Wood features - Wood processing techniques

Knots - how they occur and how they affect wood

Knots are a common characteristic of wood. They are easily known and recognised. But what are they, how do they occur, how many kinds are there, how do they influence the characteristics of the wood, how do they influence the finish? These are questions that are answered below.

photo source: diy.persiamag.com

What are knots

Knots, along with staining, cracks, resin pockets, are part of the category wood defects. Aesthetically they are not always perceived as a defect. For example, rustic furniture is even more appreciated if it has knots. Grhinzile, patch, the polishes feel even more natural when they have knots. Knotted wood also gives cabins authenticity and a special charm.

How do knots appear? Either they are the base of a branch that has stopped growing and has fallen off, or they are shoots from which branches were to grow but have stopped growing. Where the branch grows, the woody fibres in the trunk have a deviation in the direction of growth that can be as much as 90°.

Even if the branch disappears for various reasons, the deflection remains and is covered over time by the woody mass materialised in annual rings. Thus, the fibre deflection is contained within the tree and appears in the form of knots when the tree is cut and cut into planks. As a result, trees with dense branches will have more knots and those with sparse branches fewer.

photo source: arroway-textures.ch

Types of knots

There are two types of knots: fixed knots (also called growing or healthy knots) and dropping knots.

Fixed nodes come from a healthy branch that lived on the trunk for a long time before disappearing.

photo source: hobbithouseinc.com

The nodes occur when the branches break during dry, droughty periods. During these periods the tree reduces its water absorption capacity, the tissues contract and the knot becomes detached from the other woody layers.

photo source: hobbithouseinc.com
photo source: hobbithouseinc.com

Quite rarely is a third type of knot mentioned - rotten knots(unhealthy) - occurring when branches break during periods of very high humidity. The knot then becomes infected with fungi that attack the wood fibre.

Influence of knots on wood quality and finish

The technical properties of wood are affected by the presence of knots. The strength of the wood decreases at that point and the likelihood of splitting along the grain increases. If wood is used in construction, existing knots need to be carefully analysed to see how they affect its strength.

Knots influence the quality level of the timber by their number per square metre, shape, size, strength and type (fixed or falling). In the past, knots were replaced in production by twigs so that the appearance did not differ too much. Now only the dropping knots are replacedbecause they dry out, fall off and a hole remains in their place. In healthy knots, cracks can appear during drying due to the stresses created by the water outflow. Before finishing the cracks must be grouted. If the furniture is rustic, the knots may remain unglazed, the cracks increasing the value of the furniture.

Wood of resinous has a higher number of knots due to the specificity of the trees. The branches are thicker, the light penetrates hardly to the base and the branches at the base dry out and fall off as the tree grows. The presence of knots in softwood timber brings another problem. Through knots, the resin in the wood can come out even after the wood has been finished. The process can take months, even years. That's why if the knots don't come out the surface must be treated with an insulator that locks the resin inside.

photo source: directwoodflooring.co.uk
photo source: ronseal.co.uk
photo source: dealry.co.uk

The article is not an advocacy against knots, quite the contrary. This extraordinary material that is wood turns flaws into qualities. Twisted grain, staining, growth defects are a few examples. So are knots.

About the author

Mihaela Radu

Mihaela Radu is a chemical engineer but has a great passion for wood. She has been working in the field for more than 20 years, wood finishing being what defined her during this period. She gained experience working in a research institute, in her own company, as well as in a multinational. She wants to continuously share her experience with those who have the same passion - and more.


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  • Therefore, if the knots do not remove, the surface should be treated with an insulator that locks the resin inside.

    What insulator? Doesn't normal primer do that?

    • Normal primers don't do much to lock the resin inside. If there is a large bag of resin it is best to remove it because in summer when it is hot it expands and pushes the finish cracking it.
      Best insulated with polyurethane insulation. It is fluid, it penetrates deep into the wood and rays inside blocking all substances that would interact with the finish (tannins, stains, resin). There are also water-based insulators that do a pretty good job. These variants are for small bags of resin. Large bags should be removed to avoid problems.


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