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#1
11-24-2021, 08:44 PM
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Anyone know what causes these kinds of back cracks?
Edit: sorry everyone, fixed the image link.
I've noticed some old guitars have cracks on the back of the body that almost look almost like symmetrical lines. Here's a good example:  I'm curious since I have an old repaired Gibson LG-1 with marks like these and I've seen a couple used guitars recently with them too. What causes this particular type of crack? Is it a bracing issue, warping issue, neck issue, etc? I've tried to search online to learn more about it but no luck. Thanks Last edited by ilikeguitar90; 11-24-2021 at 10:26 PM. Reason: Fixing image link 
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#2
11-24-2021, 09:57 PM
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link does not work...
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#3
11-24-2021, 10:19 PM
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Quote:
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#4
11-25-2021, 06:19 AM
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Quote:
Assuming the back in the picture is solid, then it still could be due to the neck block having been stressed and the weakest grain lines gave way? It's typically seen on guitar tops more than on backs. You might glean some good insights from master repairman Frank Ford. Grab a cup of coffee and enjoy this article from his Frets.com archive! http://www.frets.com/FretsPages/Luth...btopcrack.html Regards, Howard Emerson
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#6
11-26-2021, 01:55 PM
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To me that looks like an issue with the wood grain. Those cracks are too far apart to be at the edges of the neck block, so I don't think it's a structural issue.
It's a little hard to tell, but there are some indications that the crack is right where the grain is 'flat cut', with the annual ring lines parallel to the surface of the wood. This is, of course, the opposite of 'quarter sawn', where the ring lines are perpendicular to the surface. We use quartered wood for tops in part because it has higher cross grain stiffness than other cuts, although the penalty for flat cut in that regard is not too high. The bigger issue, all around, is that quartered wood is more stable. It has lower cross grain shrinkage than flat cut wood, for one thing. The biggest problem, though, is in the way the shrinkage changes as the tilt of the annual ring lines changes. Most woods shrink more 'tangentially' than they do 'radially'; a quarter cut piece shrinks less than a flat cut one for given drop in humidity. If you have a apiece of wood that, like most construction lumber, shows significant curvature in the grain lines as seen from the end of the board, it will tend to cup across the grain as it dries. If it's too thick to be able to cup as much as it wants, or is held down flat, say by being nailed to a deck or glued to cross braces, it will crack. This is made worse by the fact that a medullary rays that help make quartered wood stiffer across the grain run through the thickness of a flat cut board. Since these rays are little bundles of cells running across the grain they are weak spots in the bond between cells, and make it easier to crack the wood, as anybody can tell you who's split fire wood. So, as I say, it looks to me as though that back was cut from fairly near to the center of the tree, with the arc of the end grain going from something like skew cut near the center joint through flat cut a few inches out, and back to skew on the other side. I could be wrong, of course; it's been known to happen, and it can be hard to tell much from a photo on line. There could simply have been a weak spot in the grain that is mirrored on both sides of that book-matched back.
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