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I have decided to heat condition some tops. Some refer to this as "cooking" or "baking" tops. Some refer to it as "torrifaction", but I am NOT asking about "torrified" tops. Torrifaction entails special procedures such as an oxygen free environment and high temperatures (400F or so). I have no facilities to perform true torrifaction procedures.

My hope is to increase stiffness, decrease internal damping, and reduce mass as some have reported from heat conditioning. I recognize that strength will likely be reduced, but with the way guitars are generally structurally overbuilt a reduction in strength does not overly concern me.

I am considering a procedure with equipment similar to what Todd Stock previously described (thread now deleted). Namely, as follows:
- Insulated shop-made box with purchased food dehydrator to circulate air and remove moisture from box.
- Electric Heating blanket inside box with automatic temperature control based on thermocouple in center of box's air space.
- Sticker spruce top wood inside box with rubber bands and weights to help maintain flatness during drying/heat conditioning.
- Heat at 180F for 24 hours, then flip stack upside down and continue heating for another 24 hours at 180F
- Remove wood and rehydrate in normal shop temp and relative humidity (mine is 73F and 43% RH) for several weeks.

Obviously, there are other temperatures, higher or lower, and periods of time. Some might use 200F for 2 hours or whatever.

What is your experience with "cooking" tops?

What equipment?

What procedure?

What results (i.e. Young's modulus before and after rehydration, weight before and after rehydration)?

I can't help you with the before and after. I don't weigh them or do Youngs modulus. I do believe that it helps the top greatly on its aging process. They 'feel' stiffer to me after they have been cooked. But then wood does get harder with age. Just try to nail 75 year old softwood. It also sets the pitch pockets so they don't surprise you when you start finishing.

I bake them in the wife's oven when she's not looking. I do it like this:
Preheat oven to 180, this is important. The rising heat when an oven is being heated can ignite or at least scorch the tops. (Don't ask how I know this)
Bake for 2 hours.
Remove and place between two 3/4" pieces of plywood and weight.
Leave like this for 3-4 days then sticker for a week or two.

Sometimes, especially with rosewood, I bake the backs also. When you think about it they are the last of the body that hasn't been baked

I would prefer to do it Todd's way but I don't have the equipment.

_________________
Joe Beaver
Maker of Sawdust

Brian Burns did a lot of testing on backed tops, and still does. He has been doing it for ten ten years that I know of. This is a link to a thread he did on Mario's site 10 years ago that answers a lot of your questions, Ed.
http://www.luthierforum.com/index.php?/topic/1343-kermodie-testing-by-brian-burns/?hl=%20baking%20%20tops

Bob

Thanks, good to know.

I bought an old oven cheap and use that. Sticker and weight the wood in the oven, preheat to 200 degrees and I also set a little surface thermometer on the wood to double check. Bake for 2 hrs and allow to cool in the oven. Sticker and weight in the shop after that.

I usually don't use it for quite a while. Maybe a year now as I have a supply of baked wood.

I started doing it after I had some pretty dramatic movement with newly cut Lutz from Shane that had been sitting in the shop for quite a while. It was 6% on the meter but not very well seasoned. The baking seemed to stabilize it. I can't comment on sound, I was looking more for stability.

I bake any wood that I think may be unseasoned. I can get 25 year old Sitka and I don't bake that. I think it does improve stability. Tone? Who knows? Count me as a fan

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Do not heat wood if there is wax anywhere on it. The wax will flow just like water and penetrate the wood. You can have a joint failure years later.

I convection bake, in my kitchen oven, as close to 212F, without going over much, for 12 hrs. I keep a themometer in the oven to determine where to set the oven. In my case the oven set on 190 will heat up to 215 and then cut out, so you need to find your own sweet spot with your oven. I have not found any tonal gains, but do it purely for crack prevention. If you can get the wood down to bone dry, and then let it come back up to normal humidity, for a week or two, then assemble, you will improve the crack resistance significantly. I assembed a guitar in BC, at 40% RH, then the guitar resided in NC for a few years, in high humidity. The top swelled in the high humidity, but since it had nowhere to go, I believe it compressed the cells a bit, effectively leaving less wood when it dried back down in a 40%Rh environment. When I moved back to Canada, the first dry winter resulted in a hairline crack in the top, at 35% RH. Other guitars I built that never left Canada, and were assembled at 40%, can handle 30% RH, and not crack. Had I baked this top prior, I dont believe it would have cracked. Also had two completed soundboxes that had a nice 25' radius to them in Canada, at 40% RH, but after the two years in humid NC, and then returning to Canada, they are now flat to slightly convex at 40% RH. Once they are taken to bone dry, and slowly rehumidifed, they dont shrink as drastically next time they are exposed to dry conditions.

