Thermally modified tonewood goes by many marketing nicknames, but its effect on tone is still up for debate.
“Torrefied” or “torrefaction” are words you’ve likely heard pretty often in the past few years in the context of musical-grade tonewoods. They refer to a heating process that changes the material properties of wood. The process was first developed to produce biofuel, and later to make construction lumber weather resistant. Because this process made wood lighter and more resistant to moisture, instrument makers took notice and began to explore the possibilities of applying the process to tonewoods.
When I first heard the term torrefaction applied to tonewood, I had a “hold-on-a-minute” thought: If it’s torrefied (which makes it water resistant), then how are you going to glue it together with standard water-based, guitar-making glues like hide glue or aliphatic resin? The answer is you can’t. So, what we’re most likely dealing with in treated musical-grade lumber is a unique process that would use cook times and temperatures different from those used for construction-grade lumber. This means that tonewoods are not truly torrefied, but rather thermally modified.
To make matters more confusing, each manufacturer uses its own trade name for thermally modified tonewoods in an attempt to carve out a unique slice of its marketing value. I’ve seen heat-treated tonewoods referred to as aged tone, specially aged, VTS roasted, toasted, baked, etc. I hope one day someone will discover how to cut the processing time and temperatures by 50 percent, so we could call it “half-baked” tonewood. Now that would be cool.
To grasp the basic process of thermally treating wood, we need to understand that wood is essentially a composite material made up of three main organic polymers: cellulose, lignin, and hemicellulose. When heated, these polymer chains begin to break down and the wood off-gasses some of these molecules. This makes the wood slightly weaker and less dense. Depending on the time and temperature, this process can improve the stiffness-to-weight ratio of the wood. But if wood is heated too long or too high, its stiffness-to-weight ratio can actually degrade. If done correctly, however, this process can imitate the effect of naturally aging wood.
There are methods for altering tonewoods through thermal modification that have been patented by Yamaha and perfected by smaller boutique makers around the country, but again, these are not the same as the processes that are being widely described as torrefaction in today’s guitar market.
Another factor to ponder: The net gain from thermal modification is very small, generally in the range of about 10 percent. This means that through this process, the wood is either getting lighter or stiffer, but either way, we’re talking about very small amounts of positive gain. This also includes any changes in the damping factor within tonewoods, so to say that applying these techniques to tonewoods is going to make a massive difference in the overall sound or volume of the finished instrument is, well, highly questionable.
That’s not to say that thermally modifying instruments doesn’t have its place in our industry. The first time I saw a piece of what was referred to as “roasted maple,” I thought it was beautiful. Since then, I’ve seen instruments with thermally modified backs and sides made by Dana Bourgeois and bass necks by Roger Sadowsky, all of which were stunning.
So, why would you want to thermally modify a piece of wood? The goal would be to make a substandard piece of wood better, or to make a good piece of wood great, right? But as of yet, there’s no substantial data available to back the statement that just because a piece of wood has been heat treated, it’s going to make a great guitar.
In the end, even though we have a handful of pioneers like Bourgeois spearheading our understanding of this science, the jury is still out concerning the pros and cons of thermal modification. One thing for certain is that nothing will ever beat an already great piece of tonewood supplied to us by Mother Nature. It’s been proven through centuries of application that good ol’ high-grade tonewoods are the way to go for longevity and great sound for your guitar.