I reached out to Andy Powers directly on this topic, as it was asked over in Taylor Guitars Owners Group on Facebook as well. I do some video work for Taylor and know Andy a bit, so I'm lucky enough to be able to reach our occasionally when questions require a "deep dive" beyond my knowledge or expertise as an authorized repair tech.
Here's what I asked:
This is an interesting topic that just came up. Someone asked about whether or not Tonerite “works” and if it’s as significant as roasting tops. My elementary understanding is that roasting is speeding up the wood curing process, which is more like hitting fast forward on aging. I’ve never seen any science on vibrating guitars and wanted to get your thoughts.
There’s also a question about whether or not roasting tops affects the longevity of guitars, meaning will they not last as long because they’ve been roasted. I would think the opposite might be true, as old guitars seem a little more humidity resilient in my limited experience. Again, curious to hear your thoughts.
Thanks for all your help on these.
Gabriel O'Brien
And his response:
Hi Gabe, These are both good questions, with fairly long answers, as well as further discovery still to be made. There is an interesting connection between a vibrating guitar "aging" as well as heat treating having an aging effect. In simplified terms, these two processes have somewhat opposite effects, judging by the intention of each technique. Torrefaction is a high heat treatment which tries to improve the resonance characteristics of a piece of wood by subtly increasing wood strength proportionate to its weight. Vibrating a piece of wood to improve its resonance characteristics is trying to make the wood more flexible and therefore be set in motion more easily. Both processes can achieve these results to a degree, both happen simultaneously, and neither are exact substitutions for what happens to a guitar naturally.
It's helpful to understand what we want from a guitar. As a player, I want the pleasing, accurate, musical vibrations which allow me to play songs. At the same time, I want to reject or remove the non-musical sounds which might interfere with this music. More of the good musical sound is good, less of the bad, inharmonic sound is also good. More of everything isn't necessarily better.
Torrefaction can make a piece of wood more resonant, but this translates to more able to resonate with the good harmonic sound, as well as the undesirable inharmonic sound. As an experiment, we tried some heavily torrefacted wood-far beyond what I'd build a guitar with. While the guitars were loud, they had so much inharmonic distortion, they were impossible to tune to accurate pitch, and had an harsh, brash sound-very unpleasant to listen to. The heat treating process we use on some of our tops will alter the wood's resonance a little, but retains more of the natural oxidation which would occur over a long period of time. With this, we experience a level of dimensional stability that serves a player and the instrument well, but allows for a more musically pleasing guitar. It seems to do a better job aligning with what occurs over time at room or stage temperatures. You might find it interesting to know we actually use a heat treating step on every single one of our tops, and have for more than 25 years now. Improved dimensional stability is achieved at far lower temperatures than is required to change the color of a top and we like consistent, stable guitars. So what appear to be raw spruce tops on everything from a baby Taylor to a presentation series guitar has actually been seasoned using a heat treatment.
Vibrating a guitar in a way inconsistent with how a guitar is played will certainly "loosen it up" to use a common term, but doesn't do it in the exact same way as a guitar which develops a preference for playing the notes originating from its fingerboard.
To use another analogy; if we started with a fresh and crisp dollar bill, then got it wet, then dried it in the sun, it turns into a crackly, crisp piece of paper. Then, if you crumple it, fold it, bend it, rumple it, over and over, it slowly takes on the texture of a limp piece of cloth. That's similar to these two processes.
Considering the longevity of the guitar, if the heat treating process isn't carefully controlled, you can certainly shorten the wood's functional lifespan. If done well, other aspects of the guitar, like the neck angle, will surely become a problem far before the top's structural characteristics become a chief concern.
-Andy
