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Hello, all together!
I want to build an acoustic guitar with a torrified adirondack top.
What are your experiences with torrified wood? I have heard that it is difficult to glue, especialy the bridge.
Any recommondations?

I have some, but have not used it yet. If you have not yet bought the wood you want to use, I have heard from a friend in the guitar wood business that over-torrefaction is a bad thing. So, if you can look into whether the torrified wood was not overcooked, you will be better off. My friend runs Appalachian Tonewood, so you can probably trust the wood you buy there is not overcooked.

I have used a fair amount of torrified red spruce tops. The only issue I have had is that they can be somewhat brittle and they will split easier. I split two while doing the ‘sheet metal’ test (if you are familiar with that) when I was thicknessing them. I.e. flexing them cross-grain before they were braced. I like the results I get and I still use the torrified tops, just have to be a bit more careful.

Related thought: I had a client who wanted an A style mandolin with torrified back and sides. I tried every technique I could find to bend the sides and finally had to admit defeat. Again, just too brittle. I had to switch to regular maple.

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Steve Smith
"Music is what feelings sound like"

My understanding is that 'Torrefaction' is a particular method of heat treatment, involving specific times, temperatures, and control of the atmosphere. Not all 'baked' wood has been 'Torrefied'.

That said; all the 'baked' wood I've encountered has been brittle. I can't say which of those pieces were properly 'Torrefied'. I have not gotten any 'baked' wood myself, but my students have used it in several builds. None of them have used it twice, to the best of my recollection.

It makes sense that 'baked' wood would be hard to glue, as it would most likely reduce the 'surface energy'. That sounds 'new age', but it's a real thing. When you cut into a piece of wood the new surface has a lot of open bond sites: places where chemical bonds have been broken and have not had a chance to latch onto something yet. It has been shown that gluing within about 15 minutes of working the surface, while the surface energy is still high, results in stronger bonds, even in 'new' wood.

These open bond sites carry a small electrical charge. The test for surface energy involves spritzing the surface with a light mist of water. Water molecules are 'polar'; they have a small electrical change from end to end, so they're attracted to open bond sites. Water on a surface with low energy, few open sites, will bead up, while on a high energy surface it will spread out into a film. I have seen water bead up on 'old' wood, and we were especially careful about gluing any 'baked' wood within a few minutes of working it, to get good glue joints, so we didn't have problems.

I'm not a 'fan' of 'terrified' wood...

Alan, I agree with the technical aspects of your analysis but in practical application I have had no glue failures with either Titebond or HHG applied normally; I do glue immediately after scraping or otherwise working the surfaces (a long-time habit from OLF/MIMF conversations long ago). My completed guitars that have torrified tops seem to be aging well with no issues after 6+ years. Baked spruce, in my limited experience, is obviously darker in appearance and I avoid that. For what it's worth I purchased my torrefied tops mostly from RC Tonewoods although I did get a few from StewMac (of the two I managed to split, one came from each). In my opinion torrefication is not a magic bullet as some seem to think, I look at it as just another build choice much like which species of spruce you select.

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Steve Smith
"Music is what feelings sound like"

Recommendation is to use a little bit more glue than normal, and increase clamping time. Torrefied wood is hygrophobic, so adding more glue is good practice, as is letting it set longer.

The warning of over torrefaction is accurate, it can definitely be taken too far. This will be hard to judge without experience with the material, but hopefully you’ve bought from a reliable vendor. It is definitely more brittle, and if you dent it, you can’t undent it with steam like normal, and if taken too far basically crumbles. But I’m guessing that you did get it from a good source and you should be fine. The torrefaction sweet spot is meduim rare, not well done. My first half dozen or so I used torrefied bracing as well, but carving the braces gave me the heebie jeebies so I went with standard bracing for the duration. I have had no consistent problems long term in the torrefied guitars that would set the wood as a problem outlier.

I’ve built at least 30-40 guitars with torrefied tops, probably more.

On a data level, torrefied tops will have about a 15% reduction from their original density, and since we know density and stiffness are closely correlated, they have about a 15% reduction in stiffness from their original. I once sent a batch of 50 tops to a guy for torrefaction, each top carefully measured for dimensions, weight, and stiffness. That’s where this data comes from. Theoretically, a properly torrefied top will have shrunk to it’s final resting proportions so should be much more stable through humidity swings.

Tonally, it works as advertised. You can expect a more mellow, broken in, relaxed sort of tone right off the bat. This comes at the expense of headroom and tonal complexity. It makes a great niche sort of tone and is a reliable pallet choice as much as choosing rosewood over mahogany.

As Alan mentioned, there is a difference between baked and torrefied. For proper torrefaction, the wood needs to go into an oxygen free environment at a temperature above where the wood would normally combust. I think most people use steam to push out the hemicellulose.

