Centuries ago, it was known that heating wood by burning its surface would prolong the life of ship hulls and fortress posts. Thermally modified wood dates back to pre-Viking times; and today, it continues. But now, we understand the actual physical and chemical changes that occur in wood due to the process of heat treatment, and that’s exactly what we’ll be covering in detail in this article.
Perhaps you’ve already heard about thermally modified wood, also known as heat-treated or thermally treated wood. This is different from pressure-treated wood, which is more common. Pressure-treated wood uses high pressure to force chemical preservative agents deep into the lumber to help extend its life. This is what you’ll find most often used for fence posts, decks and other outdoor building projects. While it is safe to use for an outdoor base of a sauna, you don’t really want to use it for the interior or other indoor projects because low-level leaching of the chemicals can be an issue.
So with those differences squared away, let’s buckle up. By the end of this article, you’ll know the answers to the 5 questions below:
What is Thermally Modified Wood?
What is the Process of Heat-Treating Wood?
What are the Benefits of Thermally Modified Wood?
Is Thermally Modified Wood Recommended for Saunas?
And...What Can You Expect from Forever Saunas?
QUESTION #1: WHAT IS THERMALLY MODIFIED WOOD?
Thermally modified wood is wood whose structure has undergone chemical changes by a controlled process of wood being heated to greater than ~356°F (or, 180°C). When the wood is heated, it releases moisture and its chemical composition is completely transformed. The key, though, is the absence of oxygen during the heating. Low oxygen content prevents the wood from burning at these high temperatures, and instead induces the chemical and structural changes that increase durability.
In addition to heat treatment, other technologies can be used to achieve this. Some wood manufacturers use water vapor, while others include hot oil or the addition of chemicals.
For the sake of this article, we are going to steer clear of any process that includes chemicals and will focus on processes that improve the natural properties of wood using just heat and water vapor (or steam).
Two such companies, Forever Saunas and SaunaLife are the most widely trusted brands of thermally modified wood for barrel saunas in the country.
Their expertise lies in their process and technology of thermally modifying wood, which they’ve been perfecting for more than 20 years. Their products can be seen in homes and public spaces in more than 50 countries around the world. It’s their particular process of heat-treating wood that we’ll dive into to answer question #2 below.
QUESTION #2: WHAT IS THE PROCESS TO HEAT-TREAT WOOD?
The exact process varies by wood species and purpose of use, but the principle for heat treating wood is generally the same across the board. Using a heating chamber with special sensors known as a thermo-kiln, the temperature is raised to approximately 374°F (or, 190°C) over the course of two days. The head reduces the moisture level of the wood to 0%. Once this 0% moisture point is achieved, the temperature is increased again to a specific level, followed by cooling and the re-addition of moisture.
In the case of products for outdoors, such as saunas, more intensive heat is needed to provide greater durability. For that, the temperatures need to hit at least 419°F (or, 215°C). However, while higher heat can equate to stronger durability, it’s important to not “cook” the life out of the wood. The key is to use just enough to improve its condition.
This might seem like a very simple process — simply heat and remove oxygen, then re-add moisture. But, this procedure is complicated by the need to achieve the same quality levels in different batches. Different batches of raw wood provide different challenges. Their properties will be affected by where it is grown, residual humidity, temperature and so much more. This is why the entire process must be closely monitored.
QUESTION #3: WHAT ARE THE BENEFITS OF THERMALLY MODIFIED WOOD?
STRONGER & MORE DURABLE: During the thermal modification process, the wood’s physical and chemical properties change — its density decreases and its structure changes. Unlike chemical impregnation, thermal modification enhances the wood throughout, not just the outer surface. The result is boards that are stable and durable in every way.
IMPROVES STABILITY: Thermal modification lowers wood’s absorbency capacity. This means when the wood is exposed to moisture, it won’t swell as much. It also improves the dimensional stability, so the wood is less likely to twist or bend as much as other wood products in outdoor conditions.
STRENGTHS BIO-RESISTANCE: Wood’s biological resistance increases as its nutrient make-up is altered during thermal modification, making it less accessible to pests and microorganisms. This increases the durability of the wood in outdoor conditions so it doesn’t need to be dosed with “protective” toxic substances.
MORE EFFICIENT: According to tests completed by CATAS (a European research laboratory) “the heat and sound insulation of thermally modified wood is superior to that of natural wood by as much as 30%.”
MORE LIGHTWEIGHT: Thermally modified wood has a lower weight and density, making the wood more lightweight and easier to work with.
