Humidity-induced arch

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Nate Scott
Posts: 74
Joined: Sat Jan 07, 2012 5:25 pm
Location: Portland, OR

Humidity-induced arch

Post by Nate Scott »

A while ago I came across a reference somewhere to a technique for arching flat tops and backs using differences in humidity/moisture content. I think it was referred to as an old-school technique, but I admit it's unfamiliar to me, so I'm looking for more details and thoughts from the illustrious minds of MIMF.

My rough understanding is that the process goes something like this:
  • 1 - The thicknessed plates are brought to a state of extra low humidity using a hot box
    2 - The plates are removed from the hot box and quickly glued to slightly arched braces in a shallow mold (less than the typical 15- or 25-foot radii)
    3 - After glue up the plates re-acclimate to the higher humidity in the shop. In so doing, they swell across the grain and take on a deeper arch that persists at normal RH levels
If I understand the technique correctly, it would seem the plates will have a built-in low-humidity buffer to help protect from future cracks. I believe the claim was that this process made for better longevity and improved tone in lightly built instruments.

I'm thinking about trying it on my current build, and I have a few questions ...
  • Is anyone here doing this or familiar with the process?
    What are the target RH levels for the hotbox (or the giant zip-lock bag - [http://www.mimf.com/phpbb/viewtopic.php?p=8175#p8311])?
    How do you control the degree of the arch? I believe the example I saw, the same shallow mold was being used for both tops and backs, but the backs got a deeper arch.
    Could this be done successfully using LMII glue instead of HHG?
    What is the expected effect on tone - I wonder if there might be a more "sprung"/lively response?
Thanks for your thoughts.

Michael Imbler
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Joined: Thu Feb 20, 2014 8:55 pm

Re: Humidity-induced arch

Post by Michael Imbler »

Personally, I wouldn't go down that road. That is known as compression crowning in the piano soundboard world, and Steinway still uses it. Most people think it contributes to cracks in the soundboard, and most modern pianos use rib crowning like we do in the guitar world. In compression crowning both top and bottom of the soundboard are in compression creating more stress on the board especially with inevitable humidity swings. In rib crowning the top is in tension, the bottom is in compression and the midpoint of the top is actually unstressed.

I understand the theory of "humidity buffer", but in higher humidity, it can actually wind up crushing and creating ridges which then turn into cracks. My Steinway has a number of cracks that have had to been shimmed.

Just my opinion, if you pursue this, I will be interested in your results and hoping that it works out for you,
Mike

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Mark Swanson
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Re: Humidity-induced arch

Post by Mark Swanson »

I don't see it. Most of us who build flat-top instruments do this all the time, we use a "drying box", and we brace our tops in a radius dish. The braces are shaped to the same radius as the dish and the tops are pressed into the dish when gluing the braces in place. So we have been doing this a long time, but the radius never increases, our idea is to keep it the same.
  • Mark Swanson, guitarist, MIMForum Staff

Michael Lewis
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Re: Humidity-induced arch

Post by Michael Lewis »

A good session with a hair drier will do the job. Remember that you want to keep the surface as dry as you can muster so it doesn't expand. This is important when cleaning up squeeze out when you might be tempted to wash the whole surface down, which would swell the surface. Careful and accurate use of the glue is the best answer, and scrape off the gelled HHG before it becomes tough and hard. You can do more clean up later after it has dried. A few times through this process and you will gain a feel for how much hot air and glue and when and how to clamp.

Nate Scott
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Location: Portland, OR

Re: Humidity-induced arch

Post by Nate Scott »

I think this may be different than what most of us are doing (or at least what I've been doing).

To clarify, the idea is to glue up at significantly lower than normal humidity levels. For example, if your shop is normally at 45% RH, you would instead stabilize the top, back, and braces in the dry box at something like 20% (I'm making a guess here - maybe it's 10% or 30%). After glue up, the plates would arch further as they re-stabilize back to 45%.

If the completed instrument was later exposed to 20% RH (or whatever), the top and back would be able to flatten out without cracking because of how they were glued up.

My understanding of the normal process is to store and glue up the parts in the same ~45% RH environment and then keep the finished instrument in roughly the same humidity range or a little higher.

The info about piano soundboard crowning and cracks is interesting and provides a new avenue for research. Steinway's are known for their tone, no? I wonder if the affect would be the same for hardwood tops and backs.

Clay Schaeffer
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Re: Humidity-induced arch

Post by Clay Schaeffer »

Hi Nate,
The idea behind building with an arch is partly to compensate for humidity changes. If lightly braced the top will flatten out when the humidity dips below what it was braced at and the arch will increase when the humidity climbs higher. 45% to 50 % is suggested as it is in the middle of the range. If the guitar lives in the desert you may want to go lower, and if in a tropical region maybe a bit higher. If you brace it at a low humidity to induce the arch you may be building in more stress than you want.

