Hide glue testing.

[Samuli Samuelsson]

Hi.

I came across a small pouch of granules that I bought as hide glue back in the day (20 years ago IIRC).

The place of purchase wasn't excactly what I'd consider a reliable source of information, and there's no markings on the pouch.

How would one go about it if testing the properties of that assumed hide glue would be the goal?
After preparing it, obviously.

I'm a BEng Mech, so material testing is something I'm familiar with. Just need some parameters to aim for and some methods to use.

The pouch is fairly small, so just bying some more from a reliable source is definitely an option, but being the junkyard-rat-hoarder I am, just tossing it would not sink in well if You alike-minded know what I mean.

[Greg Robinson]

Hi Samuli,
Prepare a joint with some pieces of scrap wood, with the same care you would use for a joint in an instrument, with a plane or scraper. Use the hot hide glue as normal, clamp, allow to dry. When dried, use destructive testing, and break the joint. If the glue is of good quality, it should not break along the glue line, the failure point should be the wood.
Hot hide glue has a near indefinite shelf life, so long as its kept dry and out of direct sunlight, so as long as it was of reasonable quality to begin with, it should be fine.

Good luck.

[Samuli Samuelsson]

Hi.

Thanks for the answer Greg.

A few additional questions if You don't mind.

Should I be interested in temperature/viscosity ratio, or about the viscosity in general?

Hide glues aren't supposed to creep AFAIK, but what about static, or dynamic stress tests?

Hide glues is an area of woodworking that I'm in the dark about, and since there's supposed to be all kinds of hide glues on the market, are all of them suitable for instrument making from the structural standpoint?



Just to be clear, at this moment I'm not that interested in (alledged) tonal variations between different brands or glues made from different animals/different animal parts, just about being able to determine the properties of any unknown batch of hide glue.

[Greg Robinson]

Hi Samuli,

Temperature is important, the protein chains will begin to seperate with excessive heat. Around 70°C or 165°F is optimum. Viscosity is dependent on how much you dilute it, so is a variable, and different dilutions are appropriate for different tasks, so might be hard to quantify. Around fresh, slightly warm honey consistency is usually good.

I'm not sure what the required parameters would be for any stress testing. I suppose you could mock a bridge and simulate string loading for a static test, maybe hit it with a hammer for dynamic.

Hide glue is normally graded by gram strength, and by clarity. 192 gram strength is one of the more commonly used and available, and is significantly stronger than the wood, but anything from 120 up I would be confident with. Higher strengths have less working time. High clarity glue is preferable as it will not colour the joint, which is one of the reasons hide glue is revered for instrument building. Low clarity can discolour joints. As instrument building is one of the largest consumers of hide glue, almost everything on the market is adequate. There are ultra high-quality options like isinglass, which is made from sturgeon bladder, and used by glass artists, but it is very expensive and offers no real benefits, while having a shorter working time.

Hope this helps.

[Steve Senseney]

I agree with all that Greg stated.

The only thing I would add is that high glue is very forgiving. It can tolerate being more dilute or more concentrated.

The only absolute, is make sure the material is liquid as the joint is closed.

I understand destructive testing. It is fun!!

Rather than make formal testing, get the glue and start playing with scrap. It has unique properties. Since starting building guitars, I use hide glue a lot more on all of my wood projects.

[John Hamlett]

Hide glue is graded by "gram strength", though that has very little to do with joint strength, for our purposes as luthiers at least. The main difference we notice when working with glues of different gram strength is working time, especially open time. We're generally better off with glue that is a little slower, and that is medium gram strength, for the most part.
Gram strength testing involves mixing the glue with water by weight, heating then cooling and testing the resistance of the jelled (not dried) glue to a specific size and weight plunger. I'm sure you can look up the exact procedure if you want to try to figure out the gram strength of your unlabeled old glue, but if it gives you enough working time it's probably fine to use. I would expect destructive testing of joints to give similar results regardless of the gram strength of your found glue.

[Michael Richwine]

Hi Samuli,

There are ultra high-quality options like isinglass, which is made from sturgeon bladder, and used by glass artists, but it is very expensive and offers no real benefits, while having a shorter working time.

FWIW, we've been making our own isinglass glue for over 15 years now, importing dried sturgeon swim bladders from Russia. Working time is actually quite a bit longer than medium gram strength hide glue, and once it's dry, it's impossible to open a seam without taking wood with it. Can't pop a violin fingerboard off like you can with hide glue after a few years, and opening an instrument is a real trial, even after 10 or 15 years.

Actually, I much prefer medium hide glue, but the boss sets the standards in the production area. Fortunately, I can use what I like in my little shop. Sturgeon bladders are getting near impossible to get these days,so the production guys may end up using regular hide glue after all.