FYI: Humidifying to close cracks.

[poul hansen]

I humidified a small solid wood guitar in a plasticbag with a hygrometer inside it, to monitor the humidity there, which gives a hint of wood absorption.

I first tried to just put a big thorougly wet cloth inside the bag.
It took 3 days to go from 45% to 55% and then nothing more.

I then blew steam with a steamcleaner inside the body, lying in the bag.
It went to 99% and i closed the bag. Within 1 day it went to 71%, probably as the humidity was absorbed into the wood.

I repeated the process to 99% and in 2 days it went to 81% and it looks stable there now.
This is what I was aiming for, as 99% might also affect the glue.

In a normal environment I have found that a 10% change takes 24-48 hours to equalize inside a guitar.

[Simon DS]

How big is this plastic bag we’re talking about, Poul?
Would it be big enough for a sound studio, chairs and a coffee machine?

[poul hansen]

As the subject says: "....to close cracks...."

[Simon DS]

So the cracks didn't open again as the instrument dried out? That would give someone a surprise!

I had a friend who tried this on a bowl-back but the whole thing just popped apart. The pressure he said.
And someone else who tried it on a guitar using wood oils or something similar to remain in the wood to prevent later contraction but he said the guitar sounded leaden afterwards. (Though maybe months or years later it would improve?)

[poul hansen]

I glue the cracks before it dries very slowly. And on tops I glue cleats in as well of course.

Too high temp. and humidity might soften the glue but on bowlbacks I tape the back with tight painters tape, which will actually pull the cracks together when humidifying and then I glue them but they will actually almost keep their new shape afterwards even without gluing. I do however keep them humidified 1-2 weeks and taped a month after.

[dhergert]

Maybe this applies, maybe not... If it doesn't apply, sorry for the disruption.

We live in an arid environment in Southern California and sent my wife's 1989 D-41 to a prominent Martin Certified luthier in Utah for repair of a number of old top cracks, bridge work, a neck reset and general setup work...

He contacted us before doing the the work with some top repair options. One option was to humidify the top to try to close the top cracks, while the other option was to splint the cracks. This was in addition to cleating the top cracks on the inside (and of course he uses hide glue and applies finish to the too-cracks after the work). Because we knew (1) the top-cracks were symptomatic of this vintage of Martins, and (2) that the instrument was coming back to an arid, low humidity environment, we suggested it might be best to go with splints instead of humidification. He agreed.

Immediately after we got the guitar back, I observed "swollen" areas around the top cracks. Within a few months they had significantly flattened out, and now, a year and a half later, the repaired top cracks are just about level with the rest of the top. I can still feel them, but only slightly.

Of course now that the top cracks are repaired and stable, along with the bridge, neck and setup work, the instrument sounds and plays wonderfully.

I think either the steam method or splint method would have worked for the top cracks on this guitar, but given that the top wood on this guitar (and many other of its vintage) was never well aged before the instrument was built, and that it was coming back to an arid environment, the splint method was the right repair to make. But I do think it depended on those factors.

Granted that even provided properly aged wood, a flat-top instrument is a different animal than a arch-top instrument. But I do think some thought should be made about where the instrument is going to live after the repair. If it is a difficult battle to keep an instrument humidified in the environment where it will live, it's probably a good idea to make repairs that are appropriate for that environment.

[MikeEdgerton]

Humidifying before a repair is always a good idea and the Martin Guitar Repair manual that came out decades ago has an entire section dedicated to using humidity to clamp cracks shut. It works most of the time. Don does have it right though. Give some thought to how it will be stored after the repair.

[pops1]

I like to glue in plane shavings into the cracks. I put them in dry and as tight as I can and then flood with hide glue and wipe off excess. I've been doing this for years with great success and not having the cracks open up. They will be in the same environment that made them crack so if I humidify they will crack again. I have had to deal with cleats that others have put in and the crack opened up and now can't be closed because of the cleats. I had to cut the cleats out under the top. Using the shavings, which is like a small splint, I don't feel the need to use cleats. I haven't had one open in years. On a violin I prefer to open the seam, close and glue the crack, and then reglue the seam, so the tension from drying out is non existent. That is not so easy on a guitar. or a mandolin.

[Chris Pile]

Next time don't use a rag. Use a sponge, and squeeze most of the water out of it first.

[Richard500]

Tried this just once, on a fabric-lined bowl back, and within a day, had mold spots everywhere. Anyone have a recipe for avoiding this?

[poul hansen]

First I blow out all the old dust, then I keep the humidity at around 80% and have no problems.

But if I'm impatient I give the instrument short doses of steam, until the crack closes, glue it and leave it. That gives a very time at high humidity, so mold can't build.

For long term I would mix some Rodalon, desinfectant, moldinhibitor to the water. I haven't tried it though. Or lightly spray inside before starting.

I also had mold one time, when I let a humidifier continuously blow into the bag, and I FORGOT IT! So it was 2-3 days at 100% and condensation accumulated at the bottom, so the sides split.

[rcc56]

Super-humidifying cracks is touchy work for reasons that have been stated above.
And it has been my experience that if you try to force a crack closed, it may open again later, or a new crack may develop next to the one you have repaired.

If I do use humidity to close a crack, I generally just use dampits for a few days, or perhaps a sponge in a small dish in a flat top guitar.
The humidity in my home/shop doesn't get below 40% unless we get an unusually bad cold snap.

After the instrument has been humidified for a few days, I remove the humidifiers and let it sit for a few more days. This allows the instrument to reach equilibrium with what we consider to be safe environmental conditions for a stringed instrument.
Only after the instrument reaches equilibrium do I execute the repair.

If the crack closes easily, no problem. If it is still open so much that it would have to be forced closed with high clamping pressure, I install a splint, and level and touch up the splint best I can.