Concave acoustic tops- new build

[Simon]

Hi, I'm new here and very much an amateur builder. I have successfully completed one all mahogany flat top, primarily with the help of Jonathan Kinkead's book as well as various other resources. I am now working on two more small guitars as well as doing a bit of restoration on a couple of old ladder braced guitars.

My problem is that the tops I have made have become concave and I don't know why. They are ladder braced tops which inevitably have a little less comprehensive structural strength but I don't think that can explain it. Both tops are spruce. I have cut and carved spruce braces, with good tight grain perpendicular to the tops. The braces were perfectly flat when I made and glued them. When I returned to them to continue the build they both have a significant dip across (mainly) the lower bout such that a straight edge laid across the edges has a gap of 3 to 5 mm in the middle where the bridge would be.

I have had a similar issue with one of the guitars I am restoring. It came to me as a bit of a wreck, with the back off and very concave. Two braces were beyond rescue and I replaced them. I reshaped two others before re-gluing as they were in a pretty bad state. I arched the braces to create the appropriate gentle convex contour in the back. Despite that, the back has ended up concave a few days later.

My questions for the many more experienced luthiers here are therefore (1) any clues as to what is going wrong; and (2) Am I right to assume that these tops are a write-off and not worth proceeding with?

Thanks folks...

[Roger Häggström]

The wood and/or the humidity around the guitar was not dry enough when you glued the braces. When wood dries it shrinks and no brace can hold the top flat. When gluing tops it's important to keep the humidity in the air and wood as low as possible. Water is in the glue and in the damp cloth to clean up the glue excess too. Use as little water as possible when gluing and give the wood a good heating with a heat gun to make the surface really dry before doing the assembly. It's a good thing to give the top a bit of curvature, if the top dries up it won't crack until the dome has become flat from the shrinkage.

To rescue the top you need to reglue the braces, new or old, this time in dry conditions.

[Simon]

Thanks for the insight.  It was all done in the same room..  The wood was in there for a long time before I cut and glued the braces, and stayed there afterwards so the humidity ought to be broadly the same. It was also the same room I did my first guitar in- but perhaps that was a different time of year.  I think I'll get a humidity meter and see if I can un-titebond the braces!

[Roger Häggström]

Yes, a humidity meter is essential :-)

I like cheep analog ones the best, they can be hand calibrated and the needle corrected. Put the meter in a glass jar with salt covering the bottom about 0,5 cm with just enough water added to make a dry "slurry". Like snow you can make snowballs with. Put the meter on top of the salt "snow" and close the jar with an airtight lid. Let the jar be closed over night and the meter should show 70%. Adjust the needle and you have a meter that shows a correct (enough) value. The cheap digital ones can't be trusted...

[Roger Häggström]

Looking at some examples on the net about calibrating humidifiers and it seems that the humidity should be 75% and not 70.

[Mark McLean]

Yes Simon, as Roger said, this is probably a humidity issue.  Every beginning luthier goes through this.  We all start out focuing so much on the build tasks that we tend to ignore "peripheral" issues like humidity control in the workshop.  And then every one of us has an experience like this that teaches us an important lesson.  For me, it was also on guitar 2.  

Here is the thing - wood is very hygroscopic, it loves to absorb water from the atmosphere.  This was the function of the wood fibres when they were a tree and they still do it when they are on your workbench.  This makes it dimensionally unstable, swelling when it gets exposed to some atmospheric moisture and shrinking when it is dry.  If you work in a space where you can control and measure the humidity level (measured in relative humidity %) it is best to keep things steady at around 45%.  But if you are building in a draughty shed or garage you don't have the choice.  You then need to adapt your building to the prevailing conditions.  Having a reliable humidity meter to measure RH in your workspace is a critical thing.  

During the build process the crucial stages are:

1.  When you glue braces

2.  When you glue the back or the top to the sides

The serious mistake is to do either of these steps when the humidity is high and the wood is relatively swollen (even though it won't look any different).  Later when the wood dries and shrinks (which for a flat plate like a soundboard is mainly a shrinkage in width across the grain) the effect is that free plates turn concave, or if you have glued them to the sides they may crack.  My bet is that you glued braces to your two soundboards on a rainy day - and now it is dry?  If your workspace is not RH controlled there are just some times when you can't undertake these stages of the build.  You need to wait until RH has been <50 for a few days.  Do something else on the wet days (like neck carving, inlay work, read a book.......).  Once you have an instrument past the critical points ( i.e. as soon as the box is closed) you don't have to worry so much about RH.

You don't need to throw the soundboards away, but you do need to remove the braces and start again.  You could plane them off, or if you used titebond or hide glue they will probably come off with heat/steam and a palate knife (but be careful of grain runout)  Don't dispair, you will get past this setback!