Cracked top on a classical. Causes apart from humidity?

[Simon Magennis]

A guitar a work colleague has had for about a year suffered a cracked top yesterday. I will be getting it back next week to have a look. Looking at the photo and considering the weather we have at the moment, I am pretty sure that the immediate cause is a a drop in humidity due to weather plus central heating. The crack runs from the end block to the bridge and onto the rosette. I had a 1984 Spanish guitar from a reputable builder crack the same way here in our home a couple of years back at the same time of the year during a cold snap. First time it has happened one of my builds however.

What structural elements, timber characteristics, that might contribute to this? Really I want to figure out if it is something wrong with my building or just a case of sh%t happens. It has a pretty thin top. Its about number 15 for what it is worth.

My plan is the get the guitar back, put some thin animal glue on the crack timber and then re-hydrate. If it closes up 100% then I will apply a bit of heat to reactivate the glue. Maybe put in done or two very small patches one each segment.

Click on the photo to get a reasonably sized image.

I have already learned something from it and will learn more by repairing it. As he paid very little for it and I am quite OK to give him his cash back, there is no major harm done but I want to avoid this if I start charging real money for guitars in the future.

[Barry Daniels]

These kind of cracks are common when the instrument is exposed to low levels of relative humidity. It was probably not related to the way you built it or the wood you used. Does your client monitor and control the humidity in his house? If not, he may need to.

Before you put glue into the crack, get the crack to close by placing the guitar into a humid enclosure.

What kind of a finish do you have on the guitar? You might want to consider putting a sealer on the inside of the top for future builds.

[Simon Magennis]

Thanks Barry.

I reckon the guy never gave humidity a thought. In this climate it is not usually a concern. You only find out how important it is if something goes "pop". Winter here often turns up some dry cold days and the buildings are generally well heated so indoor indoor humidity can drop quite fast. I guess people with expensive instruments do take care but others will not and more often than not stay lucky.

My reasoning behind putting in some thin glue before (or during) the humidifying process is that the crack on the 1984 Spanish guitar I have closed up so completely that I have difficulty finding it let along putting in any glue. I have no idea how I would get glue into a completely closed crack.

[Barry Daniels]

The way to work glue into a closed crack is to lay a bead on the top surface, then take your fingertip and rub along the crack while taking the other hand through the soundhole to press upward on the crack. This flexes the crack open enough to allow glue to enter. When the glue starts to get a bit sticky, take a slightly damp rag and wipe it clean. The following day you can glue some cross grain cleats on the inside.

[Simon Magennis]

Thanks Barry. That is very helpful. I will be getting the guitar back on Weds. With a bit of luck 2 or 3 days hydration might get it closed up, assuming there is no other issue.

[Barry Daniels]

Another technique is to use a small rubber suction cup which can force glue into the crack.

[Simon Magennis]

I got the guitar back yesterday. A few hours humidification made a fast and very clearly visible difference. I decided not to humidify overnight as I want to check it every two or three hours. Fingers crossed.

[Simon Magennis]

After a few days re-humidifying, the crack is closed up so tight that it is mostly invisible - mostly it is just given away by the fact that the shellac is cracked on top of it. I am now letting it dry.

I have tried working in some fairly thin hhg into the crack as described above but I reckon very little went in as there is virtually no movement on the crack now.

The crack almost parallel to the centre fan brace but moving slightly further away as it gets towards the end block. There is very little space for cleats especially between the sound hole and the bridge. Would putting some paper or cloth strips that actually go up onto the braces, or at least touch them, be a reasonable option instead of cleats?

Thanks.

[Barry Daniels]

I would add some tiny spruce blocks right up against the fan brace.

[Simon Magennis]

Thanks Barry.