Post
by Mark McLean » Sun Jul 17, 2022 1:46 pm
Hi Jurgen
Most of us have experienced something like this. Humidity issues are a serious concern when you are building with solid wood, and unless you manage the humidity during certain stages of the building process you will get issues like this. It should be fixable.
First lets go over what caused it. Sorry if you know this already - but other people will be reading this also, so it is worth explaining. I am certain that this happened because you performed one or more of the critical stages of the construction at a time when there was high humidity. The plate expands in width (only slightly, but significantly) when it absorbs water from the atmosphere, and it shrinks again when it dries. Imagine if it is humid on the day that you glue the back onto the rims. It all goes well and looks great. Then, maybe 6 months or 6 years later, the guitar experiences dry conditions. Moisture content escapes and the plate wants to shrink by maybe 1 or 2%, but the outside edges are firmly attached to the rims and cannot contract. If you have built it with some arch in the back the first thing that you will notice is that it goes flat, because the arch is there to provide some wriggle room for expansion and contraction. But if it keeps shrinking, something has to give. It will usually crack at the weakest point - which might be your centre joint, but in your case it is the natural weakness where the sapwood joins the heart timber. To prevent this you need to avoid high humidity when building, and avoid it getting dry later. Ideal conditions for building are at Relative Humidity (RH) of 40-50%. Factories doing high volume production keep the whole production environment in this controlled humidity range all of the time. For people like us this is a difficult thing to achieve. In reality there are only 2 critical stages of guitar construction where it makes a big difference. These are the day that you glue up the braces, and the day that you glue a back or a top onto the rims. So, you can get by without fancy climate control if you just pay attention to the prevailing RH, measure it carefully right in your workshop, and choose the right time to do those important steps. Just make sure you do those jobs when RH has been stable at 40-50 for a few days. Don’t glue those steps when it is humid or rainy. You can do just about any other part of the build, just NO BRACING, NO CLOSING.
OK, so you broke the rule, and now the damage is done. Can it be fixed? The good news is that nearly everything can be fixed. You have a few options, going from the simplest to the most complex in order:
1. Rehumidify the guitar. The crack will probably close and you can glue it, cleat it or glue a joining strip inside, and it will look OK. BUT - if the guitar gets dry again it will be susceptible to cracking again, maybe in a different place, because you set it up for this problem on the day you made it. If you know it is going to live its future life in a high humidity climate you might get away with this, but it is a risk.
2. You could graft in a sliver of additional wood to effectively make the back a fraction wider, but without needing to deconstruct it. Do you have any offcuts from the back? If so you could cut a long thin splinter and graft it in and you would probably get a pretty good cosmetic result. If you want to do it this way I can give you some more tips about how to go about it. It would make it more stable in the future. The risk would be an imperfect cosmetic outcome, but it will be stronger than method 1.
3. Fix the underlying problem. Take the back off, remove the braces, repair the crack by humidifying, gluing and maybe filling the defect. You might need to rejoin the back with a centre strip (as you mentioned) to restore it to correct width. Rebrace it and then reglue it to the rims (and make sure you do these two steps at RH 40-50%). More work, but best possible outcome.
Whatever you do, get yourself at least two RH measurement devices and watch them carefully. If you want to get into controlling RH in your workshop there are lots of good threads at ANZLF to advise you.
If not I can try to make a sketch
