I was rinsing out the stiffened washcloth I use for wiping my glue brushes when a thought occurred. If I was to take a completed lute and immerse it in the laundry sink, in about five minutes the entire thing would delaminate and undo ninety hours of work. I started mentally calculating. Following the swim I'd be left with 87 pieces of wood, 23 pieces of gut and one hunk of bone. Oh, and one brass screw.
Carrying the thought further, I realized that with some patience and appropriate drying time it should be possible to put the whole thing back together again into a recognizable facsimile of the original.
After this conclusion, I meditated on the fact that I'm engaged in producing what is for all intents and purposes - a biodegradable object! This led to pondering the paucity of original lutes that have come down to the present day, when one takes into account lists like the Fugger inventory- hundreds of instruments. All to have vanished with the passing of time. Exceedingly fine, handcrafted ephemera. I don't quite know why, but it made me feel good to realize things from this perspective.
My current instruments are stuck together with protein-based, animal derived glues. Ostensibly this makes them easier to repair, and it's a historically informed approach to working. I've been using hide glue and a less well known substance called "fish glue". Hide glue has a reputation. It's decidedly old fashioned stuff that requires soaking and heating. It has a very short working time before it cools and seizes up. I've read that it stinks something fierce. I don't find this to be the case, at least not with the refined glue I use. I do recall the somewhat fetid stench of "rabbit skin glue" used back in art school days for making gesso and wax encaustic painting. This is altogether different. Fish glue is made from fish skins and is remarkably similar to hide glue when dry. It's benefit? - No heating necessary. No stink here either, unless one forgets to change the water in the brush-rinsing jar for a week or so. Even then it's not fishy, more like a hockey equipment bag.
Hide glue isn't all that hard to work with, really.
The glue pellets are soaked in cold water to soften for a few hours. (Cold water is important!
Hot water cooks the granules before they can soak up the moisture. They become case-hardened little pearls not unlike the tapioca balls one finds in Tai bubble tea.)
Here's my rig. Small quantity of glue in a baby food jar nestled in a double-boiler situation on the old hotplate. I cook it with care, adding finest Swiss chocolate and rich, creamery butter...
Well, to be honest I just make sure it doesn't go above 150 degrees F. I adjust the viscosity by adding water from the pot until it drips from the brush in a continuous stream the consistency of light machine oil.
Why use fish glue instead, if hide glue's not so bad? Mostly because I have a basement shop. I've managed to regulate the humidity but it's still a cool place. I could get around it by heating parts to be joined, but I've found the fish glue makes tricky assemblies less nerve racking.
Time for some informal, decidedly unscientific testing to compare the adhesives I use in the shop. Aliphatic glue -yellow carpenter's glue, works fine for putting jigs together. It does have a reputation for drying to a somewhat rubbery state. It also tends to let go if left in a hot car. I've seen the effects. I also find uses for cyanoacrylate superglue in several viscosity's. It works well in repair situations. This photo isn't the greatest. The warm incandescent glow hides the colours but I didn't feel like going back and shooting it again. When dry, the carpenter's glue is indeed, yellow. It sticks reasonably well to this slick piece of melamine laminate but with a little work it bends away from the surface. Fish and hide glues are both pretty pale in comparison and decidedly brittle -they crackle like hard candy when stressed. The superglue is water clear.
There's a well established opinion in luthiery circles the the brittle nature of animal glues makes for better transmission of sound across joint areas. There's no "give" to impede vibration. It's also supposed to be easier to reverse if things need repair. I'm not so sure about this, personally. I've removed dozens of steel string guitar bridges using a regular clothes iron to heat them up. The two bridges stuck down with hide glue took a lot more effort. Heat wasn't enough. I had to get in with a palette knife and water before they'd give way.
Let's make like Professor Julius Sumner Miller and devise a practical experiment. I've glued up similar lengths of beech to a block with the various glues. I'll use these hefty c-clamps on the end of the lever to put some strain on the joint. I suppose this is a problem of torque, though I have no idea of what the rotational distance is, nor the clamps' mass in newtons to calculate force, so I don't suppose I'll publish this in "Science". I'll give credit for the setup to repair guru Frank Ford, who did this with hide glue and Titebond a few years ago.
All loaded up and in the oven at about 200 degrees F. (Incidentally, the temperature your guitar would experience in the trunk of a great red Cadillac just outside Barstow...) As suspected, the aliphatic yellow glue gave way first, at about the six minute mark. The others just kept on holding. I raised the temp to 250. Still nothing, other than a pleasant roasty Christmas tree smell. I spritzed a little water in there as if preparing a rustic Italian crusty loaf. That did it. In two minutes both the fish and hide glues let go with a satisfying "clank!".
So, what of the cyano? It did continue to hold for an additional five minutes, when I reminded myself that this wasn't being funded by a research grant and I turned off the oven. When things had cooled, I inspected the joints.
The superglue holds fast, but the bond was super weak. I nudged it and it fell apart. The hide glue let go entirely. The fish glue seems better, but it too is loose. I suspect that it re-gelled when it cooled down. The carpenter's glue has little strands connecting the two halves of the joint. Will I switch to super glue for all my adhesive needs? No. The expense, toxic fumes, embarrassing finger-sticking calamities, all mitigate against adopting it for general purpose use. I still love it for sticking down loose frets and filling cracks in polyurethane finishes though!