Understanding Timber Defects: Shakes Explained

Timber is a natural material, and sometimes it develops defects during its growth or processing. Identifying these defects is important in construction and woodworking as they can affect the strength, durability, and appearance of the wood. The question asks about a specific type of defect where there is a longitudinal separation within the wood, occurring between the annual rings. Let's examine the options provided to find the defect that matches this description.

What are Timber Defects?

Timber defects are imperfections that reduce the quality, strength, or usefulness of the wood. They can be natural, caused by growth conditions, or artificial, resulting from improper seasoning or handling.

Analyzing the Options for Longitudinal Separation

• Knots: Knots are formed where branches grow out from the main stem. They are areas where the wood grain is disturbed and can affect strength and workability, but they are not longitudinal separations between annular rings.
• Rind gall: Rind gall is a swelling on the surface of the timber caused by the action of insects or by cutting. It is an external defect and not a longitudinal separation within the wood's structure.
• Shakes: Shakes are separations or fissures within the timber that run along the grain, typically between the annular rings. These are indeed longitudinal separations. There are different types of shakes, such as heart shakes, ring shakes (or cup shakes), and star shakes, but all involve this fundamental separation. Ring shakes, specifically, are longitudinal separations that follow the line of the annual rings.
• Twisted fibers: Twisted fibers occur when the wood fibers do not run parallel to the main axis of the trunk but instead grow in a spiral direction. This defect affects the strength and can cause warping, but it is a twisting of the grain, not a separation between layers.

Based on the definitions, the defect described as a "longitudinal separation of woods between the annular rings" is clearly identified as Shakes.

Summary of Timber Defects

Therefore, the defect that causes longitudinal separation of woods between the annular rings is known as Shakes.

Revision Table: Key Timber Defects

Additional Information on Shakes and Timber Defects

Shakes are a significant defect as they can weaken the timber's structural integrity and make it difficult to cut and finish. There are different types:

• Heart Shake: Starts from the pith (center) and extends outwards radially. Caused by shrinkage of the heartwood.
• Ring Shake (or Cup Shake): A separation that follows the line of the annual rings, often occurring between two growth rings. This is the type specifically referenced in the question. It can be caused by stress during growth, strong winds, or frost action.
• Star Shake: Starts from the bark and extends towards the center in a star-like pattern. Usually caused by rapid drying of the outer layers, especially in felled timber exposed to hot sun or frost.

Understanding these different types of timber defects helps in selecting the right quality of wood for different applications and knowing how defects might affect its performance.