Testing hot hide glue strength

[eliya]

I made a small batch (1/2 oz glue to 0.9 oz water) of Milligan & Higgins 192 gram strength glue. I followed Frank's quick method for making the glue. I stirred the granules and water together, let it sit for about an hour, at which point it looked like fish eggs. Then I cooked it in "double boiler" to 140 degrees. It looked pretty good and it had the consistency it should. It's thin but not too thin, it's somewhere between what and good maple syrup.

I decided to test it on two small scrap pieces of mahogany. They both have been sanded, so you don't really feel the grain so much when touching them. Anyway, glued them up (grain perpendicular) and clamped them overnight. I couldn't break the pieces apart with my hands the next day, but I was able to when I held one piece with pliers and tried to pry the other piece with another set of pliers. I should note that the piece broke not because I pried it, but because I put pressure perpendicularly to the grain, which made the piece "fold" and break. The break was very clean with no splinters or fibers sticking out.

Because I'm new to hot hide glue, this test seems inconclusive and I'm not sure what to make of it. So I wonder what are some ways people here use to test their glue?

[Doug Berch]

I can't quite visualize the experiment you performed but good for you for testing a new adhesive before using it on an instrument!

When the pieces broke apart did they break on the glue-line or somewhere else? Hide glue can sometimes be popped apart under stress even with a good joint. A friend who did violin work told me if he could get a knife into a seam he could sometimes remove a top or a back without using heat or moisture; he could just pop the joint apart using the knife as a wedge. 

Sometimes hide glue can begin to gel before the joint is firmly together. In a room at around 70 degrees Fahrenheit I usually figure I've got 30 or 40 seconds to get the joint together once hide glue is applied. If I heat the work with a lamp or heat gun I can extend that a bit. If the glue began to gel before assembly the joint may appear okay but full adhesion of the two pieces did not happen and stress can pop the joint.

So there are several variable to consider and again, I can't quite picture how you put those pieces together and how they broke apart.

Hope this helps. Hide glue is great stuff once you get the hang of using it!

[eliya]

Thanks Doug. Here's a picture of how the pieces were glued. The two pieces in the picture aren't glued, and the ones I did glue were smaller, too. Probably about 1/2"x1/2" in size, each. The pieces were clamped where my fingers are in the picture. The clamp went on very quickly, within 30 seconds of when I brushed the glue on.

So now if you look at the picture, one set of pliers was holding the bottom piece, and I was grabbing the top piece with the other pliers, and pulled up. There was no prying or wedging. What happened, I think, was that the the pliers snapped the top piece. Not sure if that makes more sense now.

30 to 40 seconds? Yikes! I thought I had a minute or two. I guess I to have a lamp nearby at all times!

[Doug Berch]

If the joint broke at the glue line it could be that the stress broke the joint. Hard to know.

I'd suggest trying some thicker scraps, 1/2"-3/4" or so thick, an inch or so wide and foot or so long each. Glue them face to face for about half their length (six inches of each piece glued together face to face). Brush the hide glue on quickly, press the two piece together and clamp them up. The next day clamp the end of one of the pieces in a vise or to a bench and whack the other with a mallet until it breaks. 

If the break is at the glueline and both pieces are smooth there is a problem with either the glue or the technique of gluing. If you get some splinters and the wood breaks more than the glueline or you just get broken wood all is well.

It can take some time to get the hang of using hide glue so don't get frustrated and have fun!

[eliya]

Hi again, Doug. I performed the test you suggest but with a slight variation. Here's what I did.

I got two 1.5" wide and 3/4" thick foot long pieces of pine and glued them together so that 6" were overlapping. Of course, I heated the pieces prior to the application, and I applied the glue quite liberally. See the first picture.

I didn't clamp the end of one piece in a vise because I don't have one at home, but also because I knew I wouldn't be able to quantify how much force was applied to the joint. What I did do was balance my weight on the joint, i.e. one foot on each end, and rocked on it until it broke. The break is very clean with minimal wood tear, see the second picture which shows both pieces after the break.

To be honest, I don't think I can easily quantify how much weight the joint was able to handle. I couldn't add weight gradually, and technically the two overlapping pieces with me standing on top is a lever, and I'm not sure how to take that into account. Anyway, I weight about 175lbs, so that's about 87.5lbs of weight on each side of the joint, but since the force on both sides was directed downwards, I'd like to think it can be considered as 175lbs of weight applied to the joint. This is far from the most accurate measurement, though. Perhaps the way the joint broke can tell us more about the glue's strength.