In practice, you can do it at home by wrapping the wood in foil (oxygen free environment) and sticking it in your oven. Your wife won’t care cause it smells great.

This may not meet the technical requirements for torrefaction, but, in the small experiments I have done, there was no perceivable difference between the wood I sent out to get done, and the wood I did myself. Might look different under a microscope, but the property changes were about the same. I put a thread here once if you can find it.

My have I blathered. More glue, clamp overnight, lol.

Good info Ed. I also do not use torrified braces, just plain red spruce.

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Steve Smith
"Music is what feelings sound like"

I've built 4 dreadnaughts with torrefied Sitka tops, and 3 of those with torrefied braces.
It is indeed somewhat hydrophobic is pretty brittle.
StewMac has the best torrefied brace stock (tight grain, perfect quarter).
I am not terrified by torrefied......

Here are other observations:

• When applying glue (titebond or HHG), gently brush it onto/ into the surface with an acid core brush trimmed for stiffness.
• Take a little extra "dwell" time to make sure the wood joint surface is "wetted" with glue and doesn't pull away from the edges.
• Don't flex an unsanded full duplex top (square, not trimmed)- wait until it's trimmed to size.
• Sanding produces a flocculent and not saw dust- it floats everywhere so turn your room filter on.
• Put the top on the radius dish when you carve the braces to avoid cracking the top.
• Be extra slow and careful when carving the scoops between the peaks- errant grain direction on the downslope of the scallops can split REALLY easily.

This current build,14 fret OM, because of the cost and availability of torrefied brace wood, I didn't use torrefied braces on the back and didn't use it for the upper bout braces (sound hole strips, UTB, popsicle) or side slats.

The first guitar I built (my very first guitar I ever built) with torrefied top and plain brace wood, continues to improve in voice (complexity and sustain) to this day. I like using torrefied and can hear a livelier and more responsive guitar from start to finish. I've played a lot of new guitars, and pretty sure they were not torrefied anything...

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Measure Twice,

Karl Borum

Pacific Rim Tonewoods commissioned a report on thermal modification of wood for soundboards from the University of Dresden in 2019. You can download it for free in the following link.

Comparison of different techniques of thermal modification, regarding the improvement of acoustical properties of resonant soundboard material Scientific Report by order of Pacific Rim Tonewoods Inc

Abstract

In the present work different technics of thermal modification (TM) were compared. It was reviewed how the thermal treatment influences the sound properties of Sitka spruce samples for guitar soundboards. Therefore natural frequencies and damping was measured by acoustic resonance analysis. With this data elastic constants were calculated in the longitudinal and radial dimension. All treatments increased the sound quality, defined as a higher ratio of stiffness and density. At the same time some treatments achieved a lower damping. Within a ranking the method of treatment 2 achieved the best results.

Another source is a Handbook from the International ThermoWood Association.

My take-aways were:

1. increased resistance to humidity.

2. reduction in strength (ultimate breaking point)

3. stiffness (flexural modulus) was the same or slightly reduced. So you might have to increase the brace dimensions to achieve the same natural resonance frequency.


Greg

Thanks for sharing that paper.

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Measure Twice,

Karl Borum

I forgot to note that when gluing to torrified wood I always coat both surfaces and I also use a fairly stiff brush for that.

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Steve Smith
"Music is what feelings sound like"

Torrefied lumber is essentially halfway to charcoal, so naturally it will be brittle and have different working properties as compared to unbaked woods.

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"Act your age, not your shoe size" - Prince

Torrification is not baking or roasting. Torrification of top wood is done in an oxygen-reduced atmosphere at moderately elevated temperature (190-250C) in an autoclave - generally to exclude as much oxygen as possible and support post-conditioning rehumification. The charcoal process is similar, but generally done with enough oxygen present to support incomplete combustion to drive out moisture and other volatiles in one go.

Difficulty in gluing torrified wood as mentioned elsewhere in the thread is documented, with perhaps the best of the cautionary tales Martin's use of Level 3 torrified tops from their vendor on the 50th Anniversary D-35E. While Greenridge provided one of the VPs at Martin (and we would assume the Repair Shop) with methodology on how to successfully glue bridges to highly torrified spruce (our present method for 315g HHG plus full removal of any AR glue residue), the factory ultimately decided to re-top warrantied guitars with more moderately torrified tops, and follow-on VTS offerings used level 1 or 2 tops (I am not sure of their supplier for the D-35E tops, so the various levels of torrification will likely vary across suppliers). Using 315g HHG for this highly stressed joint also eliminates issues with both hot and cold creep seen with Titebond or similar AR glues, so something else to consider when working with torrified wood or other hard-to-glue timbers.