What Is Thermally Modified Wood?
Simple answer: Thermally modified wood is essentially identical to any kiln-dried lumber you may already have. To remove the free water from the wood, green (wet) wood is placed in an oven, and the temperature is increased in accordance with standard kiln drying schedules. Throughout this procedure, fluid is applied to control drying and guard against harm to the wood fibers.
Once all free water has been removed, the kiln temperature is then increased to above 200 Celsius, which is a significant increase over the usual kiln drying temperature for timber. At this high temperature, oxygen levels in the kiln are lowered to prevent combustion, and the bound water inside the cell walls is baked away. This includes any hydroxyl groups that might later interact with moisture. The extractives (sugars, oils, resins, etc.) also bake off at temperatures above 200 Celsius, leaving the wood entirely dry and rot-resistant. The hemicellulose and sugars have the ability to crystallize and become entirely inert.
In order to prevent cell collapse and warping, moisture is then reintroduced as the oven temperature is lowered. This allows the wood to adjust to the surrounding air and humidity (also known as “case hardening”). This final procedure virtually eliminates your typical wood movement that is caused by seasonal moisture variations, and this process in the end leaves the wood extremely stable.
The hydroxyl group negative ions (found in typical kiln dried lumber) would be normally just waiting for a water molecule to join them so they can build new water-bound molecules deep inside the cell walls. However, because the high kiln temperature baked away these hydroxyl group negative ions, the regular expansion and contraction that one would typically perceive in wood is gone with the absence of these ions.
As with any wood, there is some early movement and typical warping during the last process of conditioning in the kiln. The important thing to remember is that the wood is essentially stable after that initial warping and re-humidification. The seasonally low tangential and radial movement is now so small (0.1%) that it is insignificant. The cellulose and sugars are baked to increase the wood’s hardness, which is what gives the color its black hue. The same species of wood that was first placed in the kiln is still present, with just a little reduction in bending strength as a result of internal hardness and a decrease in weight due to the absence of moisture.
All of our thermally altered wood products are created by GWood Pro and begin as FSC timbers. We are an exclusive distributor of GWood Pro and its parent firm TanTimber’s thermally modified timbers (TMT). TanTimber is a member of the International ThermoWood Association and complies with all of the strict requirements set forth by the organization to guarantee constant product quality.
Thermally Modified Decking & Siding
Thermally modified decking & siding is made of 100% wood, just as the name implies; it is just dried differently, but nothing else is added. The best external material for decking and siding is thermally modified wood. The thermal modification method enables us to employ much more widely distributed (and reasonably priced) FSC certified hardwood and softwood species like Ash, Oak, Iroko, or Pine in place of tropical hardwoods or naturally rot-resistant species.
Drying Ash, Oak, Iroko, or Pine in a controlled kiln atmosphere at even higher temperatures than normal changes the cell structure to create wood that is dimensionally stable and pest/rot-resistant. After drying, the wood exhibits almost the same working qualities and takes on a darker, roasted golden brown tone. Even if the seasonal movement problems are gone and all the sugars and water have been baked out, the wood is still the same species; as a result, bugs don’t bother it and water beads up and drains off. In other words, the wood is the same but now even better.
It is a method that was used by the Vikings and Japanese woodworkers in the far east thousands of years ago. ThermoWood is now the industry standard for assuring quality control and consistency throughout the drying process, after the process was perfected in the 1970s. Siding or decking that has undergone thermal modification is an excellent option for exterior products that will be subject to abuse.
Thermally modified decking and siding will perform well on oceanfront properties or those that are frequently exposed to the sun, because they can withstand the elements well and be finished to maintain their color or allowed to weather to the wildly popular silvery gray that we have come to expect with current exterior favorites like Ipe or Teak. Thermally modified lumber can be used even in ground contact conditions or in close proximity to water, such as docks and poolside decks, unlike some of the other popular species.
Thermally Modified Ash Wood
If you have ever worked with White Ash, you are already familiar with what thermally modified Ash is like. Just without the seasonal movement, it nevertheless retains the same functional characteristics. Although it is technically somewhat harder and less dense than conventional kiln-dried Ash, these differences are not noticeable when working with it. It can still be machined, connected, and finished like Ash using glue, nails, and screws. The color change is the most noticeable. Regular kiln dried Ash has the appearance of white wood, but after thermal modified, the wood now has a deeper brown tone. If desired, it will still gradually weather to a silvery gray, but the natural hue can be preserved with annual finishing and maintenance.
Thermally Modified Ash Decking Sizes
The Ash wood can be pre-finished to order, sanded, or machined for hidden fasteners.
- 7/8″ x 3.5″ x 5-14′ long (21 x 90 x 1500-4200 mm)
- 7/8″ x 5.1″ x 5-14′ long (21 x 130 x 1500-4200 mm)
- 1″ x 5.1″ x 5-14′ long (25 x 130 x 1500-4200 mm)
- 1″ x 6″ x 5-14′ long (25 x 150 x 1500-4200 mm)
Additional sizes, thicker thicknesses that can be bonded up, and a variety of custom milled profiles and finishes are all available. Call us at (800) 638-9100 for further information.
Thermally Modified Ash Characteristics
| Character | Thermally Modified | Change from KD |
| Bending Strength (MOR) | 1,722,600 lbf/in2 | -1% |
| Stiffness (MOE) | 12,000 lbf/in2 | -20% |
| Hardness (Janka) | 1325 lbf | +1% |
| Specific Gravity | .55 | 0% |
| Weight | 33.6lbs/ft3 | -20% |
| Density | 554 kg/m3 | -12% |
| Radial Shrinkage | .1% | -2% |
| Tangential Shrinkage | .4% | -5% |
