What is Shou sugi ban siding?
Shou sugi ban is an ancient Japanese method of burning the surface of wood to preserve it. The final product is also known as Yakisugi (yaki means to cook/burn and sugi is the Japanese name for cedar), but shou-sugi-ban seems to have won as the most commonly used term in English, and it is sometimes simply known as carbonized wood.
Usually done with planks of cedar or larch, the thin film of carbon that is created during a very light surface burn protects the wood. This draws out moisture, and the resulting chemical compound protects the wood.
Cedar and larch are local, very durable and resistant to rot. This process can also be done with other common woods such as pine, hemlock, maple or oak.
How carbonized wood is made:
Wooden boards are burnt on each side, which is usually done with a blowtorch. Using a stiff brush, carbon residue is then removed from the boards and the material is rinsed with water.
Once a board is dry, a natural oil is applied to seal it. Colours usually range from a very rich natural wood colour to a deep black. The final appearance is determined by the species of the wood, the extent of the burn and how vigorously it is brushed. The colour can be further manipulated by using stain instead of oil.
- Durable and long lasting: the life expectancy of shou sugi ban is estimated at more than 80 years when properly maintained.
- Life-cycle: Wood is a renewable resource and no chemicals are required for finishing. We would be remiss not to acknowledge the fact that fossil fuels are required for the burn process, but the ensuing lifespan makes that sacrifice negligible.
- Aesthetics: You can get a variety of beatiful colours out of it depending on how deep you burn and how deep you brush, to further alter or enhance colours - and it can be stained as well.
- Water resistance: The combination of carbonized wood and oil makes the board resistant to water and mould.
- Fire resistance: Believe it or not, burning wood can give it flame retardant properties. Who knew?
- Insect resistance: Termites and other problematic insects hate burnt wood.
If the intended use is to have it outside and exposed to weather (cladding, fences, decking), it should be oiled about every 10-15 years, which is fairly standard for exterior wood treatment. It will retain its colour better and will remain water-resistant longer if it is well-maintained.
For interior purposes where it will mostly be seen and not touched, it will require virtually no maintenance.
When used as furniture, the required maintenance would depend on its use - a coffee table may take more of a beating than other pieces. But maintaining oiled wood is pretty simple; it takes little more than a quick hand-sanding if necessary, and wiping on a single coat of oil.
Shou sugi ban and torrefied wood:
There is a related wood treatment called torrefied wood or thermo-modified wood, where milled planks are baked in a high-temperature oven. Virtually all the moisture is removed during this process, which can change the crystalline structure of cellulose. This makes the wood more dimensionally stable and resistant to fungal growth but, at that point, it is less resistant to impacts and abrasions. Also, it is the charring of shou-sugi-ban that gives it its protection against UV rays. With torrefied wood, some of those advantageous characteristics are lacking; additionally, stains and oils do not adhere to it as well after processing.
How to make shou sugi ban wood:
In order to obtain a consistent-looking final finish you need to be very precise. The torch needs to be passed at the same speed and held the same distance from the wood to get a uniform burn. This can also be done using coals from a fire, but this will likely result in a far more inconsistent final finish.
Still important to be precise but not as easy to screw-up, would be brushing off excess carbon. Keep an even pressure on the brush, and always go with the grain. This stage may actually help you bring boards back to a desired colour if you over-burned them a bit in places, by brushing a little more vigorously.
When oiling the wood, spread it evenly over the surface with a rag or brush (going with the grain of course). Wipe off the excess and allow it to dry; a second coat of oil at this point is recommended.
You'd need to be okay with slight variations in colour and consistency if you want to try this yourself, but if you really want it uniform in colour, it is best purchased from a professional manufacturer.
So, this process is not complicated but should be done with safety in mind; remember you're playing with fire here. Keep some water close by and use goggles and a respirator when brushing and oiling boards.
© Shou-sugi-ban used for interior finishing © Zwarthout.com