DIY: Burning Shou Sugi Ban by the Traditional Japanese Flue Method

The Setup: Research, Preparation and Location are key

One of the most interesting things about shou sugi ban is the transformational process used to make it–burning carefully prepared planks with intense heat just enough to enhance their durability properties but without ruining them. Even though we live and breathe shou sugi ban every day, nowadays it is mainly a high-volume automated manufacturing process and we’ve never had the chance to make it ourselves using the traditional Japanese method. We’ve also been simply repeating what the Japanese side told us for our educational program in the West: that proper heat treatment takes a lot more BTUs than simply burning planks with a roofing torch. A couple of years ago we had some air-dried 1x8s shipped over from the Tokushima mill to practice burning wood for fun and to see what we could learn from the manual process. 

After years of anticipation we were finally able to try burning sugi with the traditional Japanese method ourselves earlier this month. We found the process fun, rewarding and we learned a lot from spending a day getting our hands dirty.

Sugi Shavings and Triangle Flue
We used a hand plane to prep a box full of sugi shavings to use as kindling
Binding Sugi Boards Together to form Flue
We used wire to bind the Sugi boards together in a triangular flue, but twine or other materials should work fine too

There were two hurdles that caused the delay in making this happen: a safe location and guidance from Japan during a pandemic. I’ve heard from a couple of people that the most difficult part of making shou sugi ban is finding a safe location to do it, especially in the city. We’re located within Portland metro and didn’t have a good location until recently when one of our employees’ parents finally gave us permission to make it at their farm about an hour east of town. We also wanted our Tokushima mill boss Mr. Itotani (we call him Mr. Yakisugi) to be here when we did it, but after waiting over a year for the pandemic to play it’s course and allow travel we finally gave up on waiting. So in the end, even though the Nakamoto group’s history and technology are core to the yakisugi story in Japan and overseas, our exercise turned out to be an amateur fire drill with a bunch of American weeaboos trying to figure out how to heat treat this stuff without burning it into a charcoal mess or setting the farm on fire. 

The Process:

Early on a clear Friday morning, six of us loaded up 80 boards of unburned sugi stock, an eclectic mix of tools and a bunch of camera gear, and set out to the country. Check out the video recap we made for a look into how challenging it is to make charred wood siding by the traditional Japanese flue technique: 

We ran into several challenges throughout the day, some expected and some not so much. But with some effort we were still able to consistently make good quality shou sugi ban. The most difficult part of the process was getting the wood boards to catch fire quickly and burn evenly–too little tinder and the boards won’t ignite, too much and all of a sudden you’re spending half your day preparing tinder. We also found it difficult to get the edges of the wood boards to burn evenly, since it is tough to get a perfect triangle shape and the flames don’t reach into each acute corner without some manual help. It takes a knack to wedge a flat bar between the edges of the boards and wiggle them around to get the wood edges burnt evenly. Another unexpected lesson we learned through trial and error is that when flipping the flue 180degrees halfway through the heat treatment in order to burn both ends evenly, it must be done quickly and smoothly. Otherwise the flames get starved of oxygen and snuff the fire out, resulting in unevenly charred wood siding.

Triangular Flue DIY Method
Getting the flues to catch fire quickly for even burning was the toughest part of our day
As soon as the boards catch fire, a huge column of dark smoke starts pouring out of the top of the flue
We tried a variety of flat bar types to straighten the boards and open up the gaps while they were burning. This controls the flame and allows the fire to reach all the way to the edges of the boards for even heat treatment
Bringing the sufficiently charred boards to horses for killing and quenching

The Takeaway:

All in all, we learned that making shou sugi ban is not so easy without an experienced hand present, and it took us about half a day of practice to get to the point where our boards consistently had the proper degree of char end to end and edge to edge. Here are some lessons we walked away with:

  • The Japanese flue method creates a contained hot box with fantastically high temperature, a scale more intense than merely burning a piece of wood with a roofing torch
  • It would be a very involved operation to do this at scale. A few hundred square feet would be fun but even with experience and preparation burning thousands of square feet of wood would require an intimidating amount of effort
  • The planks are better milled a little long and then defected afterwards, since the ends of the wood boards easily get over-burnt near the fire at the base
  • The base fire needs good air flow since the long flue drafts a huge volume of air when the fire really starts to roar
  • Gray smoke and flame are good, white smoke is bad (since it means the flame went out)
  • Don’t kill your fire once the plank faces ignite and before the full planks are burnt completely. Since charcoal has a much higher temperature of combustion than wood, getting the fire started again is challenging
  • Since it is difficult, if even possible, to burn the board edges deeply this might explain why most old shou sugi ban we’ve seen in Japan is board and batten. It is logical to put batts over the inconsistently burned edge joints
  • The intense heat causes the boards to warp and cup, so that they need to be stickered under weight to become flat again. This is another reason that the planks must be thin stock material. Thicker material would be more likely to split or check during and after installation, plus it would be more likely to overpower fasteners and move dimensionally when exposed to fluctuating atmospheric conditions over time
  • The back sides of the boards get really hot when heat treated properly (use heavy gloves!)
  • The fire quenches itself as soon as the flue is opened up, and minimal water quenching is necessary
  • Quenching with water does not immediately make the planks lie down flat again, but it probably helps them set straight if quenched with water and then stickered to dry under weight
  • 8-12 foot board length range probably works best simply due to flue handling logistics. Shorter planks are definitely easier to heat treat
  • None of us got burnt and we never lost control of the fire. We also did not get covered in soot as we had expected
Cutting off the wire and laying the boards flat for quenching
Once the boards are laid flat, most of the flame dies on its own. We quenched the boards using a watering can to stop any smouldering
Our very own handmade yakisugi still steaming after the quench!
We got the timing down for wood heat treatment, but still had a hard time getting all of the boards to burn evenly to the edges

Stay tuned as we take on the next step of the process soon: brushing our newly created Suyaki down to Gendai and Pika-Pika!


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