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I braced 3 tops in March this year when the humidity was low (25-30). I wondering if i can glue them on the box now while the humidity is higher (45 - 55)

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Mike McNerney

Probably. But it depends on exactly how you clamp it. If you apply spool clamps in the center and work your way out, it will crack in low humidity. If you apply spool clamps at the widest points and work your way to the center, you'll get a big bubble that can't be squashed all the way flat. Best is to clamp the widest points first, then center, then half way between, and continue dividing in half until there are clamps everywhere. That will compress the wood some, creating a similar condition to gluing in low humidity and then having it expand later. And the compression will be spread evenly across the width of the top.

The only humidity that is truly important is the humidity present when attaching the plates. This event "sets" the plate into a rigid location from which it cannot move or you have structural problems.

In other words, if you attach the plates during low humidity - then you have to keep that guitar at low humidity for the rest of its life. Obviously, the lutherie industry chooses to do gluing operations in the low 40% range because that is the humidity range that can be obtained in almost all environments with a bit of care. Truth be told, instruments probably would sound a bit better if we made them and kept them at lower humidity levels but that is not a practical or reasonable solution for the general populace.

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Regarding your situation, yes - you can glue them on and as long as you keep the humidity where it is now , then you'll be okay. Nevertheless, you are adding tension in your top by having the bracing glued on at a much lower RH. Personally, I would remove the bracing and redo at the current RH. I can't tell you what sort of problems you might encounter but it certainly goes against conventional lutherie wisdom to brace at a drastically lower RH and then glue the plates to the rims at a higher RH level. That said, if the plates look okay and they aren't distorted, then you might be okay but I certainly wouldn't want to risk it.

Hope this helps !!!

If you have successfully completed "boxes" at the lower humidity levels you braced these at, gluing the plates on at 45-50% should not present a problem, and in fact is probably better than having completed them at the lower humidity. Gluing them up at the recommended R.H. should create less stress overall in the completed box, but won't change the stress induced by bracing at lower than recommended levels. Personally I would rather brace at R.H. levels that are a little low rather than a little high, and quartersawn wood has less movement than flatsawn, so I would be inclined to continue on with it.
If you want to crunch some numbers and see what the wood movement will be this site might help you:
http://blog.carbideprocessors.com/wood/ ... expansion/

Lots of misinformation.
RH only really is critical when bracing. I have glue plates when RH was all over the place. What is more important is how the plates are cured and stored.
Why is the bracing the critical point with RH ? It comes down to this. Once you glue the braces to the plate you lock the cross grains of the plate. This is where the stresses collect and make the top rise and fall. Yes plates will move in RH changes but that is from the top cells expanding and contracting from the moisture.
Once braced these stresses build up and when they get high enough will cause damage to the top. I like to brace tops in the 40% range. I store my wood in 45 to 50% .
Once the plates are glued you can close the box . It is just common sense to work in the 50% range if possible. If your making one in Arizona well that is a different story. They are dry as dust there.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

To add to John's point - the average monthly relative humidity in Ottawa is 63% (the wettest month is January when indoor humidity is lowered by heating), Where John and I live it averages about 68% R.H. and in Florida it is closer to 75%. Daily fluctuations and indoor humidity levels can vary greatly. Assuming the guitars will "live" in a climate similar to Ottawa, building at a slightly less than ideal R.H. may be O.K.