While it sounds simple in concept, there is a good deal of science behind the thermal modification of wood. As with all scientific processes, there are multiple variations available to achieve the same or similar results. Let’s look at an overview of thermal modification and how wood is modified by most processors:
First, the wood itself. There are hundreds of species of wood and each has its own characteristics that make it either a good or bad candidate for modification. Here at ThermaPro Wood, we use several species but the majority of our prefinished building materials are made from Southern Yellow Pine because it is a fast growth highly sustainable species that is ideal for modification. Other species well suited and popular for modification include European or Scots Pine, Radiata Pine, Nordic Spruce, Ash and Poplar.
Second the kiln. Thermal modification requires a special sealed-environment kiln that is computer-controlled. The modification level and programming details vary by batch based on the wood species and its intended use. Thermal modification kilns differ from standard wood drying kilns in that they operate in a sealed vacuum environment. Â Once the batch of wood is sealed in the kiln, it goes through a phased process of introducing high heat and steam. This is a very precise physiochemical treatment process utilizing automated control equipment to ensure that each batch of the same species is consistent, and each species is treated accordingly to achieve the desired physical, chemical, and moisture level changes.
During the modification process, the biological and physical properties of the wood are permanently changed. The scientific detail is quite in-depth but the two main changes that occur are hydrolysis of the hemicellulose and cellulose, and the conversion or caramelization of the Polysaccharides (sugars) within the hemicelluloses. Achieving Hydrolysis makes the wood hydrophobic meaning it does not absorb as much water.  By reducing absorption sites for water molecules, you reduce its propensity to rot and minimize the expansion and contraction of the wood which is the cause of warping, cupping, and cracking
The ‘polysaccharide conversion’ process dehydrates the hydroxyl groups within the polysaccharides and condenses those contained in the lignin.  These sugars are a food source for bugs and fungi, therefore removing them makes the wood less attractive to insects and fungal growth. This part of the process creates that rich brown color in thermally modified wood which varies by species and heat levels obtained in each program.
It’s important to note that during thermal modification these changes are permanent and happen throughout the entire piece of lumber without the use of chemicals resulting in an eco-friendly process and product. We’ll go into the details of carbon storage and reduction in a future post.
Third, let’s briefly review the benefits achieved by thermal modification. The key benefits are:
Reduced water absorption equaling resistance to rotting and decay.
Reduced water absorption minimizes expansion and contraction by up to 90% so it doesn’t warp, cup, or crack like non-modified wood.
Conversion of sugars means wood-eating insects and fungi don’t see the wood as a food source.
Density of the wood is increased which improves its insulating properties.
Increase in wood density also improves its acoustical properties.
Lifespan is lengthened beyond that of chemically or pressure-treated wood
Completely non-toxic and chemical-free process provides you with an eco-friendly building material.
These physical changes and benefits make thermally modified wood the ideal choice for manufacturing decking and porch boards; cladding, rainscreen, and trim for outdoor structures; or extreme environment applications like saunas and wine cellars. It is also much loved by guitar aficionados due to its acoustic properties.
Finally, some quick history: The process of modifying wood with heat goes as far back as the Vikings charring the exterior of their wooden ships to improve its durability. The first written reports of more modern thermal modification took place in 1915 in Madison Wisconsin however it never achieved mass commercialization. It wasn’t until environmental concerns arose in the 1990s that the technology was resurrected as an alternative to chemically treated and non-sustainable building materials. Today, thermal modification is a well-established commercial process and a popular cladding product used throughout European and Scandinavian countries due to its eco-friendly properties. Commercialization in the US is in its infancy stages, here at ThermaPro we are hoping to help change that.
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