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Has anyone here have any experience torrifying wood? I happen to have a large kiln that I use for the burn-out for lost wax bronze casting and I also happen to have a vacuum pump that I use for vacuum laminating and de-airing silicone rubber. I could probably use all the stuff I already have to bake a few pieces of wood with a little good guidance. Anyone done this and if so, what is your setup like?

https://www.youtube.com/watch?v=EXH-9gXr6Ps
https://www.youtube.com/watch?v=7CA6HbwWe7M
https://www.youtube.com/watch?v=MwN47RDEodA

This person did some homebrew easy-bake oven type experiments. I'm not sure I would try this (oven fires have been known to burn down houses). If you have a gas grill and a good thermometer you might be able to adjust it to a reasonable temperature. Wrapping the wood in aluminium foil to exclude most of the oxygen and placing it between baking stones to help conduct the heat more evenly _might_ be worth a try if you have some low cost tops to experiment with.
Some people have suggested baking tops at 250F for a couple of hours to "preshrink" them. I have done this to well seasoned wood and it doesn't appear to have any deleterious effects and my in fact do some good. I'm not sure I would go much beyond that.
If your oven can hold a vacuum then you might be able to exclude the oxygen using argon gas.

Well the guy in these videos is just baking the wood in a kitchen oven without reducing the oxygen. That will dry the wood out completely and will brown it, I don’t think that it would be strictly considered torrified.
I could easily bake the wood in my kiln but removing the oxygen will allow for higher temperatures without combustion. Some people talk about using argon and others talk about a vacuum chamber but I can’t quite see how the heat is supposed to transfer to the wood in a vacuum. Does anyone have any experience with this?

I would be curious to know if there is a zone of torrefication as wood slowly combusts. Excluding most of the oxygen with tinfoil and stoneware might allow the wood to "char" more slowly if you can precisely control the heat. There are videos on charcoal making which show methods to limit the oxygen, but they go a bit farther than guitar tops need.
Kiln drying wood is something that even the professionals don't get right all the time. Different species have different requirements. Torrefication is probably even a bit harder. Aside from risking a few low quality sets for curiosity value I'm not sure it's worth pursuing.

I view "torrified" tonewood as a fad. I won't use it.

Believe me, I wouldn’t have even considered using it if it weren’t for the fact that I have this large kiln. It’s gas fired, and I can regulate the heat relatively well. But my wife happens to be a ceramicist and she has 4 large programmable electric kilns that are really accurate. But I’m not sure I could convince her to let me use any of her kilns for my experiments.

That is a pretty cool kiln! It looks like it could be used as a forge with a little modification. I don't know much about programmable electric kilns. I inherited an electric kiln from my mother's ceramic shop. It used cones to regulate the temperature, which didn't give "to the degree" temperature control. Most kilns are designed to operate at very high temperatures and have a schedule for ramping up and down the temperature for the types of clay being fired. I can see where programmable kilns might be an asset.
You might have better luck regulating the temperature of your gas kiln. If you could make an air tight metal box (maybe use an oven gasket on the lid) and pack it full of wood and noncombustible filler ( unglazed ceramic tile?) to limit the air space, you may be able to do some homebrew torrefaction.
In the charcoal videos they were burning off the vapors coming off the green wood. I wouldn't think "dry" wood would give off as much, but you might get some pressure build up in the box - so maybe add a one way pressure cooker type valve?
The nice thing about being a Hobbyist is we don't have to make this stuff pay....

Well this ‘fad’ has been around 20+ years by now. And it works as advertised as far as giving a vintage broken in kind of vibe. Great for some goals, not so much for others...

Fads have been around a lot longer than that:
https://www.youtube.com/watch?v=sfAbQVpHst4

After a little research I discovered a couple of things about this process. Most importantly I'm spelling it wrong. Its Torrefaction, not Torrification. Secondly, its been around since the Vikings. They were using it to preserve the spruce and pine they were using to build their boats and ships.
Anyway, I've been doing some experiments using my wife's most run down kiln. After a couple of tests, I think I've had some success.
I'm using spruce and maple in my experiments.
My first firing, I ramped the kiln up to 500 degrees Fahrenheit over 2 hours and then held it a 500 degrees for 4 hours, at which point the kiln was allowed to cool until the next morning. About 22 hours in total.
Here are the results.

As you can see, its a little overdone. Even though it looks black as charcoal, its actually retained a lot of its integrity. In fact, the samples seem to have become harder. I have a hard time making a mark with my fingernail in both the maple and the spruce. The spruce lost 18% of its weight. The maple lost 28%

Next firing I reduced the time. I brought the kiln up to 500 degrees over two hours again and then held it at 500 for just one hour. Then let it cool until the following morning. Again 22 hours in total.
As you can see, the results look much better. I cut the pieces in two to check that the wood had been processed right through. This firing, the spruce lost 11% and the maple lost 12%

Looks promising. How did you keep the wood from igniting at that temperature?

An interesting read on Torrefaction gases:
http://biomassmagazine.com/articles/163 ... taken-off/

For my next test I wanted to see how much tensile strength the samples had retained. As you can see, the samples have shrunk noticeably, so I expected that the processed pieces would snap first. I should have planed down the unprocessed pieces to the same shrunken down dimensions before I tested them side by side.

Clay. S, I’m using a method that I assume the Vikings were using. I’ll give you details when I be done a few more tests.

