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Torrefied Tonewood


Berl Mendenhall
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Stewart McDonald is selling, what they call, Torrefied Tonewood, for guitar makers (not sure about mandolin makers).  It's wood that has been baked in an oxygen free environment.  I have a good friend who is a top mandolin maker who bakes his wood.  He says that guitar and mandolin makers call it "setting the pitch".  He seems to think it helps the tone of plucked instruments.  I thought this might be of interest to some here.

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I saw the wood in my morning email and was interested.  I don't play or know anything about guitars, but I enjoy the the mail and email I get from them and the videos with Dan Erliwine (sp. wrong) . We should have another rousing thread on wood treatment coming up!  I have a feeling that there are, as always, many valid paths to a valid product.

 

 

DLB

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Coincidentally, I was just reading over the Yamaha patent for wood modification (http://www.google.com/patents/US6667429) just before logging onto MN and seeing this thread.

 

A quick google search also shows quite a lot of activity in the guitar world using thermally processed wood.

 

This is something I have been doing for a number of years now, as the observable results seem to be "good", if you think that reduced density, higher modulus and speed of sound, and lower damping are good things.  Aesthetically, darker and opaque wood look good to my eye, although there is such a thing as too dark.  And probably such a thing as too brittle and splitty.

 

Also coincidentally (or not so random?) I have been getting some inquiries of late about my wood and processing, which got me motivated to upgrade my chamber to a much larger size capable of doing many more sets of wood at once, as well as 1-pc backs and larger viola wood (cellos ain't gonna happen for me).  I don't know if I'm going to go more commercial with this processing or not, but at least it's going to be feasible in a few months, once I get the new chamber up and running.

 

As for the wood itself, there are definite measurable changes in the billets, as I mentioned, but much harder to pin anything down quantitatively once the instrument is assembled.  I recently built a violin from unprocessed but well-seasoned, good wood, and that basically removed the last of my lingering doubts about the difference.  Not that the natural wood violin is bad... but it does sound "new" to my ear.

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Lots of big guitar factory companies bake the wood. It make be a cool way of saying we kiln dry wood. If you make 150- 200 hundred guitars per week, how long will you realistically have to store wood to air season it? 

 

That said it might not be a bad idea. But this is like a 15 year old topic in guitar making. 

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Lee Valley Tools has been making tool handles from torrefied maple in recent years. One of their claims is that the processed wood is nearly moisture free after processing, and has a greatly reduced tendency to re-absorb moisture, or dimensionally change with humidity changes. How could that not be a good thing for the longevity and stability of musical instruments?

http://www.leevalley.com/US/wood/page.aspx?p=69619&cat=1,41504

http://www.silvatimber.co.uk/media/pdfs/torrefied-wood/torrefied-wood-explained.pdf

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Do you evacuate the chamber Don? I'm curious how well the heat would transfer to the wood in a vacuum state. I imagine argon or some non-oxidizing inert gas could be used instead of vacuum and help with the heat transfer.

 

There is no heat transfer via conduction in a vacuum I think. Only radiation.

 

What about pure nitrogen?  People put that in tires I think.

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Yeah... I failed to mention that processed wood has a far lower EMC that normal wood, so should be far less prone to environmental changes.  And it's pre-shrunk, so shrinkage cracks should be a non-concern.

 

Baking wood is easy to do, and may do a little something.  Certainly if you get it hot enough, it should darken somewhat, although I'd worry about surface oxidation and a lighter color at depth.  I didn't see much in the way of positive physical changes from the oven-bake type of cooking.  The Yamaha patent, Plato process, and other processes (including mine) involve an environmentally controlled chamber to exclude oxygen, and usually INclude steam at elevated pressure.  

 

With pressurized steam, there isn't a problem with heat transfer.  Even with vacuum, if you have a very slow process, the radiation transfer is enough (unless you're using chrome plated wood in a polished chamber).

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I guess the steam IS the heat source in this case. How long do you cook the wood for?

 

My mentor put some billets into a commercial kitchen steam kettle for about 8 hours or so. At the time, he thought he was on to something and seemed to be encouraged by the results, however he lost interest in this after a short while. He's passed away now, so I can't ask him about any more details. I do remember the residual liquor left afterwards was the color of whiskey, indicating that there was something removed from the wood.

