Re-engineering an ancient art
“The technical advances of thermal modification are a game-changer for both tonewood suppliers and guitar makers.”
Two guitars side by side, identical but for soundboards made of thermally modified vs natural wood, will not only look different; they’ll produce different tone. The thermally modified top will be slightly lower in density, and slightly stiffer across the grain because of intrinsic changes in the wood induced by the modification. It will also have lower damping as a result of lower moisture content. All other things being equal, this would result in a guitar with greater responsiveness, louder average volume, and slightly greater sustain.
The aging process of wood has come a long way since the Vikings torrefied wood to build their ships. Also known as Torrefaction, thermal modification was often referred to as roasted, baked or cooked—and for good reason. An early iteration was developed by a Finnish fire chief who knew a lot about fire and a little about wood. Using a rudimentary container and the concept of steam injection, this inventor proved that you could make wood resistant to decay without using toxic chemicals. Fast forward a few decades and many patents later, the process of thermal modification has evolved into a mainstay for luthiers wanting to craft vintage looking and sounding guitars.
Ask the Expert
Thermal modification works primarily on hemicellulose, and the hydrogen bonds between hemicellulose and cellulose. If we heat wood for a number of hours to about 350° Fahrenheit, in the absence of oxygen, some of the hemicellulose will degrade, and some of the water will be driven from the weak hydrogen bonds, converting these to a stronger covalent bond. If you have read about “cross linking” with thermal modification, this is primarily what is meant.
1) Density decreases: Primarily due to the partial loss of hemicellulose. Most studies show that 5 – 8 % of the hemicellulose can be lost without sacrificing too much strength or structure.
2) Stiffness increases: The replacement of hydrogen bonds with covalent bonds between hemicellulose and cellulose increases stiffness, especially “across the grain” of the wood. One potential downside to this is that the wood also becomes more brittle.
3) Moisture Content decreases: All wood will absorb atmospheric moisture, proportional to relative humidity. The ability of wood to absorb moisture is decreased in thermally modified wood, because of the partial loss of the hydrogen bonds. Thus, not only will thermally modified wood have lower moisture content at a given relative humidity, but that wood will be less susceptible to swelling and shrinking with changes in relative humidity.
4) Damping decreases: Damping, or internal friction, refers to the speed with which a vibration returns to zero. For example, a car’s shock absorber has high damping, a brass bell has very low damping. In wood, damping is strongly dependent upon moisture content. High moisture increases damping, and dry conditions decrease damping. Thermally modified wood will always have lower moisture content than natural wood at a given relative humidity, and thus that wood will have lower damping.
Two guitars side by side, identical but for soundboards made of thermally modified vs natural wood, would produce different tone. The thermally modified top will be slightly lower in density, and slightly stiffer across the grain because of intrinsic changes in the wood induced by the modification. It will also have lower damping as a result of lower moisture content. All other things being equal, this would result in a guitar with greater responsiveness, louder average volume, and slightly greater sustain.
Thermal modification can be easily “overdone”, with either higher temperatures or with longer treatment times, resulting in wood that loses stiffness and strength. Even in the best of circumstances, thermally modified wood is more “brittle” or susceptible to cracking and fracture. Pacific Rim Tonewoods is currently conducting research on all aspects, good and bad, of thermal modification, with one of the leading experts in this field, Professor Alexander Pfriem of Berlin, Germany. Through this research, the first of its kind, we are comparing the various methods of thermal modification, and optimizing the “recipe”. In so doing, we are increasing the quality, consistency, and reliability of the thermally modified spruce that we supply to the acoustic guitar community.