Are Torrefied Tops the New Industry Standard? Dana Bourgeois Weighs In

Every acoustic guitar top must be dry, stiff and lightweight. For centuries, luthiers chased this holy trinity of adjectives with various methods of carving, bracing, and drying with air or kilns.

In the past decade, however, a growing cohort of builders has applied the benefits of a technique used in charcoal manufacturing and the treatment of oak wood flooring to supercharge the pursuit of dry-stiff-light.

That process is called torrefaction. The result - torrefied wood - is sometimes also referred to as roasted or tempered.

In a nutshell, torrefaction involves heating wood in the absence of oxygen to remove water and volatiles. Electric guitar necks have been getting this treatment from certain builders for some time, making them lighter, stiffer and more resistant to temperature and humidity changes.

When applied to new acoustic guitar tops, torrefaction gives them a similar molecular composition - and therefore tonal response - to vintage guitars tops after decades of aging.

First seen on high-end boutique models, torrefied tops have now made their way into model lineups across the price spectrum, including Santa Cruz, Collings, Martin, Taylor and even Recording King, as this RO-T16 model shows.

All of these manufacturers, however, are following the lead of Dana Bourgeois, the first luthier in the United States to regularly use torrefied tops on his models. Since starting Bourgeois Guitars in 1993 out of the old Roy Continental Mill in Lewiston, Maine, Dana has emerged as one of the leading experts in choosing tonewoods, voicing guitar tops and torrefaction.

We recently jumped on the phone with him to get his thoughts on torrefaction spreading throughout the industry and the details of getting it right.

The main goal of using torrefied tops is to get something close to “vintage acoustic tone” right out of the box. That can feel like a buzzword sometimes.

What specific characteristics of vintage guitars are we talking about?

The best vintage guitars have a quick response, dry sound, good fundamentals without too many overtones, even balance string to string, and strong projection.

They just have an immediacy. You pick up a nice vintage guitar and the sound just explodes. There’s nothing holding it back. Even a vintage Gibson that isn’t very loud will have a wonderfully quick response.

Dana Bourgeois of Bourgeois Guitars

And that’s due more to how wood changes over time rather than a case of they don’t make ‘em like they used to?

Correct. Pre-war Martins did not sound the way they do now when they were fresh out of the factory. There are also some acoustic guitars being made today that are of equal or better craftsmanship than some vintage guitars but still can’t get that response.

To understand why, you need to think about what happens to the wood over time.

Freshly cut wood is mostly made of cellulose with lignin, which is a particular type of resin that holds the cellulose together. It has a fairly large content of volatiles - pitches, sugars, oils, and other chemicals that are byproducts of a living tree. It also has considerable water content.

These volatiles initially have a damping effect when a guitar is first made. They prevent the top from vibrating as freely as it could.

Over decades of air-drying, these volatiles will oxidize [combine with oxygen], gas off and mineralize. The slow chemical reaction emits a gas and leaves a mineral residue. The lignin also sets like a glue, holding the cellulose together.

Trees also have vessels within them that bring water from bottom of the trunk up through the tree. These vessels have valves. These also oxidize and mineralize, closing the valves. This changes the cellular structure of the wood from something like a sponge, which is open-cell, to something like styrofoam, which is closed-cell. This eventually makes wood more resistant to temperature and humidity changes.

Three main things happen over time as the volatiles oxidize, the lignin sets and the cells close: 1) Sound velocity is improved because of diminished damping. 2) The wood becomes lighter and vibrates more freely. 3) The wood becomes stiffer.

When wood is fully cured and volatiles are fully oxidized, it becomes a better material for musical instruments.

All these things happen naturally, but it takes decades of exposure to air for wood to fully cure. Humidification slows this down. Torrefaction speeds up that process. It changes the chemical content of the wood to make it resemble what it would be decades from now.

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How does torrefaction do that?

Torrefaction is the same process as making charcoal: Wood is heated beyond its combustion point but is denied oxygen. Without oxygen, the wood itself can’t combust. The gases coming off the wood combust. If you heat it long enough, it becomes charcoal.

The trick is to heat the wood at just the right temperature for just the right amount of time. Structural tests have revealed that there’s a different “recipe” of heat and duration needed for each kind of wood.

