WoodSense: Thermally-Modified Wood

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This article is from Issue 101 of Woodcraft Magazine.

Thermo-Woods, hard and soft. Thermal modification works with most hardwoods and softwoods. Three of the more commonly available species include southern yellow pine, poplar, and ash.

Roasted through. The thermal modification process imparts a brown cast throughout a board, though some species get darker than others. Thermo-poplar for example resembles black walnut whereas soft maple retains hints of its original lighter hue.

Roasting wood for durability, stability, and darkened good looks

In the last decade or so, the lumber industry has developed a process that takes wood a step beyond kiln-drying: thermal modification. This process makes wood highly rot-resistant without having to infuse it with toxic chemicals as is the case with pressure-treated lumber. Before lumber can be thermally modified, it must first be properly kiln-dried. Depending on the species and type of kiln, this includes the wood being heated to between 140 and 160° F for a specified length of time. For the thermal modification, the wood is transferred to a specialized kiln where it’s heated to between 300° and 325°F, in essence roasting it. This process converts the sugars and starches within the wood’s cells into a caramelized substance that is no longer palatable to the fungi that cause wood to decay. Also, the cell walls become crystalized, preventing the wood from absorbing water, which helps to eliminate the second condition necessary for rot—a moist environment.

History in woodworking

Thermal modification isn’t a revolutionary idea. The Vikings, for example, were scorching the hulls of their ships hundreds of years ago to thwart rot and to keep barnacles and other marine life from hitching a ride. Native Americans also used fire to harden and cure their arrows and other wooden tools they used for daily life. And in the 18th century, the Japanese developed what they call shou sugi ban, a technique of preserving wood by charring it with fire. The main difference is that, instead of fire, the thermal heat-treating process involves computer-controlled roasting, with specific programs developed for each wood species being modified.

Availability

While nearly any wood can be altered in this way, the more commonly available thermally-modified hardwoods are poplar and ash, while softwoods include spruce, and southern yellow pine. Much of this material is milled to standard sizes for use as siding and decking. Southern yellow pine is also available as 2× stock. Rough sawn 4/4 and 5/4 stock in widths to 15" and lengths to 12' are generally available, ranging in prices from $5 to $7 per board foot. Modifying thicker lumber, which is difficult due to its deeper internal moisture, and usually results in internal checking.

Working and finishing

Thermally-modified wood is 75%-85% more stable than regular wood, making it less susceptible to expansion and contraction with changes in relative humidity. This stability, along with its rot-resistance, makes thermo-wood an excellent choice for exterior applications. Even with direct ground contact, the wood has a 25- to 30-year life expectancy. One popular application is its use in making raised garden beds. Despite the wood’s constant contact with soil, no chemicals will leach out contaminating your vegetables.

There are four levels of modification with #1 being the lightest and #4 the heaviest. The more modification, the more rot resistance, but at the cost of degrading the wood’s strength. The third level treatment is a good compromise. However, any level of modification leaves the wood more brittle and prone to splitting. Specialized load charts are available for structural situations. And while the wood is rot-resistant, it is still subject to ultraviolet (UV) degradation from the sun. For a “natural” look, apply a UV protective coating. (See page 26.) Use an oil-based primer before painting thermo-wood. Water-based primers do not adhere properly because of the changes to the wood’s cellular structure. When gluing, leave the clamps on longer than usual – overnight is best. Titebond II works well, as do two-part epoxies.

Thermally-Modified Uses

  • Outdoor furniture 
  • Decking
  • Siding
  • Raised garden beds

A pin-striped table. In ash and other open-grained woods, the thermal-modification process really enhances the contrast between the early and latewood as seen in this unfinished table. Sanding to a higher grit produced the color difference between the top (120 grit) and legs (220 grit).

Dark and dusty

Having worked with several species of thermally-modified wood, both for this issue (see page 31) as well the table below and other projects, I have several observations. First, it’s much nicer to work with than pressure-treated lumber. Not having to worry about the preservation chemicals is delightful, and the stability is astounding. It just doesn’t warp. Kerfs don’t close up at the table saw, workpieces stay flat after planing, and they don’t shrink after installation. On the downside, the wood is quite dusty and crumbly when cut. Crosscutting produces ragged corners on both faces. And chiseling across the end grain requires very sharp tools or else the grain tears. Regardless of species, the boards seem harder than their unmodified counterparts. I noticed this hardness in particular as I turned the bowl on the opposite page. But the wood sands well and, afterward, the surfaces feel like burnished leather. Perhaps the most remarkable trait is the odor. When handling, cutting, and sanding thermally-modified wood, it smells strongly of burnt toast or an old campfire. I don’t find the smell unpleasant, but it does linger. As I write this, I can still tell I was working with thermo-wood yesterday even after several hand washings and a shower. 

—Ken Burton, senior editor.

About the Author

Jeff Schucker owns Bailey Wood Products in Kempton, PA, a full-service wood distributor, sawmill, and mill shop started by his great grandfather in 1928.

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