Thermally modified siding and decking is made of pure and natural wood, as suggested by its name. Nothing was added to the wood; however, the drying process was different. The majority of decking and siding manufactured from thermally treated wood is considered to be of superior quality. The thermal modification process enables us to use FSC-certified, more accessible hardwood and softwood species such as Ash, Oak, Iroko, and Pine rather than more expensive tropical hardwoods or other more costly, naturally rot-resistant species. Ash, Oak, Iroko and Pine are significantly more readily available and cost less than tropical hardwoods.
The cell structure of Ash, Oak, Iroko, and Pine can be modified by drying the wood at much higher temperatures in a controlled setting. This produces a wood that is dimensionally stable and resistant to both rot and insects. After drying, the wood has an appearance that is deeper, more of a baked golden brown, while retaining almost all of its original qualities for working with the wood. A board of thermally modified lumber is still the same type of wood, but it does not experience the seasonal movement problems that a board of the same type of regular lumber frequently does. Moreover, all of the sugars and water have been baked out of the wood, which then causes insects to ignore it while simultaneously causing water to bead up and flow off. To put it another way, it is the same wood, but of a higher quality.
The Scandinavians and the Japanese woodworkers of the far East were the first known people to use this technique, which dates back thousands of years. But the ThermoWood technique, which was developed much more recently and was perfected in the 1970s, is today considered to be the gold standard for assuring quality control and consistency throughout the thermally modified lumber drying process. Decking or siding that has been thermally treated is an excellent alternative for external wood construction that will need to be able to withstand significant wear and tear.
This type of wood can withstand the elements and can be finished to maintain its color or left to weather to the immensely popular silvery gray that we have come to expect with current exterior favorites like Ipe or Teak. Oceanfront properties or those that are regularly exposed to the blazing sun will perform well with thermally modified decking and siding. In contrast to some of the other species that are now popular, thermally treated lumber can even be utilized in applications where it will come into touch with the ground or with water directly, such as docks and poolside decks.
Thermally Modified Ash Lumber
If you are familiar with White Ash lumber from previous work projects, then you already have a good idea of what to expect with thermally modified Ash wood. You don’t have to be concerned any more with the seasonal movement issues typically experienced with regular Ash as the thermally modified version of Ash no longer demonstrates that kind of movement, though it otherwise possesses the same functional beneficial qualities of its species.
Although it is technically somewhat harder and less thick than typical kiln-dried Ash, one seldom perceives these differences when working with thermally modified Ash, because these differences are so minute. It is still possible to grind it, connect it, glue it, nail or screw it, and finish it in the same manner as regular Ash wood. The color change makes the most significant distinction. You no longer have the light-colored appearance typical of Ash wood; rather, you have a darker brown tint. It is possible to finish it and maintain it annually in order to keep its natural color, but if that is not what you want, it will still fade to a silvery gray.
Thermally Modified Ash Decking Sizes
The decking lumber may be sanded, machined for hidden fasteners, or pre-finished according to the customer’s specifications. Other sizes are available for use as cladding or in glued-up thicker sizes, in addition to a wide variety of custom milled shapes and finishes. Contact your J. Gibson McIlvain Company representative for details.
What Is Thermally Modified Wood?
It’s a simple question with a straightforward answer: thermally treated wood is mostly identical to the kiln-dried lumber that you can currently purchase. The green (or wet) wood is put into an oven, and the temperature is raised in accordance with the standard drying schedules for kilns in order to remove any free water that may be present in the wood. Throughout this procedure, moisture is applied in order to control the drying and prevent damage to the wood fibers.
With thermally modified wood, after the free water has been removed, the temperature of the kiln is increased to more than 200 degrees Celsius, which is significantly higher than the normal kiln drying temperature for lumber. In order to prevent combustion from occurring at this high temperature, the oxygen levels in the kiln are lowered, and the water that is bound up inside the wood cell walls is then baked away. This includes any hydroxyl groups that have the potential to connect with water vapor at a later time.
Above 200 degrees Celsius, the extractives, which include sugars, oils, and resins, among other things, bake off, leaving the wood fully dry and resistant to decay in every way. At these high temperatures, sugars and hemicellulose have the potential to crystallize and become rigid, which then renders them fully non-reactive. After that, moisture is reintroduced while the temperature of the oven is lowered; this allows the wood to acclimate to the air and humidity of its surroundings, which helps to prevent cell collapse and warping (also known as “case hardening”). This final stage renders the wood extremely stable and virtually eradicates any movement in the wood that may have been caused by variations in annual moisture levels.
The hydroxyl groups are negative ions that are just longing for water molecules to come along, so that they can form bonds with them and create more water that is bound deep within the cell walls. However, with thermally modified lumber, the hydroxyl groups are gone, and because of this, the regular expansion and contraction patterns that one would normally observe in wood are no longer present.
In the final stage of the kiln process, where the wood is being conditioned, there is some early movement and the standard warping that you would expect to see from any type of wood. The initial warping and re-humidification of the wood are necessary steps, but after those are complete, the wood is essentially stable. The tangential and radial movement is currently at such an all-time low seasonally (0.1%), that it is essentially nonexistent.
Baking the cellulose and sugars results in a darker hue of wood since it increases the wood’s hardness, which in turn contributes to the increase in the color’s intensity. There is a minor decrease in bending strength due to the internal hardening, and there is a drop in weight due to the absence of any moisture; nonetheless, you still have the same species of wood that was placed into the kiln at the beginning of the process.
GWood Pro is responsible for the manufacturing of all of our thermally modified wood products, which begin life as FSC timbers. GWood Pro and its parent business, TanTimber, are the manufacturers of thermally modified timbers (TMT), and J Gibson McIlvain Company is an exclusive distributor of these products. TanTimber is a member of the International ThermoWood Association and complies with the stringent criteria that have been established by the association to ensure that all of their products have a high level of quality consistency.