Cupping with Mahogany

[limehills]

So I'm new here and seeking advice. I am working on a Mahogany Dreadnought and run into some problems, the back has cupped quite badly, and wasn't like that a few days ago, I'm not really sure what to do about it, I also experienced cupping while bending the sides. Anyone else experienced this issue or know how to fix it?

[kiwigeo]

My first thoughts...humidity changes between time you glued in the bracing and the time you took the photos.

[GregL]

Hi limehills,

I agree with Martin - humidity change. What's the weather been like near you lately?
And also - what sort of Mahogany is that?

Thanks,
GregL.

[Mark McLean]

Every few months someone will post with the question “why has my wood become bent”. The answer is almost always humidity (specifically, the immediate humidity in your workspace - influenced by climate and also by air conditioning, central heating etc. ). You braced your back and glued it to the sides when the relative humidity (RH) was high (so the wood was a bit swollen - but imperceptible to you). It has now dried out and shrunken. Are you measuring and controlling RH in your workspace? If you are, and you know it was Ok, then we can talk about other rare diagnoses for this. But it is nearly always RH issues.

If it is a himidity problem - what can you do? 1. This back probably needs to be removed from the sides, the braces removed, and do it again (sorry for the bad news). 2. Get a humidity meter. Only glue braces or attach plates when RH is less than 50% (stable for at least 24 hours). If you are doing a lot of guitar building you will want to control RH to keep it stable (eg running a dehumidifier in your workshop if you have a humid environment).

[limehills]

cheers for the responses. I'll have a go at fixing it, lets see if i have any luck!

[limehills]

Surprise surprise took it back to the workshop where i originally glued it and within a few hours it had just about returned to its original shape. I was thinking of doing something slightly dodgy and gluing a small brace inside to stop it from moving again instead of disassembling it, or alternatively if i put a sanding sealer on it like shellac would that stop it from going anywhere?

cheers
Laurence.

[simonm]

The older generation of German luthiers, pre-aircon, pre-dehumidifiers, were of the opinion that you prepared everything and then did all the bracing, top and back, and closed up the box within 24 hours. In other words, they locked everything in before the humidity could change much when there was a good weather window. If the weather was unfavorable, they did stuff that wasn't real effected by the humidity. There is always something productive to do in a pro-workshop.

[Mark McLean]

Lawrence
The concern her is that this could keep happening. When your guitar gets a bit dry it will develop shrinkage and cupping again. But when it absorbs some atmospheric moisture it recovers. Why worry? Well,when it is dry the action is lower, buzzing strings appear from nowhere, and (worse case scenario) the back or sides can crack. If the recent episod was provoked by extreme hot/dry conditions which are not likely to occur again - then you might be OK to consider this a one-off event (but carefully protect it from low RH in the future). However, if this happened during pretty normal conditions, which are likely to occur again and again - then you should still be worried.

This is why the canny luthier only does bracing and plate gluing on days when the RH is low. The guitar becomes "programmed" to the conditions at the time of these procedures. If you brace when dry and the guitar subsequently experiences high humidity, it will cope with that perfectly well. But if you do the critical steps when it is humid, subsequent dryness can be a big problem (" wet-to-dry it will die, dry to wet it will live yet").

[Allen]

What Mark said.

If you glued up in high RH then this instrument is destined for failure. Don't worry, you wouldn't be the first to happen to. My first guitar top and back turned into potato chips the day after I glued the braces.

[limehills]

so I glued more bracing in as a quick and easy fix (this one is for me so I'm not too worried) and left it a few days in the room with low humidity where the cupping originally occurred and it hasn't gone anywhere since, I think what Mark previously said makes sense about the abnormally high humidity, so i glued the top and and no movement since, fingers crossed it doesn't go anywhere in the future.