Thanks, Bob.

I cut and pasted Brian Burns report from 2005 in italics below.

Average changes in the 9 baked soundboards, after allowing 2 weeks for re-acclimation to an equilibrium moisture content:

Density went down 1.2%.

Long grain stiffness went up 4.7%.

Long grain stiffness to density ratio went up 4.8%

Cross grain stiffness went up 8.3%.

Cross grain stiffness to density ratio went up 8.4%

Q went up 21%

Drying Spruce: Nature or Nurture?
Bob Taylor
June 16, 2010

Guitar making is a mystery to many of us and embellished with lore. I once spoke to wood technologist Bruce Hoadley, author of Understanding Wood, and mentioned that we guitar builders think that as wood gets older the resins break down, thus causing a guitar to sound better and to maintain stability, to which he said, “That’s great. What resins are you talking about?” To which I answered, “Well, I don’t know.” I’m a dork.

He went on to say that he thinks it’s like chicken noodle soup, which people say makes them feel better when they’re sick, and it probably does, but let’s just go with the idea that it does, and not try to explain it because we don’t know what we’re talking about. So, he believes that older guitars sound better, but he cautioned me to be careful about saying why.

That was about the time I wanted to find out about spruce shrinking and if there was something I could do about it. So, I’m going to get all technical on you and tell you about some experiments I conducted about 15 years ago and how those led to my current techniques for drying wood.

Testing the Field
The scientific method is to have a hypothesis, to test it with an experiment, and then to accept the results of that experiment. That’s hard to do because often the results aren’t what we want to see and so we often are tempted to ignore them. But this was a dispassionate idea, which was to simply find out how much spruce shrinks after being dried, either naturally, or through a forced process (thus the title of this article).

Step one was to take a group of tops, air dried and kiln dried, equalize them to 47 percent relative humidity (RH), then cut and mill them to the exact thickness of my dreadnought model. They were then measured for width to the thousandth of an inch and weighed on a gram scale. Next step was to equalize them to 30 percent RH. “Equalizing” expresses the state where wood has absorbed or given off the necessary moisture to be even with the surrounding RH.

Next was to weigh and measure each piece. The results were that they lost between a 1/16" and 1/8" in width and lost significant weight due to water being removed from the lower humidity. An eighth of an inch is enough to crack some guitars, so what to do?

During this time, I also asked for the help of every guitar factory you can imagine. I asked them to send a top to me that was ready for bracing, seal it in layers of plastic wrap, and then overnight it to me. I measured and weighed each piece as soon as I opened the pack, and proceeded to equalize it to my factory, then measured again. By doing this I was able to compare the humidity levels of all the shops and factories around the U.S. at the time. Then I proceeded to find out if any of their drying methods worked better than those I had been using. They didn’t, and their spruce exhibited the same shrinkage factors. This sampling included 20-year-old tops and tops which were salt-water cured, a method we guitar makers have heard about that some people say Stradivarius employed. None of them performed better than the others.

Lessening the Shrinkage
To keep this article short enough to read, I’ll fast forward to the final drying method that I came upon. That is to heat the spruce to 200 degrees for 30 minutes, or in other words, bake it. I call it oven-roasted. I do know that the pitch in spruce will “set” at 180 degrees, but more than that, nearly every bit of water is driven out of the wood through this process. The next step is to re- equalize the spruce to 47 percent RH, which hydrates it back to “normal,” but at this point it is permanently smaller than it was before it was heated. Upon exposing these pieces of spruce to 30 percent RH, they now only shrank between 1/32" and 1/16"! That is half as much as before and enough to eliminate most of the cracking that might occur on a guitar that is dried due to exposure to low humidity.

I shared with the results with all the builders who contributed tops to the experiment. I immediately employed this method of drying and have done it ever since.

