time of felling. Live knots come from green branches whose tissues have fused with the surrounding wood. Dead knots originate from dry branches whose tissues are separate from the adjacent wood. Live knots remain within the wood, while dead knots can become loose and leave holes. Knots affect mechanical strength, especially bending, depending on their size and location. They are often accompanied by pitch or resin, which can cause finishing problems.
Knots result from poor forest management and can be prevented with proper pruning.
Ring Shakes
Ring shakes are separations of wood elements along the growth rings, visible in the wood’s cross-section. They can be caused by growth stresses, frost, or excessive wind stress.
Bark Pockets
Bark pockets originate from cambial injuries, resulting in partial bark inclusion within the wood. This affects mechanical strength and can limit the wood’s applications.
Kino and Resin
These defects involve the accumulation of exudates in cavities. Kino occurs in hardwoods, while resin occurs in conifers. They affect the wood’s appearance and limit its use due to reduced mechanical strength.
Eccentric Pith
The pith is a central core of young tissue. While typically located in the geometric center of the tree trunk, it can sometimes be off-center, known as eccentric pith. This occurs in trees growing in constant wind or at the edge of forests, where growth is more intense towards the light source. Eccentric pith is linked to reaction wood formation and can complicate sawing operations.
Reaction Wood
Reaction wood forms in specific areas of leaning trees or near large branches. It has different color, texture, and properties than normal wood. There are two types:
- Compression wood (conifers)
- Tension wood (hardwoods)
Compression wood forms on the compressed side of leaning trees or the underside of thick branches. Its characteristics include:
- Reddish color
- Higher proportion of latewood and higher specific gravity
- Higher cellulose and lignin content, making it unsuitable for paper pulp
- 10 to 20 times greater longitudinal shrinkage than normal wood, making it prone to warping
- Smaller tangential and radial shrinkage than normal
- Affected mechanical strength, especially the modulus of elasticity in bending
- Greater hardness than normal wood