Whether you are considering building your own wood furniture or creating decorative carvings, you need to think about knots in the wood. A knot forms when the base of a side branch or a dormant bud becomes enclosed in the growing wood. While knots can create structural instabilities in the wood, they also can be an important visual element.
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An encased knot is caused by the tree having grown around a dead branch. There is likely a dark ring of bark around the eye of the knot, and because of this, the knot might easily slip out of the wood. These can be attractive, but you should avoid them when you are looking at structural supports because of the potential for the knot to slip out of the wood.
An intergrown knot forms around the living base of a tree branch. The wood of the knot still alive when the tree is cut, and typically you will see that it is surrounded by bands of new growth. These knots are attractive and can be used in many types of building and carving because they are secure. An intergrown knot is also called a tight knot because of how well it is secured to the wood around it.
If the board is cut along the length of an encased knot rather than through the cross section, it's called a spike knot. Instead of being a round knot, it's a thick bar. This type of knot can be attractive, but the newer the growth around it, the more loosely the spike knot can sit. In many cases, a spike knot will be secure at the base and loose at the edge of the board.
A cluster knot is a set of two or more knots that are not part of the same original limb. They are divided by wood fibers. It is considered a cluster knot only when the knots are small and spaced close together. Cluster knots are not allowed in header and beam grades of wall logs and can impact the final stability of the wood.
When the wood inside the knot is soft, the knot is likely to be decayed. This means the knot is going to slip out of the wood soon. If this is the case, the best course of action might be to slip the wood out and fill the knot hole. In cases where a large knot takes up a central place in the wood, be careful because the wood might splinter around the knot.