AESTHETICALLY APPEALING: Thermally modified wood has a beautiful, deep color that creates an elegant look.
SAFE & ECO FRIENDLY: If no chemicals are used in the thermal modification process, thermally modified wood is safe and there’s no need to take precautions or handle them as hazardous waste.
QUESTION #4: IS THERMALLY MODIFIED WOOD RECOMMENDED FOR SAUNAS?
Thanks to its versatility and durability, thermally modified wood is suitable for both indoor and outdoor use, in both private homes and public places. And yes, that means SAUNAS!
If you’ve read our “Most Popular Sauna Wood Types Explained” blog, then you know that in general, pine is one of the cheapest options for sauna construction due to its tendency to bend and warp more than other sauna woods. But, if the pine is thermally treated, then it’s much stronger and more durable and becomes a whole other ballgame. Because thermally modified wood transforms the weaknesses of all wood types, it opens up a world of sauna options.
And that goes for all wood types, not just pine! Take European Spruce for example.
European Spruce, or Nordic Spruce, is a wonderful sauna wood -- it has beautiful coloring and a subtle aroma. But because it is a coniferous lumber there is a chance for some sap seepage during heat cycles. And although this is harmless, thermally treating the lumber prevents it completely.
Not only that, thermal modification lowers any wood’s absorbency and improves its dimensional stability, which means it won’t expand or contract as much in outdoor or high-moisture conditions, making it more durable and stable with a stronger structure and higher resistance to rot. So the answer to the question: “Is Thermally Modified Wood Recommended for Saunas?” is a resounding YES!
With that, we’re very excited to share that we now offer outdoor saunas made with thermally treated spruce.
QUESTION #5 WHAT CAN YOU EXPECT FROM FOREVER SAUNAS?
We love Forever Saunas because they share our values of high quality and sustainability. When sourcing wood, Forever Saunas sets specific, strict requirements to ensure the highest quality with the smallest ecological footprint.
So they source raw materials from places that employ conscientious forestry and sustainable production. And they also extend their values to include those who they do business with. That’s why, whenever possible, Forever Saunas chooses business partners, manufacturers, and suppliers whose customs and practices have been approved by recognized international organizations. Also important to note, like all of the sauna brands we offer here at My Sauna World, none of Forever Saunas’ products use raw materials sourced from tropical woodlands or rainforests.
Forever Saunas’ quality is ensured by a specialized thermal modification process, developed and refined over 20 years. Their knowledge, technology and procurement principles make it possible for less costly wood types to achieve a resulting aesthetic and function that’s equivalent to precious tropical wood. This can be seen in their barrel saunas made with thermally treated spruce. The deep tone created by heat treatment complements the wood’s attractive pattern for an executive-like appearance.
In addition to the next-level thermally modified wood, Forever Saunas give you your choice of HUUM heaters and Harvia heaters, both which are pretty amazing. They are elegant in appearance and they even offer wifi controls for super convenient automation. Learn more in this “How to Automate Your Sauna”
Their minimalist design and organic shape accentuates the natural beauty of sauna stones. The one in the images shared here is known as the DROP stove, and is inspired by the pure element of a water drop. Yes, it’s absolutely gorgeous, but it’s not designed just for aesthetics.
This particular shape of this sauna heater also significantly increases the capacity for stones, fitting up to 132.3 lbs (60 kg) of stones or (this capacity exceeds most wood-burning stoves!).
Why would you want to fit more stones? The more stones in your sauna heater means more steam, for a longer period of time.
In addition, HUUM stoves are built using very little metal so the heat primarily comes from the stones and very little from the metal. According to the craftsmen, it is built this way to maximize “the good negative ions from the stones, like in traditional saunas, and keep the positive ions from the metal at a minimum.” Together with a high stone capacity this is HUUM sauna heater’s secret to healthy, mild and long-lasting steam.
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Plus, HUUM heaters have also developed a mobile app, so you can start heating your sauna from the comfort of anywhere.
At My Sauna World, we’re always happy to expand our sauna offerings to include more high-quality options, and we believe Forever Saunas and SaunaLife do just that! We can’t wait for more people to discover everything thermally modified wood saunas have to offer.
Plus, our goal is to ensure that we have an unrivaled range of options and education so that you can design a sauna experience that’s just right for you and feel confident about your decisions. So, if you have any questions about thermally modified wood for your next sauna, or simply want to chat with us about all the sauna options out there, please feel free to connect with us using the chatbox on this page, or by contacting us here. We love to talk shop and answer questions!
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