Cracks due to "drying out".

[HonketyHank]

I am just going to cut to the chase here. My first try resulted in a quasi-dissertation.

Cracking does not occur just because the wood is dry. It occurs because the wood has dried out non-uniformly, thus introducing stresses. If the wood is dried slowly so as to allow time for moisture to migrate through the wood to the surface and escape, the non-uniformity of moisture content is minimized throughout the piece of wood (eg, a mandolin).

A major barrier to the escape of moisture from the wood of a mandolin is the exterior finish. Thus moisture can escape more easily from the interior than the exterior, producing non-uniform drying and thus stresses and perhaps cracking.

Is this non-uniformity enough to cause cracking? I don't know. Does it make sense to apply a moisture barrier to the inside surfaces of a mandolin prior to assembly? If not, why not?

[Bill Snyder]

Common answer is "NO". The wood is thin enough that the lack of finish is of minimal consequence and the trouble it makes for any future repairs trumps any benefit it might have.

[Dale Ludewig]

Cracking occurs when wood movement is restricted and its moisture content becomes less. You could take a carved top or back, unattached to anything and rapidly change its moisture content, either up or down. It won't crack, it will warp. Unless it's perfectly quartersawn. Then it's likely to just get wider or narrower. But glue it to something else that isn't moving in the same direction at the same time and you've got problems. Getting away from instruments, look at old dressers and buffets where the builder or later owner didn't understand this. They glued or nailed down to boards going across the grain. The top's cracked. Wood is going to move and you can't stop it. And if necessary, it will pull itself apart. Guitars tend to crack up by the headblock, the tail block, and the famous cracks where the pickguard was glued down. Even the bridge can add to the problems.

Mandolins are no different.

[sunburst]

Never mind...

[Steve Ostrander]

I just went through this with my guitar last year. I was told that the bridge grain runs crosswise to the top grain and thus when the bridge is glue to the top, it can cause the bridge to split, as mine did. I get that, but apparently a "lifetime" warranty isn't really a lifetime warranty, even when you humidify your guitar.

[HonketyHank]

I buy into Dale's answer quite readily. And Bill's rings true, too. Steve (and Dale), that sounds similar to what happens to old bowlbacks with glued down pickguards.

I just knew I couldn't have magically solved a problem that has been around for centuries. Thanks.

[multidon]

I agree that finishing the inside would not help much, if at all, but I did own a flat top mandolin by the late Walt "Gypsy" Kuhlman, and its interior was shellacked. I asked him why, and he said it was to control the escape of moisture. He also added that it was an "Old World" technique. That makes me wonder if anyone else here has encountered a European made instrument with interior finish.

[Marty Jacobson]

Cracks occur not because wood moves when it dries (or takes on more moisture, for that matter). Wood finishes are permeable to moisture. If you finish every surface of an instrument, the wood will still eventually reach the moisture content that unfinished wood would reach in the same environment. The mechanism is wood moving, which is different across the grain vs. along the grain, but even if it were the same amount in both directions, problems would still arise from the wood changing size/shape.

There are many makers who coat the interiors of their instruments. It does look nice, and certainly won't hurt anything. It might make repairs more difficult, since internal areas would have to have the shellac or whatever sanded off before a cleat could be glued on.

[Wes Brandt]

Something that hasn't been mentioned in this thread is that: the difference in expansion and contraction due to humidity is on average 20 times more across the grain than with the grain with the woods we are using… this is THE main reason you have to pay attention to humidity when building.

With guitars, having braces crossing at 90 degrees ...or closer to 45 deg for top braces, its not surprising to see lots of movement, sinking, rising, cracks and splits, specially as the humidity drops significantly below the humidity level that it was built at. Most guitar shops make it clear what humidity they build at.. usually the low 40s

With carved mandolins, you have no back braces (usually) and if your using tone bars, they don't cross the top grain all that much… X braces much more so ….but what is not as obvious, the side/rim wood grain cross's closer and closer to 90 degrees as you closer to the end blocks… which can partly explain those cracks near and lose plates at the end blocks. Still the plates are "locked" at the perimeter and the plates will expand and contract with the humidity, causing action changes if nothing else.

[Stephen Perry]

Piece of wood drying tends to crack regardless unless pretty thin, maybe even then. I have had wedges of fresh cut check at the ends. When I cut and split myself I leave extra at the ends and seal the ends.

[Wes Brandt]

Drying and seasoning wood is a skill that is used by wood suppliers at all levels… if you know what your doing and the wood doesn't have tensions built into it, which sometimes does ...you can dry wood with no cracks… but you have to control the humidity/temperature around the wood, dry it at the right speed and cut the wood correctly ...either with or without a kiln.

Or you can put it outside and cover or in a shed or the attic and let it go and then, then whatever the weather does is how your wood will turn out cracked or not. Some people do this with in mind the pieces that survive will be the most durable and sort of stress tested… and maybe even de-stressed ...could be pretty wasteful though if the weather isn't cooperating.