David Malicky
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Location: San Diego, CA

Re: Humidity-induced arch

Post by David Malicky »

A late reply but I can add to this one--we've used a humidity-induced arch for our student project guitars for about 4 years. Bob Taylor suggested it to us. We've done 32-48 guitars each Fall semester this way, roughly 140 guitars so far. At first, we used a box and a space heater or hair dryer, measuring top shrinkage along the way and stopping when it shrunk some amount like 0.08" or so. But I found this method was pretty tough to control accurately or uniformly. Then I built a humidity-controlled cabinet that holds RH to +/-3%. Our ideal RH is not very low, about 38% (for Sitka tops; WRC would likely be lower). At first we used ~30% but we got too much dome and the guitars sounded tight. I played with different RHs and found that for our braces and process, bracing at ~38% will give a ~25' arch at 50% RH. It's essential to brace quickly (we use CA), like within 20 minutes, as the thin top will quickly suck up moisture after leaving the cabinet. The method is good for our 'mass production' purposes since it lets us use flat bottom braces and we don't need a bunch of radius dishes, but the $ and effort to build a good RH-controlled box is substantial. It's also helpful if you can't control your shop humidity (like us)--the critical steps are done at the correct RH (then we keep the braced tops in a sealed bag until assembly). For one-off builds, a simple option is to put the top in a box/bag with a saturated salt solution... 3 options are potassium carbonate at 43%, sodium iodide ($$) at 38%, or magnesium chloride at 33%. http://bio.groups.et.byu.net/EquilibriumSS.phtml Maybe use MgCl2 and wood glue -- the water in the glue and the slower process would probably be a good match for the top at 33% just out of the box.

Nate Scott
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Joined: Sat Jan 07, 2012 5:25 pm
Location: Portland, OR

Re: Humidity-induced arch

Post by Nate Scott »

Great information - thank you David.

Very interesting that you are getting that much arch at just 38% RH. I began experimenting with the technique a few weeks ago, and based on your input I suspect I may have gone too dry. I glued up several test braces and spruce scraps that had been acclimated to 20% RH and observed that they did in fact arch further after glue up. I went ahead and glued up a sitka top with flat bottom braces about two weeks ago. The unbraced top was down about 25% using a plastic bag and desiccant packs. I had the shop at about 40% when I did the glue-up with LMI glue. We'll see how it comes together. I haven't glued it to the rim yet, but the top alone does sound a little tight and the arch is around 25-feet with the shop at 45% and some carving left to do on the braces.

I guess as I loosen the braces, the top may arch further. We'll see. Any experience with that?

In your class, are you using an arched glueing mold and if so what radius?

Nate Scott
Posts: 74
Joined: Sat Jan 07, 2012 5:25 pm
Location: Portland, OR

Re: Humidity-induced arch

Post by Nate Scott »

Also, have you arched backs using the same method? What RH are you using?

David Malicky
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Joined: Sat Jan 07, 2012 11:11 pm
Location: San Diego, CA

Re: Humidity-induced arch

Post by David Malicky »

Nate,

I haven't checked how the arch changes with later carving, but I would assume it flattens some. We glue on a very flat table. Flat bottom braces. The result is really a cylindrical arch. We glue to a flat rim for simplicity, so that cylinder then makes somewhat of a dome.

Our student guitars are all BB ply backs and we keep those flat. For my solid-wood backs (I've done 5 so far), I've used about 30-33% RH, but have not studied it systematically. I'm mainly trying to give it more insurance against a depression and cracking, since back woods move move than spruce and cedar... same reason normal back radii are tighter.

AFAIK, the method works very well as protection from cracks. I have about 10 guitars at home using this technique and none are in cases. We get very low RH once in a while in San Diego. I get depressions of course, but no cracks on any. I'm sure it's possible to crack them, but I have no evidence of it.

All this said, I can't assume that the method is optimal acoustically, compared to radius dishes. Given that the great guitars are built flat or radiused, and that mine are not in the great category (though we simplify a *lot* of other things), the safe assumption is that radius dishes or building flat are optimal. The counterargument is that since flat works well for many, a critical question is: "at what RH was it built flat vs played?" Probably built flat at 45% or so, played from 40% to 60%. Building flat at 38% and getting a tiny arch at 45% is equivalent to building flat at 45% and getting a tiny arch at 52%. I don't hear reports like "my Martin sounds much different at 45% than 55%". So we build at a known 38% and get at least that control and a bit of insurance.

Also keep in mind that RH is difficult to measure accurately with inexpensive meters, especially below 40% or so. Many are calibrated to be accurate at higher RH, if they are calibrated at all. We use a $200 building sensor (a lot less $ on ebay); my Caliber IIIs meters usually read low by 10-15% around 38% real RH.

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