Good stuff. I still maintain that superglue is an exremely poor choice for a crack in a porous, light-colored wood like mahogany, especially the water-thin kind, especially as applied by the punter him- or herself. Mignal correctly identified a cosmetic issue that arises when the glue wicks into the grain; as it’s wicking into the wood, it’s leaving the place where you want it to be as well. Even if you get it to hang around, it’s not the kind of bond you want in this situation. Are the superglue advocates maintaining that the repairman use superglue to glue some sort of reinforcement to the inside of the crack with superglue as well?
As a repairman, you have a responsibility not just to satisfy the immediate needs of the customer, but to consider future owners (and repairmen) of the instrument as well. While superglue is handy for fingerboard repairs, fretwork, etc., it shouldn’t be allowed to literally creep into areas of the instrument where it doesn’t belong. Cheers,
Rob, in order for glue to work, it has to enter the adjacent wood. Yes, a thin CA will wick in further than an alpha resin or hide glue, but any glue will penetrate. If is doesn’t penetrate enough you end up with a weak joint.
In repairing a crack, exactly what type of bond do you want? I noticed in a previous post someone commented about being able to take it apart and put back together. Why would anyone want to do that? A crack repair should be as pemanent as possible. While I can understand the need for that type of bond say on a top which might need to be removed for interior repair, a crack is not a joint one will be doing that sort of work on.
Again, in my first reply to this I said “if you don’t care about the appearance.” then the OP could try a thin CA. OTH, I have done virtually invisible field repairs (and a couple of invisible shop rapiars) using CA glues, so I will disagree with your comments that it is a bad choice in any case.
Superglue soaks in way too deeply into porous woods like mahogany. That’s no good, period. As to taking the joint apart, it’s more a question of if/when the crack opens itself in the future due to wood movement. What then? Superglue can’t be cleaned out, while aliphatic resin and hide glue can be washed out to redo the crack cleanly. You can’t glue glue to glue! Sorry, mate, but you’re way off base here. Cheers,
This is a non starter, or else someone has been not gluing things up right, for the crack should never open up again.
Simply put, the glued joint is stronger than the wood itself. The goal of clamping is to force the glue as far into the wood as possible in order to provide a create the bond.
FWIW, I just walked out to the shop and had the following conversation with my apprentice:
“What happens when you glue two pieces of wood together and then after it has set, flex the joint?” His reply was “The wood is going to break.”
“Where?”
“It’s going to break away from the glue line…” The he got this concerned look on his face and said “Well, it will break away from the line, unless I screwed something up…did I?”
Regarding being off base, I’ve been a carpenter for 30 plus years, have been building instruments for 15, and doing it full time for 5. I have built well over a thousand instruments and have only have one glue failure in that entire time. And that was when I was at an outdoor festival, on a day when it was over 90 degrees. I didn’t notice that bottom end of a full size harp was in the sun, and it hetaed up enough to cause the glue to let go.
We’re not talking harps here, we’re talking mandolins, which are quite fragile. They’re generally pretty lightly built, and they’re under enormous tension from eight high-tuned strings. I love your optimism about the lifetime of your repairs, but having seen too much of what happens to instruments in the real world, I know that even a correctly done repair can come undone. Going back to an earlier point, using cleats is one way to avoid this happening, but even a cleated crack can, through various means usually related to changes in temperature and humidity, open up again.
I still don’t think it’s responsible to use superglue to glue the kind of crack we’ve been discussing. Superglue is wonderful stuff; it has earned its place on my bench for its many sterling qualities, but it has no place being bled into cracks in any porous wood. I hear what you’re saying about the glue join being stronger than the wood, but the gluing surface is so tiny, edge to edge as it is, and the forces can be so great as that thin piece of wood changes shape, that no one can guarantee that a repaired crack will never have to be redone. Superglue being present in the grain of the wood will hamper any effort to address this situation.
All our best intentions, wishes, cherished beliefs, and prayers, and those of our apprentices, aren’t enough to guarantee that our work won’t have to be redone at some point. We, as responsible repairpersons, must consider that possibility when choosing materials, no matter how fervent our wishes that it weren’t so. Cheers,
It’s only in theory that cracks never re-open, reality is certainly a little different. So we have a $3,000,000 Strad violin with a similar problem, a crack to a rib. What are you going to glue it with? Superglue? Titebond? It’s pretty obvious to me and most expert restorers which is the correct glue to use in this situation. Hide glue. If you don’t know how to prepare Hide glue then go buy some fish glue. They are both reversible and yet more than strong enough for a repair such as this. That’s why both makers and repairers have been using it for centuries. Titebond isn’t bad but it still doesn’t have the same properties as Hide glue if you ever need to re-glue that very same crack.
PS. Remember that guy who wrote a book on guitar making and was telling everyone how strong his epoxied neck joints were? Fancy doing a neck reset on one of those? Me neither.
On the 22 string harp I make, the total string tension is about 575 lbs, on the 29 string harp, it is approx 900 lbs…and this tension is tensile, not shear as on a mando, guitar, dulcimer etc. IOW, that is 900 lbs trying to peal the soundboard right off the instrument. That is a far greater amount of stress than on even a tightly strung Mando.
However, I should mention that the bulk of my work is not in harps but is on Mountain dulcimers, where I am working with wood just as fragile as that on a mandolin. The typical bookmatched back on one of my dulcimers is about .10 inch thick.
I agree with you that a cleat or some sort of internal brace would be a good choice. However, if you are experiencing glue separation on your repairs I would suggest you review your procedures as the wood around the repair should fail before the repair.
Obviously this is not something we are going to agree on, so let’s just agree to disagree and move on.
My goal in posting on this thread is not to convert other repairpersons to my way of thinking; rather I’d like to try to prevent some poor penitent mandolin player from taking what I consider to be very poor advice. Cheers,
Jeez, I’m sorry I mentioned CA glue…
I should mention that when I use this on my whistles, it’s often for an uncompleted whistle which has developed a crack before turning down to final OD. By the time the whistle is finished, the repair is usually invisible unless you’re really looking for it (depending on the wood).
On a finished whistle repair, I’ll drop the glue into the crack as I’ve noted. When it’s fully cured, I scrape all the excess off, then remount the whistle on the lathe and refinish the area. Again, I can usually get this to be virtually invisible.
BTW-- we have no idea of the quality of this mandolin and how much effort and expense it’s worth putting into it. Of course, it’s value to the owner may exceed it’s “book value”.