Did it help the sound? I can’t tell, but it didn't hurt it. It certainly helped the shrinkage factor. Does it still shrink? Yes, but less than before. Point is, there was enough scientific method employed to come up with a drying method that proves to be better than air drying when it comes to the stability of the wood.

When it comes to the mechanical performance of a guitar, I think that an approach like this works very well. We use a similar method for designing the sound (making samples with small changes), but the results are much more subjective because it’s a value judgment in the end. I suppose we could use some kind of meter to tell us what the guitar sounds like, but that just makes me cringe, so I’m not signing up for that. I suppose that’s why guitar making is considered a blend of art and science.

Bob Taylor is the co-founder and president of Taylor Guitars. He built his first guitar as a teenager and has since gone on to establish Taylor Guitars as one of the world’s premier acoustic, acoustic/electric and electric guitar manufacturers.

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Ken Cierp

http://www.kennethmichaelguitars.com/

So if one were to cook some tops, what would the jig look like, and how many would you cook at a time?
To keep from making potato chips, I am imagining a piece of 3/4" plywood the same size as the plates, a set of 1/2" x 1" stickers every 2", 4 plates (2 tops worth) with stickers in between, then a piece of ply on top, then clamped.
Bake at 200 for 2 hours.
Then the whole rig moved untouched into the workshop for an extended period for re-hydrating.
Would this approach bake them properly, or is that too much sticker?

The dimensional stability aspect of this approach looks worthy of pursuing...

That's pretty much how I do it.

_________________
It's not what you don't know that hurts you, it's what you do know that's wrong.

Many thanks for the advice Terence!
I have avoided this in the past due to dubious tone improvements, but the dimensional stability aspect has me very interested.
I have had a few of my early builds self destruct in my sons and nephews poorly humidified environments in northern Alberta....
I think it gets to almost zero indoors without a humidifier sometimes when it is minus 40 C....

I think this comment about "dubious tone improvements" is interesting and it seems to be a common statement. So I am not picking on Rob here, but just wanting to open a dialogue about this.

While luthiers have difficulty agreeing on many things, I have seen almost universal agreement that stiffer less dense tops are better than less stiff heavier tops. If you are reading this and think that heavier less stiff tops are better or think that it does not matter one way or another, please speak up so I will know that there is more disagreement here than I thought.

Now, if the vast majority of luthiers agree that less dense more stiff tops are better. This better must be better tone or volume or sustain, I assume. If not, what is better?

One thing I see reported consistently is that heat conditioned tops end up stiffer (say 10%) and less dense (say 1.5%) than when they started. (Improved Q for more sustain and improved dimensional stability are additional improvements from heat conditioning being reported.) Since the top is stiffer and less dense than before heating, isn't it better by definition?

I may not be able to hear the difference since it's impossible to hear the top before and after heat treating, plus the change is small. Yet, would we not pick stiffer and less dense tops over the opposite?

What am I missing? Please help me understand the argument that heat conditioned tops are not improved when their stiffness and density are better. If you say that they are less strong, then when has one catastrophically failed?

Robbie,

I use 3 of these to hold the top set for baking


set up like so


Don't clamp to tight as it will leave impression in the tops.
I bake at 200F for an hour, let cool in the oven, then repeat.
I then let it acclimate for a day before removing the jig.

Bob

Thanks for the pic and the process Bob, exactly what i was looking for.

Rob

Ed I think that one of the things that you are running into is some of the guys (meaning men and women of the OLF) may be reluctant to speak in subjective terms without empirical data to support one's conclusions.

Although this never stops some.... from sharing when someone asks a question such as this excellent question it's understandable that their intention and hope is that someone will participate with a more objective approach with hopefully some testing to describe and some data to share.

Just like with Strad violins centuries can pass and it remains true that we really don't have a clue.... about some of the whys associated with the strad sound. Nor are these instruments even entirely original these days either....

I can tell you that when I started baking (my tops, I have been wake and baking myself for decades and I still sound like crap likely...) I noticed a difference in the resulting instruments. But I can't prove it - period. This makes me reluctant to offer opinion or worse, contribute to the furtherance of snake oil when even though I may hear a difference I may also be in error and wrongly attributing what I hear to one variable and not others.

When these questions get asked and when the truth may be that no one really has a legitimate scientific argument to support an absolute conclusion most folks simply go off and explore on their own. That's what I did with the science/art/etc. of voicing.

