Manufacturing Process

The means to produce yakisugi involves several steps outline as follows:

• Log Selection: Harvested trees are reviewed and graded for quality, straightness, and size (diameter). Grading quality The wood that is used for Japanese yakisugi is “Cryptomeria japonica,” which is commonly known as Japanese cedar or sugi cedar. These trees grow abundantly throughout Japan with planted forests in the southern part of Hokkaido and in Okinawa in the northern part of the main island. Sugi cedar has several characteristics, making it particularly suitable for this product while other species tend to fall short. In particular, sugi cedar is well-suited for the intense heat mineral-rich fiber. Species such as pines, larches, or firs have more pitch and less minerals, so they have a greater tendency to check when charred and are not as durable. Other species of cypress, cedars, or red- woods may have good durability but when subjected to the charring process, they do not retain the beneficial soot layer as long as the sugi cedar does. Japanese or sugi cedar has a number of additional desirable characteristics, ultimately leads to boards that include standard select (some knots) or premium clear (no knots). Straightness of the logs helps with the internal integrity of the wood and the final grain of the boards. Size determines the yield of the log into different sizes of boards.
• Rough Milling: The graded logs are fed into milling and cutting machines to create rough moulder blanks. Their size will vary depending on the need, but they are the rough boards that will eventually become the finished planks.
• Drying: Once the rough blanks are produced, they are then stacked and separated (e.g. “stickered”) so they can be air-dried. Some wood siding manufacturers use kiln drying over several days at low heat. However, that is more energy intensive and may or may not produce the best results compared to air drying which can slowly and completely dry the wood.
• Finish Milling: Once dried, the rough boards are ready for their final profile milling. Widths can vary by standard lumber sizes (i.e., nominal 4”, 6”, 8”, etc.). In Japan, the standard thickness is 3/8” (10mm), but 9/16” (15mm) is also available in the U.S. since that is as thick as can be heat treated with good yield. Heat treating a plank thickness over 5/8″ (e.g., 3/4″) by the traditional Japanese charring process doesn’t work well since the interior moisture content will stay around 12% while the surface moisture content goes down to 0%. This results in a higher incidence of crooking, warping, twisting, or checking. Milled profiles typically include shiplap or square-edged shapes. Tongue and groove profiles are not produced since they are not recommended for good weathering of the wood.
• Heat Treatment: Once fully dried, the boards are ready for heat treatment. This step is done in a kiln, preferably engineered to use an afterburner so natural gas consumption is minimized and carbon emissions are reduced. Enough contained heat is applied to the boards so that the surface hemicellulose wood fibers are burned off. The result is an inky black, thermally modified surface. Note that all yakisugi is burned deeply to heat treat the wood. Cosmetic “shallow” or “light” burning is similar to aburi, a different traditional Japanese wood darkening technique done with a torch on furniture or interior millwork. This carbonizes the less dense earlywood growth rings only, not the dense latewood.
• Surface Treatment: After burning, different surface treatments can be used to achieve varied appearances. There are three common choices. First, the sooty, textured surface can be left alone (i.e., unbrushed) for protection and character. Unbrushed yakisugi generally takes 40-50 or more years for the black color to weather in a gradual erosion process into looking like brushed yakisugi. It will start black then gradually turn various colors over this period and may or may not stay black. Second, the boards can be wire- brushed lightly for a smooth and more refined appearance exposing the grain while keeping a moderately dark color. Third, it can be further wire-brushed to show more of the grain texture and color producing a distinctive, topographic texture that is lighter in color than the other two options.
• Finishing: Yakisugi does not require any additional finishing or oiling and historically oiling has not generally been done. However, most yakisugi in modern construction has an oil finish applied during manufacturing that bonds the soot onto the wood surface so that it does not come off when touched. Additionally, the boards will change color over time as the charred surface naturally erodes and creates a patina on the wood. Some designers or building owners prefer to have a particular color secured and choose to have an oil or stain added to the surface. This can be done by a manufacturer as the boards are produced, in the field during construction, or several years later by the building owner when the desired patina color is achieved.

Photo by xoMe Photography

The use of yakisugi wood in buildings can be creative, dramatic, and sustainable.

Photo courtesy of Nakamoto Forestry North America

Properly manufactured yakisugi produces very straight, durable, and hardened wood planks that can be used for exterior and interior applications.

Understanding the manufacturing process above, there are different ways to tell heat-treated yakisugi vs “faux” finishes that are lightly burned or embossed. One is the striped grain patterns that are negative images of each other on the surface of the wood. Another is how surface checking from full burning has a different texture than lightly burned, although a solid coating can conceal that. Such faux finishes can be appropriate for interior uses, but on exteriors, they may weather quickly or have unknown aesthetics over time.

DESIGN CONSIDERATIONS

With an understanding of the yakisugi products, we can turn our attention to the different ways they can be used in a design, whether for exterior or interior applications on residential or low-rise commercial buildings. A fundamental design choice will be the orientation of boards related to their use. Yakisugi planks can be installed as exterior siding in a horizontal, vertical, or diagonal pattern whether across an entire facade or broken up into different sections with different patterns as appropriate to the building design. The same is true for interior paneling and horizontal surfaces; the pattern can be at the discretion of the designer.

