Just Wood: Thermally Modified Timber


Wood as a building material has many virtues including its impressive strength to weigh ratio, relatively high insulating value, beautiful grain patterns, warm color, and soft tactility. However, despite these admirable qualites it also has one key limiting factor: it’s natural propensity to absorb and hold moisture, resulting in swelling, warping, and ultimately deterioration and decomposition. For this reason, denser, slower-growing hardwoods such as mahogany, teak, and oak have historically been the most sought after species due to their limited absorption capacity, superior dimensionally stability, structural strength, and aesthetically consistency in color and grain.

However, softwoods such as pine, fir, spruce, and birch make up the vast majority of wood used today. Because of their inherent vulnerability to water, aging, and decomposition, wood used in exterior applications are typically either impregnated with a copper-based solution such as chromate copper arsenate, alkaline copper quaternary, or copper azole, or sealed and finished using petroleum-based polyurethanes, lacquers, and varnishes. These chemical treatments essentially transform a completely organic material into hazardous waste once it’s building use has expired. Alternatives to these metal and petroleum-based treatments are slowly gaining in popularity due to increased environmental considerations. One particularly promising alternative is the use of heat alone to transform the chemical and physical make-up of wood for exterior architectural use.

Finnish Pavillion at the 2000 Hannover Expo

The benefit of thermally modifying timber has actually been know for over a century, and the detailed chemical processes involved in thermal modification is something well known to wood scientists. However, only in the past decade has a systematic testing and analysis made thermal modification a commercially viable process. Developed largely in Finland over the past ten years, thermally modified timber uses kilns set to a temperature of around 400° F for anywhere from 20-75 hours. During this prolonged exposure to high heat, the wood undergoes a series of complicated chemical and physical transformations that together dramatically increase the wood’s weather and fungi resistance, dimensional stability, and visual consistency.

The wood’s resins become chemically transformed by the heat into a natural water repellant that is essentially baked into the structural cellulose of the wood. Furthermore, the hemicellulose sugars which play very little structural role but are the most easily digested part of the wood and thus the primary attraction for water-born bacteria and fungi, is broken down and crystalized within a more rigid and inelastic structural matrix, greatly reducing the wood’s water absorption capacity. These chemical and physical transformations at the cellular level result in a building material with significantly decreased water absorption rates, increased resistance to fungi and biodegradation, and improved dimensional stability with a limited decrease in structural performance (likely due to decreased overall density). Furthermore, the natural chemical transformations in the wood result in a deep, rich coloration throughout the full cross-section of the timber. The thermal modification of fast-growing softwood species such as spruce, pine, birch, and aspen seems to offer an economically and ecologically viable alternative to both the chemical impregnation of softwoods and the unsustainable harvesting of tropical hardwood for architectural applications.

Twelve years of direct contact with the ground has not affected this piece of thermally modified wood by Stellac Wood. Photo by Seppo Paavilainen.

B.F. Tjeerdsma, M. Boonstra, A. Pizzi, P. Tekely, H. Militz.  Characterization of thermally modified wood: molecular reasons for wood performance improvement. Springer-Verlag 1998.

Kekkonen, Paivi M., Ville-Veikko Telkki, and Jukka Jokisaari. Effect of Thermal Modification on Wood Cell Structures Observed by Pulse-Field-Gradient Stimulated-Echo NMR. Oulu: Department of Physics, University of Oulu. 2010.

Wikberg, Hanne, and Sirkka Lissa Maunu. Characterisation of thermally modifed hard- and softwoods by 13C CPMAS NMR. Helsinki: Department of Chemistry, University of Helsinki. 2004



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