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I think the big companies do the process more for warranty than sound, it ensures the wood s more stable and can be shipped globally. It's less prone to crack due to environment shifts. I know from doing repair work if you call Taylor ( for example)  and give them a serial number they are very helpful with replacement parts and warranty polices. I think cooking the wood takes a lot of wood cracking headaches out of warranty policy fulfillment. 

 

The sales pitch that it benefits sound may be true as well. But traditionally there has been a stigma against kiln dried woods, one of those mythic things, so I think that it's a way to work around saying kiln dried. I would not hesitate to use this form of "kiln drying", but in the myth land of instrument making there will be those who disagree and say air dying is best. 

 

The question that is vital is to know:

 

Does this process simply accelerate the natural process of long term air drying?

Or does it actually impart some sonic advantage to the wood that air drying cannot accomplish?

 

I think this is hard to tell. Let's hope it is the later! But Meister Don is on the case. 

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Processed wood is not old wood. They are different. Nevertheless, Don's Plato process has advantages. I heard his results at the VMAAI and they are quite good.

 

I have done a fair about of wood processing eons ago. I don't do it anymore, but that is no reason for others to stop. I stopped because I wonder whether some of these processes set into motion long-term slow wood decomposition. Some chemical treatments do this, but I have no idea what heat and pressured steam do. Don is confidant that it is safe.

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The question that is vital is to know:

 

Does this process simply accelerate the natural process of long term air drying?

Or does it actually impart some sonic advantage to the wood that air drying cannot accomplish?

 

I don't think this can ever be answered conclusively without actual before/after measurements from natural aging over 100 years or more.  So we're left with only theory.

 

Part of the theory is the effect of elevated temperature, which the basic rule is 10C rise in temperature = factor of 2 in the rate of reactions.  Using this, if you have wood at 200C, it should "age" 2^18 times as fast, or about 30 years per hour of exposure.  Of couse, there can be reactions which don't happen until you get hot enough... like if you take wood to 300C in pressurized oxygen, it will basically explode, rather than just age faster.  Pressurized steam also is a monkeywrench in the simple rule, as it undoubtedly accelerates the hydrolysis reaction.

 

As Bill said, kiln drying is a different process with different intents and different results.  Kiln drying is just to bring green wood down to a low moisture content in a controlled manner to avoid damage from splitting and warping.  The temperatures are much lower than the types of thermal processing intended to modify the chemistry of the wood itself.  There could be some gray area too, as there is no hard barrier to separate one process from the other, but normally kiln drying is for huge piles of wood in large unpressurized rooms, while processing requires a pressure chamber.

 

I won't be providing many details about time, temperature, pressure, or what equipment I'm using, for the same reason Joe wouldn't be expected to give a recipe for his varnish.  Although I'm more in hobby mode than business, I want to keep the latter option viable.  Besides, if you don't know what you're doing, fooling around with pressurized steam can be very dangerous. I was fortunate to not be in the shop when one of my earliest experiments turned into a steam-powered rocket.  It took a long time to clean all of the fiberglass insulation off of the walls.

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Baking wood has been used for a long time. Vuillaume used it for a while in the mid 19th century. I had the misfortune of doing a restoration on one of his baked wood cellos and it was like planing stale bread. The wood simply crumbled.

Yes. I've had the same experience, but I wondered if I was dealing with nitric acid, rather than baking. 

 

I think if I were baking spruce fronts and wanted to exclude oxygen, I'd put them in baking bags, or pack them in sand.

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Baking wood has been used for a long time. Vuillaume used it for a while in the mid 19th century. I had the misfortune of doing a restoration on one of his baked wood cellos and it was like planing stale bread. The wood simply crumbled.

 

Vuillaume has been brought up quite often as a cautionary tale against doing stuff to wood.  However, I have never seen any details of what his baking procedure was.  There are all kinds of temperatures, times, and conditions that it could have been, and what he did does not necessarily translate to what anyone else might do, and there is virtually no possibility that he used a controlled atmosphere chamber.

 

I think if I were baking spruce fronts and wanted to exclude oxygen, I'd put them in baking bags, or pack them in sand.

 

There is still the oxygen in the trapped air spaces in the cells.  I don't know if that's significant or not, but the only way to get rid of it is with a vacuum chamber.

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