Optimal humidity for rehydrating ~30+yr old uke, 70% OK?

Wukulele · Sep 25, 2018

I'm rehydrating an early-ish Kamaka White label soprano, using two dampits inside the ukulele & simple humidifiers outside, all now sitting in a plastic tub inside a plastic bag. I'm planning on having a crack repair done in 3 weeks or so.

I put a hygrometer probe inside the uke. Is it safe to push the humidity inside the uke past 70%? I don't want to rehydrate it too quickly?

My guess is that the uke's likely been living in a hot-summer Mediterranean climate (Sierra Foothills) for a few decades & now lives in a temperate marine/oceanic climate, a few miles from the beach.

Thanks in advance.

Timbuck · Sep 25, 2018

I rehydrated a uke last year co's it had an open crack in the top about 1mm wide after a week of the damp treatment it closed right up tight I then proceded tp wick in some Titebond... the next day it looked great almost invisible..a few years ago I did the same on my vintage Martin Soprano with good results co's it still is ok today...But! this one sprung apart again as it dried out again

I'm now considering putting a new top on it.. or giving it away to let some one else have the problem.

Wukulele · Sep 27, 2018

Let me rephrase: what'sthe fastest (yet safe) rate of REhydration & max level of humidity for a uke w/a history as I described.

I'm afraid that Timbuck's response wasn't helpful, esp w/o details on humidity levels, if uke was enclosed securely, ambient temps/humidity etc.

Timbuck said:
I rehydrated a uke last year co's it had an open crack in the top about 1mm wide after a week of the damp treatment it closed right up tight I then proceded tp wick in some Titebond... the next day it looked great almost invisible..a few years ago I did the same on my vintage Martin Soprano with good results co's it still is ok today...But! this one sprung apart again as it dried out again I'm now considering putting a new top on it.. or giving it away to let some one else have the problem.

ProfChris · Sep 27, 2018

Wukulele said:
Let me rephrase: what'sthe fastest (yet safe) rate of REhydration & max level of humidity for a uke w/a history as I described.

The correct answer is that no-one knows. Wood is not an engineering material, every piece behaves differently.

If it were my Kamaka, I'd humidify it at around 70% because once repaired it will live at around 55% on average. I'd expect that to close the crack in a week or so. Once the crack was closed (see, no time stated) I'd give it an extra day or two, checking to make sure the top wasn't swelling upwards excessively, then I'd glue the crack. Then I'd cross my fingers.

Wood expands and contracts all the time as humidity changes. Over time, it tends to re-expand a bit less than it contracted. So if the crack wants to gape when at 55%, my glue joint will fail or more likely it will crack elsewhere. A different piece of wood might be fine.

If I were repairing it I'd want to monitor it as it humidifies, as that might tell me whether my repair will hold. But I'm not a pro repairer with limited space.

So no numbers, but an explanation of why you can't be given numbers.

BlackBearUkes · Sep 28, 2018

I let instruments rest in my shop before doing repairs like this. My shop humidity levels are 42-48% at all times. Wood moisture content should be at 6-7%. You need good equipment to measure accurately.

Wukulele · Sep 28, 2018

Thanks, this is helpful even w/o many numbers. The numbers you did post are similar to what I’ve been doing. This uke will be handed over to a professional luthier in several weeks as they’re booked out until then. I wouldn’t be surprised if they use glued cleats to stabilize.
Wood may not be (the most stable) engineering material but having general guidelines still allow the chance of better results than going about things blindly.

ProfChris said:
The correct answer is that no-one knows. Wood is not an engineering material, every piece behaves differently.

If it were my Kamaka, I'd humidify it at around 70% because once repaired it will live at around 55% on average. I'd expect that to close the crack in a week or so. Once the crack was closed (see, no time stated) I'd give it an extra day or two, checking to make sure the top wasn't swelling upwards excessively, then I'd glue the crack. Then I'd cross my fingers.

Wood expands and contracts all the time as humidity changes. Over time, it tends to re-expand a bit less than it contracted. So if the crack wants to gape when at 55%, my glue joint will fail or more likely it will crack elsewhere. A different piece of wood might be fine.

If I were repairing it I'd want to monitor it as it humidifies, as that might tell me whether my repair will hold. But I'm not a pro repairer with limited space.

So no numbers, but an explanation of why you can't be given numbers.

Wukulele · Sep 28, 2018

Helpful, thanks. At least one of the hygrometers in use is traceable. I thought about getting a wood moisture meter, but didn’t look beyond what’s carried by home hardware shops, since I’m not a luthier. But if there are recommended brands/models, would love to know what they are. I have other wood musical instruments & it’s possible the uke lineup in my household will change over time.

BlackBearUkes said:
I let instruments rest in my shop before doing repairs like this. My shop humidity levels are 42-48% at all times. Wood moisture content should be at 6-7%. You need good equipment to measure accurately.

Jerryc41 · Sep 28, 2018

I've seen bits on YouTube about re-hydrating.

I'm guessing slow and steady is a better way to go. The moisture in the wood will be more evenly distributed.

Optimal humidity for rehydrating ~30+yr old uke, 70% OK?

Wukulele

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