[Lawrence Dell]

I'm not sure how people test its strength and I'm no expert, but I received some good advice from a violin maker friend in regard to the use of hot hide glue. Keep in mind that he goes into convulsions just thinking about using white glue or epoxy, even for guitars.

He told me to make a diluted hide glue solution, always coat both surfaces to be joined and allow time to dry before using the glue at full strength.  Again, coat both surfaces and clamp tightly.

I dilute the full strength glue with an additional 3 to 4 parts water, and when it's applied it's easy to see how it is absorbed into the wood, creating a perfect surface for the glue at full strength, with very little glue being further absorbed.  It works.

Using hide glue takes time but I think it's well worth it. Patience!

 

 

[Andrew]

Thats an interesting anecdote Lawrence. Seems to make sense. Eliya, Id like to point out that freshly scraped or planed wood surfaces should present a better gluing surface than sanded surfaces - sanding supposedly tends to disturb/tear/loosen the wood fibre, which is not ideal for many wood glues. Of course I personally have never looked with a microscope, so for me its still theory, but Ive never had a problem with scraped/planed glue lines.

[eliya]

Thanks Andrew, I've read that too. My reasoning is intuitive (which means there's a good chance it's not true). Sanded surfaces are smoother, so the glue (and other piece of wood) doesn't have a lot to catch too.

Lawrence, that's interesting. So you coat the wood with diluted glue, let it dry, then coat with the full strength glue and clamp? Technically speaking, the glue in the joint is still diluted (although not 3 to 4 parts water diluted), which means the glue strength went down, implying the joint will slightly weaker, no?

Doug, I've followed your advice and I have two scrap pieces glued and clamped together. I'll test it tomorrow and report back.

[Lawrence Dell]

I'm not sure, Eliya, what's happening on a molecular level but I kind of doubt that the joint is being diluted or made weaker.  All of the water eventually evaporates anyway.  I just decided to take the advice of someone who's been making and repairing fine instruments for many years and who learned his trade and gluing techniques from a master European luthier.

I suggest that you try both methods and see if you can determine which is weaker.  Sanding verses planing doesn't really make any difference.  Smooth is important.  Good luck.

[eliya]

Lawrence, I hope you didn't take any offense. I certainly meant none! I was just wondering if the fact that two different viscosities (not sure if that's right term) of glue are mixed has any effect on the strength of the glue. I will try your method and see how it works for me. Thanks again.

[Russell Vance]

Hello Eliya,

There is a attached "primer" article regarding temp and humidity for your info, and then please Google "Hot hide glue preparation and application" and then read the spec sheets on Fanklin's web site.  

Glue types in discussion form both molecular and mechanical bonds and these bond occur in a ratio that is related to the substrates being glued and the type of glue and thickness etc of the glue being used.  The preparation of the surface is a function of what glue you intend to use.  

Generally a fresh skimmed flat and square surface is a default preparation if you are in doubt.   Obviously, a lot of acoustic sound boards are finished with a thickness sander  so they are glued with a sanded surface rather than a hand or machine planed surface - so sanding is not such a bad thing anyway providing it's fresh and well cleaned with compressed air etc.

Most commercially made guitars today are glued up with a Titebond glue or an equivalent and most old guitars are still doing sterling service after 50 years - gluing stuff together isn't really rocket science you just have to learn how to do it properly,  Which is why you should read tech sheets from the guys who make it.

Glue technology is a complex but understandable subject and you should read up on it before mucking about with suppositions and uncontrolled trials.   It's interesting to do but it's all been the subject of controlled and comprehensive trials before.   Fine Woodworking has a good article in it's back editions about the subject as well.

Rusty.

[Ned Knepp]

Rusty, 

That's an interesting read. I sometime delve into furniture repair/restoration and the bit about using liquid HG on stressed joints in furniture that is in use is interesting. Of course the RH matters but where I live high RH isn't an issue. Thanks.

[Paul Verticchio]

Excellent advice, as usual, Rusty!! :)

The thing I'm 'not getting' is the OP's ultimate goal. Is this just aimless experiences shared or outcome based experimentation? A plan is easier to execute with a definitive goal in mind.

Also: re the pliers: there are NO forces placed on an instrument that equals that kind of brutish force. 

Like I said, perhaps it's just me but the relevance of the discussion eludes me beyond: "You need to increase your knowledge base, study & experiment".  I think it's an artifact of growing older.

Thx again, Rusty :)