For example, Adirondack spruce requires a different temperature and duration than oak. Oak flooring is actually the largest commercial use of torrefied wood. When you put a floor in a new building, you don’t want it to expand and contract. Torrefaction solves that problem.

You have to be careful with spruce. Tests show that as temperatures increase, spruce will become slightly stiffer up until a point. Beyond that point, the stiffness crashes. If you cook it long enough, the cellulose structure will break down.

It’s better to undercook than overcook. Partly torrefied wood is still good. You’re still removing materials that you don’t want in the wood.

Bourgeois Guitars Aged Tone Fully Torrefied Guitar

It sounds like it’s really an art of finding that Goldilocks zone of heat and time for each species of wood. How much trial and error did it take for you to dial this in for your guitars?

When we first started attempting torrefaction, there were a lot of failures. Bridges lifting, tops cracking easily. I still have the first torrefied guitar we made back in 2012. I don’t think it sounds as good as it should by now, because we were still learning.

I’ve learned to compare samples before and after torrefaction, as a litmus test of sorts. I take a strip off the edge of the spruce before baking, and then cut other strips after torrefaction. You can break the strips and look at how long the fibers are. You want to see longer fibers that match those of the untorrefied wood.

The sweet spot is also different depending on where you put it in your kiln and how you pack it in there. That’s been a learning curve as well.

Hearing that will probably get the wheels of the DIY crowd turning. Is torrefaction something they could do in their own garages or basements?

Well, you need the proper equipment. We have our own scientific kiln. The hardest part is getting rid of the oxygen.

There are three ways to do that. You can either load the kiln with steam (at near 100% humidity), fill the kiln with nitrogen - which is what we do - or deep fry the wood. Don’t laugh about that last one. It’s actually done over in Europe.

Before you raise the temperature to the combustion point, you have to bring moisture content to zero. Wood with moisture torrefies at a different rate, so if there’s any residual moisture in patches, it’s going to show.

It actually looks kind of cool. We have all sorts of failed experiments that have streaks of brown and yellow and white.

Mahogany is harder to torrefy, as it can get brittle and hard to bend. We’re still trying to dial in the process in for that. Maple is much harder to overdo.

Where did you first get the idea to start using torrefaction for tops?

Several Canadian and European builders used it first. The concept is actually well tested in electric guitar and bass world.

Music Man, I believe, was one of the first to use torrefied maple necks. Tom Anderson also started using this method for his electric guitars. But Roger Sadowsky is up there as well as one of the first adopters for his basses.

This is not a new idea. Torrefaction as a process is actually patented by a Finnish company named ThermoWood. They manufacture most of the commercial kilns that are setup to execute their prescribed process (with steam).

We use a scientific oven that’s digitally controlled, with an added nitrogen function. We used steam early on, but ruined a lot of expensive wood that way.

So the idea was there. It made sense to experiment with acoustic guitar tops and see what the results would be.

Bourgeois Guitars Aged Tone Fully Torrefied Guitar

Is the industry moving more towards torrefaction?

Absolutely. I think it’s here to stay. We’re just starting to figure out what can be done. I’m guessing someone will develop a kiln soon where a small builder can do 10-15 tops at a time using a fairly foolproof process.

Luthiers should ask themselves, “How does torrefaction work for me as a builder?” You need to think about how it works with your style. People always ask me if pau ferro [an exotic tonewood] is good for building guitars. I always say that it doesn’t work with my style of building, but there are some great pau ferro guitars out there.

We’re like chefs combining fresh ingredients. The ingredients are very important. But the signature dish is dependent on the original recipe.

Why do you think torrefied tops have caught on so quickly?

Torrefaction can’t make bad wood sound great. It will, however, make all wood sound better than its original starting point.

Mostly though, there’s a market preference for vintage. So boutique builders are building in that direction. That’s just where the market is right now.

You see hipsters playing B-25 Gibsons and Style 17 Martins, the still affordable vintage guitars that haven’t been snatched up by collectors. So this [torrefied wood] fits right in.

If you were building in a real contemporary style, looking to take acoustic guitars in an entirely different direction, you might legitimately decide torrefied wood was inferior. But for now, we lionize the response of vintage guitars, their look and their characteristics. Expect to see more and more torrefied tops in the near future.

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