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I have a couple of very dry guitars that need crack repairs. I've got a bunch of damp sponges in travel-type soap containers with holes drilled in them in the cases, and I have them in my well-humidified apartment (between 45-50%). I recall reading somewhere about a repair person or shop that will put a dry guitar in a large trash bag with something like sponges, then sealing the bag, and leaving the guitar in the bag for about two weeks (changing the sponges as necessary).

Do any of you do something similar? If so, how?

The garbage bag re-humidification technique works. I've had success in getting cracks to close within 1 week. Just be sure not to let the guitar touch the wet sponge.

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JJ
Napa, CA
http://www.DonohueGuitars.com

Marty Reynolds at The Podium in Minneapolis has a clear clothes storage bag hanging on a hook with a bunch of moist sponges in the bottom and calls it his guitar ICU. There are always a couple of guitars inside in winter months. I rigged up a similar thing in my shop with a clear plastic bag. Closed a couple of big soundboard cracks in 5-6 days. Basically the same thing JJ said.

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

Throw your guitars in a bath tub along with a radio playing White Rabbit by Jefferson Airplane.

Be aware that rapid humidification can crack lacquer. I've seen this happen on Martin guitars when using two Oasis humidifiers in a hard case.

But seriously stick it in the bathroom when you take a shower and leave it in there sealed up for a while.

Hey, Mr. Amuck

I think your bath tub/White Rabbit fix only works for dehumidifying 300 pound Samoan lawyers.

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michael propsom
www.propsomguitars.com

Thanks to all who replied. YukonArizona, thanks for the lacquer warning. Both guitars predate the use of lacquer (one's 1914-ish, one's in the early 20's, both by a single-luthier shop) so I'm hoping the finish will be OK as long make sure no water gets on the guitars.

I put myself in the tub and sang "White Rabbit" at the top of my lungs until my neighbors knocked on the walls... does that count?

I know it may be common practice, but if you "Superhumidify" an old guitar to close cracks, are you not just setting it up to crack again in normal conditions?

I believe it works on everything and everybody but I appreciate that there's another H.S.T. fan in the house.

My intent is to over-humidify the guitars then let them dry out back down to 40% RH or so. My theory is that it's easier for guitars to dry out than to "get wet," so to speak, so it would be easier to go past 40% and drop down, and know the guitar has been properly humidified, than to hope it's reached that point.

At least, that's my theory.

You mean your hypothesis?

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

...and a hypothesis that doesn't seem the match the physics very well.

This can be viewed as a pure diffusion problem. Either diffusion across a barrier for water entering/leaving the wood and straight diffusion for water moving in the wood.

Diffusion is governed by Fick's Law, and Fick didn't care if the water was coming or going.

One thing you can to to accelerate water transfer is to use a fan. This help mix up the boundary layer which will help the wood equilibrate to the surrounding air faster by assuring that the atmosphere which is contacting the surface of the wood is the same as the atmosphere in the rest of the room.

-jd

Perhaps it is a theory. I found this upon further investigation:

"Wood exhibits hysteresis, which means that if wood comes to equilibrium at a given relative humidity and temperature, the EMC will be slightly higher if this equilibrium is reached by losing moisture than it would be if it reaches equilibrium by gaining moisture. In the extreme, this effect can be as much as 3% moisture content."

http://www.fpl.fs.fed.us/documnts/fplrn/fplrn268.pdf Page 2, search for Hysteresis. I found it by googling hysteresis wood movement humidity. A lot of the cites have the exact same wording so might come from a single source.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

The Hysteresis would be the result of an energy barrier at the wood|atomsphere interface. (diffusion across a barrier)

This barrier would act to inhibit diffusion until a certain concentration gradient existed.

Two ways to affect this:

1) lower the barrier height (induce mechanical stress at the surface)
2) increase the energy level of the molecules ( add heat )

-jd

So when doing the trash bag trick, do you leave the strings on or take them off?

What does this mean?

Heck if I know. I did one guitar without strings and one with strings.

Well then I'll assume that if it worked on both accounts that it doesn't matter. Thanks for the info.