Wood, Hide, or CA for Crack Repair

[Doug Cannon]

I have five short cracks on a 40 year-old mahogany back Gibson that were a result of lack of moisture. I've rehydrated the guitar, the cracks are now well aligned, and I'd like to know what type of glue would give me the best results and whether cleating is necessary for the inside in this case.

Although I've always leaned towards Titebond for most wood repairs, I see quite a few people using Thin CA glues for tight cracks and wondered if this might be an appropriate repair for it. As I've said the cracks have closed and are clean, it's a nitro finish (Gibson), the surface is even, and I know that the CA has "wicking" or capillary tendencies.

Crack #1 (Pictures below) is under the center seam bracing....that won't need a cleat. #2 and #3 are about 2 inches long and near the kerfing and I'm wondering if cleats are necessary. There is no movement on either side of the cracks when pushing on them (There was when I first got the guitar. Thankfully there are no cracks  on the mahogany sides or spruce top.....small miracle. There are also no cracks in any of the back bracing or kerfing.

I appreciate any and all suggestions,

Thanks so much!

Doug

[Roger Häggström]

Per Hallgren did a test with four different glues. From the foreground to the background; Titebond Original, Titebond Liquid Hide Glue, Lee Valley Fish Glue and Hot Hide Glue. A string of glue was put on the same size cedar wood pieces and left to dry overnight.

As the picture shows, only the fish glue and the hot hide glue shrunk with enough force to deform the wood. Even if any glue will contract two well fitted pieces of wood when drying, this experiment shows that HHG and Fish Glue will do a better job.

[Walter W Wright]

wow, nice! 

if nothing else it suggests that fish glue is actually pretty awesome for situations where you wish you could use hot hide but the time window isn't there.

[Howard Klepper]

Interesting.  But might it not just be showing that the fish and hot hide samples had a higher water content?  The titebond line looks rather narrow compared with the others.  Were the amounts of glue in each line precisely measured?    An alternative hypothesis about the result might be that the hide and fish glue were thinner and spread out more, and that is why they contracted more as they dried. This single test is suggestive about shrinkage, but it was not direct test of any of the glues' ability to pull a joint between two wood surfaces tight. 

[Roger Häggström]

I trust Per, he is a very meticulous kind of guy. I'm sure he did the experiment the right way. Easy enough to try it out yourself anyway.

The shrinkage is why HHG and fish glue can suck two pieces of wood together. When water leaves the glue joint, the glue joint will shrink and the two wood pieces will be drawn nearer by the "vacuum" left behind.

About the only time I don't use HHG or fish glue is when I glue big patches of veneer on a thin piece of  wood. Like gluing a veneer over a big side crack or a big spruce patch under the top under the fretboard when the fretboard is a thin ebony veneer (on a Romantic guitar). The suction effect  can buckle the wood like the wood on the picture. Titebond Original won't do that.

[Howard Klepper]

I don't get the sense that you gave any consideration to my previous comment.  Yes, when water leaves the glue, the glue shrinks.  More water, more shrinkage.  I suggested that the Titebond may have had less water in it, and more solids than the hide and fish samples.  You seem to agree that shrinkage is from water evaporation.  But does being thinned with more water make glue work better?  It would be news if we could improve glue performance just by thinning it more.

Per may be a meticulous guy, but it isn't clear what he tested in this test of a single sample of each glue on one kind of wood.  He did not test the performance of any glue in an actual unclamped joint, which is what we were talking about.

[Roger Häggström]

Well. I read your post. I actually don't have a proven theory on why HHG do have the suction effect in spades you say it don't have and not TiteBond (both varieties). I'm more interested in the how and not the why...

What I do know is what the picture made by Per shows very clearly. And my own experience from both types of glues.

Water evaporation from any glue joint will certainly draw the two pieces together when the glue shrinks. But somehow HHG and Fish Glue will shrink/draw the pieces together more and much harder compared to both the Titebond glues.

I don't think TiteBond Original will get better "suction" when diluted with water. I haven't tried it though. May be a way to make that bottle last longer...

[Walter W Wright]

they apparently use hot hide glue to "chip" glass, relying on its pulling and shrinking power to literally snap off pieces from the surface of a sheet of glass.

i have a hard time imagining titebond doing anything like that (though to be fair i haven't tried it).

[Walter W Wright]

really good point about humidifying dryness cracks back to "normal" rather than to "closed whatever it takes".