The torrified maple I have used was a really nice nut brown. I was going to make a mandolin with it but, unfortunately, it didn't bend worth a hoot, it was too brittle.

_________________
Steve Smith
"Music is what feelings sound like"

Hi Steve,
You can get that nice brown color on maple by using potassium permanganate. You could use torrified maple for the back and chemically treated wood to match it for the sides.

Good idea. I use the potassium permanganate on spruce to match new braces to old wood but had not thought about using it to match maple sides. I bought more torrefied maple than I need but it does make nice head plates and other trim pieces too.

_________________
Steve Smith
"Music is what feelings sound like"

Looks like you may be pulling it off. Give me some time to find some old data sets. I did extensive testing on torrefied lumber a number of years ago including 3 point flexure on an Instron and dimensional stability in long term water soaks.

As long as you're doing very small batches I can't see why foil wouldn't trap the gasses and prevent combustion. Large scale kilns use nitrogen because other methods would be too time consuming.

I’ve not had time to work on this testing lately, but I should be able to get back to it tomorrow.

I suspect the reason the Vikings used low oxygen torrefaction was more for making planking water-resistant (hence, lighter over the course of a raid) than to make their longboats sound older, which leads me to relate the following.

Based on both our observations and inside info from the factory, Martin had some real issues keeping bridges on the 2015 50th Anniversary Edition of the D-35 with the VTS 'highly torrefied' Euro spruce top used. We saw two which had shed bridges that came in within a week or so of each other, prompting a call to the factory. Per comments re: our person and relayed comments from the repair shop, those anniversary guitars still under warranty to original owner were routinely retopped with a lightly torrified top, given a notable lack of success in initial or follow-up bridge attachment to a well or highly torrified top using AR/PVA. When last I sat in on a call with the factory, Martin was using 'lightly torrified' tops only...the lowest level of torrefaction available from their supplier.

We advised the factory of our success using 315g hot hide (all of our bridges were done with 315g - not just the ones on VTS tops...never a failure), and while they asked for our process specification, I cannot be certain they tested or used that process on any repaired or new-build instruments.

If using well-torrified wood, serious prep to raise surface energy seems advisable if using AR/PVA. While we baked spruce tops for a few days at elevated temperatures, that was more to fix any resins than to reduce the top's sensitivity to water vapor. But it seems like if able to glue things to the top, having a guitar more resistant to RH swings could be a good thing.

_________________
For the times they are a changin'

- Bob Dylan

That's very interesting. when this stuff started showing up about 10 years ago I had a swarm of industry calls about what adhesives would work because suppliers gave them reason for concern. All of my testing using PVA for ASTM D905 block shears showed it to perform very well. The wood had a lower weight, higher modulus, and that meant it resonant but more brittle than regular kiln dried stuff. The exact degree of torrefaction could make a big difference. Suppliers were talking about water beading on the surface but I never observed that. However, it takes much longer for moisture content to increase even during immersion testing. Surface energy is a good guess, because the process is cooking off a lot of light organics, crosslinking lignin, and leaving behind a relatively pure structural composite that was wood. Take it close enough to that end point and weird stuff could happen!

Great input!

What I found interesting in the discussion with the good people in Nazareth was that despite the post-joining sanding and a pocketed bridge, the highly torrified tops they were using for that specific model still saw glue-up issues. The process I used required a final scrape just moments before the joint was closed, 315g HHG applied to both surfaces, and the bridge heated to 150 def F prior to closure. I wonder just how deep the surface oxidation and resultant reduction in surface energy goes...perhaps further than the pocket depth?

_________________
For the times they are a changin'

- Bob Dylan

Drying wood replaces water with air. If oxidation is the problem would it make more sense to torrify green wood? - would there be less oxygen to oxidize the internal cells of the wood?

I got back to testing my strips of torrefied maple and spruce against unprocessed strips. This time, after I had baked the wood, I re-thicknesses the unprocessed strips so they were exactly the same as the shrunken baked strips. This time I found that the breaking point of both the processed maple and the spruce were almost the same un-baked strips. I also tested the stiffness of the samples and found that the baked strips actually got stiffer. So it seems that torrefaction makes the tone wood lighter and stiffer, but it seems slightly more brittle. The gluing issue is probably something I should test too.
Regarding the method I am using, as I said, I read that the Vikings were using this process for their ship and boat building. There was no real description as to how they were doing it, but I assumed that they were burying a hewn or split plank in about 6” of sand and then building a long fire over it for an hour or two. This process would have three advantages. Greater resistance to rot, lighter weight which would mean a faster boat, and pulling out a hot plank from the ground would allow it to be bent more easily around the boat prior to final fitting. Anyway, my assumption appears to work. I have a large steel pot that I fill with silica sand and I bury the samples in it. No vacuum, no argon, no steam. The sand deprives the wood from oxygen adequately and prevents the wood from burning. I’m hoping this will translate to a larger scale when I start using my larger kiln with actual guitar wood.

If you did it to green wood it would likely play out like overly aggressive kiln drying. I had a very interesting conversation with the head luthier at PRS acoustics several years ago. At the time they were experimenting with torrefaction quite a bit and they were finding different practical limits depending on the wood species. One in particular that stood out was rosewood, and when it was taken beyond certain temperatures (not sharing what because it's too close to NDA territory) it would always become unstable and wanted to split.