30.10.2025

“One of our earliest thermal modification kilns was 70 meters long and needed an industrial hall around it: each process phase - high-temperature drying, thermal modification, conditioning and cooling - was separate from another. This spring we launched a mini ThermoWood® chamber that can process small batches of wood with a fraction of the original volumes”, says Timo Tetri. 

Tetri was the first full-time employee to join Jartek’s thermal modification kiln business in the year 2000. Originally a project manager, Tetri has since acted as product, business development and operational manager - roles that led him to his current position as Senior Advisor. 

In the early years, he travelled the world looking for business opportunities for thermally modified wood in the US, Canada, South-East Asia, New Zealand and Europe. Today, Tetri is a sought after person at the office: people consult him on sales, business and project related questions. 

He is also a long-time chair of the board at the International ThermoWood Association (ITWA), the members of which co-operate to enhance the use of ThermoWood® products. “Now that my career is at the end of its life cycle, my chair responsibilities will soon be taken over by Tuuli Luoranen, also chair of the board at Jartek”, he says with a smile.

The beginning of the wood thermal modification revolution

 “In the early 1990’s, several large companies in the Finnish forest industry together with VTT Technical Research Centre of Finland were researching a new wood treatment method. The aim was to develop an alternative to chemical impregnation of wood and provide locally grown substitutes for tropical wood”, Tetri says.

This alternative was a thermal modification process that was designed to activate a natural reaction in the chemistry of wood itself with heat. A reaction that would help prevent the wood from decay and improve its technical qualities like dimensional stability and warping resistance. The end-product of this process is today known as ThermoWood®.

Tetri has been involved in the commercialization of thermally modified wood from the start of industrial production of ThermoWood®. 

The original patents for the industrial scale ThermoWood® process were granted to VTT Technical Research Centre of Finland. When the ITWA was established in the year 2000, members received a license to these patents, some years later the patents were purchased from VTT to benefit the members of the association. 

ThermoWood® was first produced by pioneers in small-scale mills, catering to local market needs around the world. The first Jartek chamber was delivered to its customer in 1998. In the early 2000’s the association started controlling the quality of the ThermoWood® process worldwide. 

After 2005, global production volumes started to grow. “In 2010, ITWA started a project to check if the process needed altering in any way. As it turned out, we were already doing the right things and things right. The members of the association produce ThermoWood® on an industrial scale and have seen tremendous growth in product demand since”, Tetri concludes.

Understanding the delicate nature of the thermal modification process

Every species of wood reacts to thermal modification differently, so understanding how to optimize the process for different wood types is key. At Jartek, we are the only technology provider of ITWA. Our chambers guarantee the production of a high-quality and consistent end product that is sold under the ThermoWood® brand.

“The kiln construction has to be done carefully and the applications for each type of wood chosen correctly, to make sure that the material is suitable for its end-use and meets the quality requirements of the brand. The ITWA publishes a ThermoWood® handbook, making it easier to succeed in the manufacture and use of this wonderful material”, Tetri says.

“Jartek chambers are always tailored to fit the most common wood batches and dimensions that our customer produces. This helps our customers avoid bottlenecks and unnecessary work in their production. We also help our customers find the right parameters and formulas for treating different types of wood”, Luoranen continues.

During their history, our chambers have seen a number of technological advancements: their energy sources are moving from fossil fuels and natural gas to electricity, their reliability, usability and maintainability have been improved, their use life lengthened and their control system, logic and user interface completely renewed. A lot of research has also gone to optimizing air circulation within the kiln, to ensure consistent end-product quality.

“Working directly with our customers is the best way to move the technology and the whole industry forward. We are pleased that so many of our customers are willing to grow and increase their production capacity with us. We are here to support them every step of the way”, Tetri says.

The ITWA develops ThermoWood® for the whole industry’s benefit

Only a few of the original players from the 1990’s are still invested in the business. “Especially a lot of pioneers from Germany, France, Eastern Canada and the East Coast of the US have disappeared from the market during the years”, Tetri says.

As a neutral party, the ITWA cares for the interests of all players in the market from equipment manufacturers to wood producers and end-users. After early years, the association has welcomed new members, with producers increasing their capacity and conquering new markets. According to Tetri, the association and its concept is key to growing the industry and introducing new products under the trademark. 

“The members contribute to research and testing of e.g. new raw materials and surface treatments. When everyone participates in the costs and shares the risks, the benefits are also available to everyone. Together we can move forward faster”, Tetri says.

A promise for the next 25 years of ThermoWood® with Jartek

The global production volumes of ThermoWood® exceeded 280 000m³ in 2024, when the volume in the early 2000’s was only 20 000m³. 99% of it is exported from Finland to 30-50 countries around the world. The most typical end-uses for ThermoWood® are external cladding, terraces, windows and sauna interiors, where they replace impregnated or tropical woods. In addition to ThermoWood® production, many companies also use Jartek’s thermal modification kilns to manufacture high-quality thermally modified wood that they market under their own brands, further showcasing the versatility and reliability of Jartek’s technology. 

The use of thermally modified wood is likely to increase worldwide along with the general use of wood as a construction material. The growth in demand is due to more environmentally friendly choices made both in the manufacturing and end use of wood, and the ease of disposing of non-toxic materials compared to chemically treated ones.

The biggest threats to the industry are the insecurities concerning the global economy. Another threat are the organizations that try to copy the treatment methods and sell non-standardized, poor quality wood under the ThermoWood® brand.

The association and Jartek as head of it promise to keep safeguarding the trademark and its reputation, and increase the usage of this beautiful wood material.

“We will continue being the forerunner and a responsible, reliable player in the thermally modified wood market. We will stay close to our customers and their end-customers, listen to their needs and find new ways to advance the technology - to continue ensuring the best possible end quality and future for ThermoWood® products”, Luoranen promises.

Looking Ahead – and Looking Back

After nearly three decades of advancing thermal modification technology, Timo Tetri will retire at the end of October 2025, leaving behind a legacy that has helped shape not only Jartek’s direction but the entire ThermoWood® industry. His career has spanned the evolution of thermal modification from experimental technology to a globally trusted process — one that now defines sustainable wood modification.

Reflecting on the journey, Tetri says:

“When I started, we were experimenting with what was still essentially an unproven idea. Today, ThermoWood® is an international benchmark for ecological, durable wood products. It’s been fascinating to witness how innovation, persistence and collaboration have turned thermal modification into a mature, globally respected process.

Looking forward, I see enormous potential — especially as construction and design move toward more sustainable materials. Jartek’s strength lies in our ability to keep developing reliable, efficient technology while staying close to our customers. I believe Jartek will continue to lead this transformation — combining engineering expertise with a genuine understanding of what the market needs next.”

As Tetri steps into retirement, Jartek’s team, customers and partners thank him for his insight, dedication and pioneering spirit — qualities that will continue to inspire the next generation of innovators in the ThermoWood® story.

25 years of development – from 70 metres to modern thermal modification kilns.