The next fundamental design decision is the finished look of the boards. That starts with the three different surface treatments. Is the dark look of the full charred wood desired or one of the wire-brushed options? They each have their own aesthetic appeal and can be used singularly or in combination. In Japan, the dark, charred siding is often used in conjunction with white stucco on different portions of a building facade. Other designs may be more suitable for the less dark or even the lighter appearance with bold wood grains.

Finishing Choices

The choice of using an oil pre-finish or adding oil later on comes down to the personal preference of either the building owner or the designer. Historically 80% or more yakisugi was unoiled/ untreated while in modern construction about 80% is oiled. If left untreated, weathering from the elements leaves a patina, or what the Japanese call a “wabi-sabi” aesthetic. While this concept can be different than Western views of aesthetics, it is born of a cultural sensitivity that values things as they age. The “wabi” refers to a beauty from variations or imperfections in the world that offer their own contributions just as they are. The “sabi” refers to the passage of time and change. Wabi-sabi is described as finding the beauty behind something, its atmospheric feeling, not just its external appearance. There is an element of loss, of transience, but also abundance and transformation. This traditional cultural view leads to an aesthetic for the wood, which is ever-changing, accepted, and embraced.

Photo courtesy of Nakamoto Forestry North America

Traditional Japanese use of yakisugi often uses unfinished charred wood which erodes and patinas over time and is embraced as a “wabi sabi” aesthetic. It is often used in conjunction with other materials such as white stucco.

Photo courtesy of Nakamoto Forestry North America

Traditional Japanese use of yakisugi often uses unfinished charred wood which erodes and patinas over time and is embraced as a “wabi sabi” aesthetic. It is often used in conjunction with other materials such as white stucco.

If a different appearance is sought, there are also several dozen other different aesthetic options (as many as 32 from at least one manufacturer). Any of the surface treatments (charred, lightly brushed, or double brushed) can be left unfinished and allowed to patina to a color or appearance that is desirable or finished with a coating. Color change can also be slowed down by applying oil finishes at the outset. All products can be re-oiled at any time in the future to freshen up the color and sheen as desired. There are no fast rules for how wood weathers in color since every site has different conditions. However, in general, over time, siding tends to mellow reddish brown on south-facing elevations and bleach to a gray tone on north-facing elevations.

What are the reasons to consider oiling the wood? First, to achieve the particular color the designer or owner wants with a pigment. Secondly, preserving the look longer than untreated wood. Third is to glue in the residual soot as part of the finished surface.

Note that unoiled wood will have better fire resistance since there will be no combustible oils present. Eliminating the oiling also saves money offering better overall cost performance. Nonetheless, some alkyd and acrylic products can slow flame spread.

Ultimately, the finish decision needs to be based on whether the wood is being used in an interior or exterior application. The primary differences are that exterior coatings must have a fungicide to mitigate mildew, they almost always contain visible and invisible pigments since pigments are the primary UV inhibitor, and they must be more highly elastic to compensate for expansion and contraction of wood during temperature and moisture content fluctuation. Exterior coatings are generally single or multiple coats of the same formula to increase the coating thickness and durability since oil finishes erode over time (instead of flaking like paints). Interior coatings are usually a stain coat followed by a hard protective topcoat. Exterior coatings commonly contain elastic urethanes while interior coatings contain rigid polyurethanes.

There are several types of exterior coatings that range from solid color (longer lasting) to transparent color (faster-fading formulas). Acrylics are solid coatings with the best color longevity, but they completely hide the wood color. Alkyds are semi-solid coatings that can be applied in multiple coats to increase opacity. Next in durability are tung oils and some linseeds, which are semi-transparent and show more wood grain. Finally, transparent linseed oil formulas act as a lens that enhances the wood grain; however, they allow the wood to mellow or bleach from UV exposure. Designers can, of course, select the level of opacity or transparency to match an exterior design, but that means a decision is also being made at the same time on color longevity. Less color usually means more frequent maintenance and re-finishing. Totally clear coatings are not recommended on exteriors since they don’t typically meet all of the requirements and need very frequent (12 months or less) reapplication.

COMMERCIAL PROJECT EXAMPLE #1

Photos courtesy of Nakamoto Forestry North America

Project: Gladys Valley Marine Studies Building
Location: Newport, Oregon
Architect: Integrus Architecture

Oregon State University’s Gladys Valley Marine Studies Building is a 47-foot-tall, 30,000-square-foot academic and research facility. Located on the coast of Oregon in a tsunami inundation zone, the building is designed to withstand a 9.5 magnitude earthquake. With twice its height below ground, the building incorporates state-of-the art architectural and engineering techniques to meet significant resilience goals. It also serves as one of the first vertical evacuation tsunami sites in the U.S. Yakisugi was chosen for the exterior cladding due to its resilience to the region’s wet and windy coastal climate. The majority of the cladding consists of yakisugi that has been brushed once to remove some of the soot layer and finished with acrylic black stain for maximum color longevity. As an accent, the design also incorporates yakisugi that has been brushed twice to remove most of the soot layer and then finished with a silver stain. This provides tonal and textural contrast, creating a visually striking exterior. The choice of yakisugi also reflects the institution’s deep respect for nature and the region’s marine heritage.