What is Shou Sugi Ban, how it is made and why burnt wood is resistant to fungi, insects, moisture and fire

An old method of protecting and highlighting wood design is burning. It has been practised in various parts of the world, but it was the Japanese carpenters who perfected it. Shou Sugi Ban - burnt cedar plank, in translation - is a technique in which wood is burnt, brushed and cooled with water so that its surface is covered with a layer of charcoal. The water cooling is done in a controlled way so that the charcoal layer finally cracks in a certain pattern that gives the plank a particular aesthetic. But the main benefit of this burning is the very good resistance to attack by fungi, insects and fire. Exterior cladding, fences, shinglewood subjected to this technique lasts 80-100 years without maintenance. Find out why burnt wood is so durable below.

What happens when the wood surface is burnt

To understand why burning protects wood, we need to start with its main chemical compounds: cellulose, hemicelluloses and lignin.

Cellulose is the most important structural substance because it forms the wall of the wood cell, which determines its main physico-chemical properties. It represents 40-45% of the mass of anhydrous (completely dry) wood. Cellulose decomposes at over 150°C, a property on which alcohol is obtained by dry wood distillation. It is a food source for insects and micro-organisms and is very fond of water.

Hemicelluloses are very similar to the cellulose they accompany in the formation of the cell structure. Some are linked to cellulose, forming together the skeleton of the plant, others are linked to lignin. They account for 20-40% of the mass of anhydrous wood, are more unstable than cellulose, decompose faster, are a food source for micro-organisms and have a high affinity for water.

Lignin is a complex, amorphous chemical substance. It surrounds the cellulosic skeleton giving the wood rigidity and mechanical strength. The percentage of lignin in the mass of completely dried wood is 15-35%. Lignin is water resistant, transferring this property to the cell walls.

When the surface of the wood is burnt, the cellulose and hemicelluloses decompose and the food source for insects and fungi disappears. The percentage of lignin on the surface increases, leading to increased water resistance. Burning takes place in stages, with the temperature gradually increasing as the rapidly decomposing elements disappear. The first to burn is cellulose. During its burning, lignin gradually turns into charcoal. It needs a much higher temperature to burn completely and so the rest of the wood is protected until the charcoal burning temperature is reached.

Stages of the Shou Sugi Ban technique

The Shou Sugi Ban or Yakisugi technique, as it is still known, was used a long time ago, but flourished in the 17th century. At that time it was not only used to protect houses or fences but also to create real works of art. The advent of modern wood protection materials led for a time to the abandonment of the method. But it made a comeback in the 2000s, mainly thanks to designers and architects who combined the old method with modern materials to great effect.

The classical method has the following steps:

• Shallow burning using a torch or gas burner. Burning is done carefully so as not to char the whole wood but only its surface. The wood must be burnt, not scorched.
• Extinguishing the fire and cooling the surface with water.
• Brushing to remove soot and loose parts.
• Apply a layer of linseed or tung oil for increased strength and good looks.

In order for the wood to be strong on the outside it must be covered with the resulting charcoal. Such wood can last 80-100 years without any maintenance. Removing the burnt material completely loses its protective properties and only the aesthetic appearance remains. Controlled cooling (pouring water in a thin thread, dripping or pouring in large quantities) leads to the charcoal layer breaking up in various different patterns that enhance the aesthetic effect.

The main advantages of the method are very high durability and low cost. There are wooden houses in Japan finished on the outside using this method that are more than 100 years old, have not undergone any maintenance or repairs in all that time and are in very good condition. Another advantage is fire resistance.

Disadvantages include the laborious process that requires knowledge and patience, the unpleasant finish for some due to the charcoal coating, the risk of fire or injury from the open fire and steam released when cooling with water. Also, the impossibility of repeating the effect, obtaining a different coal-burning pattern each time, can also be considered a disadvantage.

Wood species suitable for the Shou Sugi Ban technique

The established species of the technique is cedar, but the method can be applied to most wood species. Some say that in this case the "cedar" (sugi) in the name should be replaced by the name of the species used. I think this is an exaggeration.

If we are referring to the application of the method to achieve different effects, not primarily to achieve durability, tangentially cut (cathedral, flader) softwoods look best. Spruce, larița, pin look great when burned, brushed and then stained. So does Accoya and Kebonyspecies which, due to chemical impregnation, have a high durability (30-50 years). The design is very well highlighted by burning, and the colouring comes in contrast to the black outline. You can see here examples of such finishes.

I hope you find the above information useful. As usual, additions are welcome. And if you have any questions or queries, please leave them in the space below. I'm sure I'll reply.