Glue and salt

A friend recently complained to me about a (non-instrument building) glue-up with which she had difficulty. Work pieces can, of course, slip against each other when there is glue in between them, and this is what she was talking about. I told her that I had read about using a few grains of salt to prevent slipping in lutherie and other woodworking forums, but had never had enough of a slipping issue to bother (and I am talking from a Titebond frame of reference here). But it got me thinking about it...
Does anyone use the salt trick with, say, fretboards? Does anyone know if and why a little salt might not be good for the bond? Just curious - and don't want to have potentially given a friend a bad steer.

The salt trick might be simply to dry the glue out a little.
I have found gluing is MUCH easier, when I let the glue dry until tacky, rather than pushing it together right away when it's wet. Different glues take different times to become tacky, and it's important with many glues not to let it dry for too long, or it will develop a "skin" (although others will stick even after having been left a longer time)
If I push them together while wet, they slip, and take a lot longer to dry, since the moisture/solvent has less air to evaporate into.
This is especially true when the things being glued are less porous, and absorb less moisture from the glue (fretboard is a good example, since rosewood which is often used, is very oily and dense, so it won't absorb much from the glue). They form almost a seal, slowing the drying process.
I'm not sure about it being detrimental, although salt does contain chlorine (chloric acid) so it's probably better to avoid using it unless you know for a fact that it's compatible with the glue you're using.
I can imagine it making the glue more brittle, but that's just a guess.

I think I saw a discussion on this subject on the Banjo Hangout forum.

mr roper said:

I think I saw a discussion on this subject on the Banjo Hangout forum.

Click to expand...

Thanks. Found it: http://www.banjohangout.org/archive/136041

Looks like good information there, Rudy's initial post in the thread (about 5 posts down) is reassuring. So now I don't feel bad for suggesting the salt trick to my friend.

Sven told me his Grandad ld: used a few grains of sharp sand in his glue to prevent slipping during clamping...I supose salt or sugar or any gritty stuff would work.

There are usually pretty simple physical ways to prevent pieces slip sliding about. Fretboards can be glued with 'pins' or 'tabs' (the method I favour). I even once glued a fretboard by wrapping masking tape around the board and Neck. Actually masking tape can be a pretty effective way to prevent pieces sliding against each other. Unless you are certain about the long term effects of adding 'stuff' I'd be tempted to leave the glue as it comes.
BTW adding Salt to Hide glue extends it's working time but at the expense of strength. At least that's what I've read.

I have used both the sand, and cut off brad heads trick, for fun, in general cabinetmaking. When I got into lutherie in the 90s, one of the first books I sought out (nor full of stuff most would want to follow today), was the Overholtzer book on Classical guitars. He had a number of strong beliefs, one of which was to build instruments with as little built in stress as possible. He often would use rubbed joints, and when properly done, he was right, they grow together, and leave no glue line.

One of the most useful tips in the whole book, is that when clamping stuff up, one should start with modest pressure, then later apply full pressure. Later would normally mean a minute or two.

Why this works so well is time. When a .223 bullet goes through the air, north of 3000 fps and hits a blade of grass, it can "blow up" on contact. The reason, aside from light construction of some types, is that the blade of grass has no time to move aside. Try being hit by a blade of grass moving at 3000 fps, it would have some force. We often overlook these time functions. So when we clamp a joint do we fully visualize the process that will be required to get all the glue from behind the fingerboard, out to the edges, and the joint closed till it bears wood on wood? I don't think so. We are just thinking of two static states, pre-glued, and glued.

We are sorta making reference to this dynamic reality when we imagine the granules of sand or salt as helpful. Why would they be helpful? Because they reach across time, so to speak, and anchor the wood so that it doesn't move before the glue has had time to move out of the joint. Then ideally the granules will get pushed under the surface of the wood. But a simpler solution is to just give the materials the time they need to displace the glue before bearing down on the clamps to the extent that the parts skid out of alignment. If you do that, you will rarely have to position parts in advance with pins, though there can be other reasons for doing that.

Alternately slap on a bit of glue, rub the parts together, then clamp. Works every time and has done for hundreds of years!
You might be surprised just how easy some of this stuff is. I haven't used a clamp to glue on a bridge in years. I put on a bit of HHG to the underside of the bridge, position it on the soundboard and then lightly press down with the fingers. I then pull it off, apply more glue and position again. I then hold it under gentle finger pressure for 3 minutes. You can't even call it a rubbed joint. I've yet to have one fail in around 8 years of using this method. There are two things you have to be concerned with: too little glue (starved joint) and too much glue left in the joint (thick glue line).
BTW Overholtzer has some odd beliefs. I guess he forgot that the stress placed on a bridge/soundboard is in the region of 40 Kg +. You would never use an Ebony fretboard on a mahogany/cedar Neck if you wanted to minimise stress. In fact you would never use an X bracing or fan bracing at all.

Well, even if the salt trick doesn't work, you have to admit that it does make for very tasty glue.

The salt trick works fine, I've been using it for a LOT of years. It only takes a few grains to keep the wood from slipping and although pins or tabs can be added, it's MUCH faster to throw on a few salt grains in the middle of joint. DO NOT use sand! Silica grit will quickly ruin any tool's cutting edge so leave sand in the sandbox where it belongs. It has no detrimental effects to the joint and doesn't leach out to the surface over time.