As for your logic it works for me. Less mass, stiffer, and a molecular structure permanently changed by the baking process where it's likely that the cells are no longer capable of absorbing the quantity of moisture that they once did all seems to me to be a highly desirable thing.

But I can't prove it and to date no one else here has to my satisfaction proven their conclusions either.

Sorry to be pedantic as Trevor might call it but when I was in a philosophy class in university on the very first day the professor strolled to the front of the room and the fist thing out of his mouth was "hello I'm professor Genden and I believe that it's better to have sex with animals than to eat them...."

Of course the class erupted in proclamations of "gross" and "what a sick ^&*^" etc. followed by one, two, and more students getting up and walking out of the class....

I watched the prof. and he just stood there watching and grinning. So I decided to stick around and see what he did for act two.

In short order he spoke again now proclaiming "Let's try this again, my name is prof. Genden and I really don't believe that it's better to have sex with animals than to eat them but I wanted to have fun this semester and wanted students with open minds in my classes...."

Only half the class was now left... He asked if anyone else wants to leave now is the time. No one else left. The prof. closed the door and then said to us that if nothing else is gleaned from his class he wanted to arm us with the ability to ask "what's the harm" when considering doing something.... His belief was that if you can find no harm in something and you are interested in it - go for it.

By the way this guy was the national director of the humane society and a vegan sort but not pushy at all with his beliefs. He later became a friend of mine.

So again sorry to be pedantic and bring up sex with animals, vegans, likely liberals...., and skirt political issues but when it comes to baking a top - what's the harm? And yes I actually think like this....

I've' bake a few top using spacer sticker clamps similar to above -- they come out a little stiffer than others unbaked off the same billet. The odor from this process is not pleasing -- the kitchen oven is now off limits. I have an idea for a light bulb heated unit that I am sure would work well but just not that enamored with the concept to pursue at this time.

I have not checked with the folks at Taylor, but I believe they bake backs and sides as well for the crack prevention benefit.

_________________
Ken Cierp

http://www.kennethmichaelguitars.com/

Hesh,

Thanks for your message. Can you please share you equipment, methods, procedures and empirical results from your baking?

Ed

Wow it's not often that I get to quote myself....

Ed what I said was that I can't prove my impressions making me reluctant to share them - although I did anyway.

As such no empirical data for you (or me...).

As for methods I place tops in a preheated 200F oven and weight them down at once, close the door, bake for an hour, turn off the oven and let it cool naturally to room temp. Remove the tops while still weighted down and let sit in my RH controlled shop for about two weeks before building with them. I'd set-up a pic for you now but I have meat loaf in the oven for another hour.... Love meatloaf....

My impressions of the tops after baking are notable or at least are to me because the wood feels very different afterwards. It's much "scratchier" for lack of a better term and seems lighter in weight too but again I never threw it on my gram scale to check it out.

I plan on baking for new builds going forward. Not interested in torrification until I see how things pan out for those trying it now. What I am most interested in is how terrified tops do say 50 years from now so please remember to dig me up and let me know?

I lied and just found an old pic from a toot I did years ago on baking.

I'd like to try it some time but I know better than to use my wife's oven.
Like my house and don't want to lose it.
kencierp mentioned using light bulbs, I have a stash of 200 watt bulbs.
Also have a heating blanket could I use the blanket as a heat source?

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Say what you do, Do what you say.

Our production benders use either three 200 watt or two 300 watt bulbs - plenty to get to over 200 degrees. My plan/thought was to make an double walled aluminum enclosure with a bulb array like the benders, the stickered/clamped stack of material would be loaded like a toaster.

A heat blanket needs to be in full contact with the heated surface -- else it will burn itself out in a few minutes.

_________________
Ken Cierp

http://www.kennethmichaelguitars.com/

I didn't know that, Thanks

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Say what you do, Do what you say.

I know that Todd Stock uses a heating blanket successfully to heat the air in the box (together with a food dehydrator which is not hot enough), not in contact with the sound boards. Maybe there are differences in heating blankets would explain the differences.

I would not use an electric heating blanket without a temperature controller.

"Omega" blankets --------- clearly stated in the installation manual. Blankets are designed to heat surfaces not free air, so the warning makes sense to me.

_________________
Ken Cierp

http://www.kennethmichaelguitars.com/