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For those of us who use hot hide glue, and also use urea to lower the gelling temperature of the glue, we know that, if you add enough urea, you lower the gelling temperature to below room temperature, essentially eliminating the gel state. Adding urea in an amount that is 30% of the weight of the dry glue will make that happen (this is the formula for Old Brown Glue, from what I have been told). But some folks worry that adding this much urea weakens the glue, or makes it less crystalline when it dries. Plus, it does seem to lengthen the drying time.

The more interesting task is figuring out how much urea to add to merely slow down the gelling of the glue, but not eliminate it. This is the sweet spot; getting the things that are great about hot hide glue, but not having to rush to get the clamps on before the glue gels and it is all wasted effort.

For those of you who use hot hide glue, and have experimented with the use of urea to slow down the gelling (but not eliminate it), what ratios have you used, and for what tasks?

I'll start. Just this past weekend, I added urea in an amount that was 10% of the weight of the dry glue in order to glue the top to the sides in my go bar deck, which takes some time. It worked out great. I can tell no difference in the adhesion strength, and the dried glue is not gummy. It is crystalline, just like regular hot hide glue. I used the same batch to glue in a few replacement kerfed lining blocks. They are stuck on there really well, and the drying time was not different, as far as I could tell. I think 10% might be a really good balance of extending the pre-gel time but not compromising the desirable characteristics of the glue.

I look forward to hearing what your experiences have been with adding urea in smaller amounts.

That's about the ratio I use too but I use salt.

I found some information on a violin makers forum several years back when I was looking into this very topic. It was suggested by Eugene Thordahl / Bjorn Industries a supplier of hide glue, that the addition of 3-5% urea or salt would do the trick without any effect on the joint. I've been doing this using 5% salt and find it much easier to work with hide glue. This doesn't turn it into liquid hide glue, it extends the working time. Mr. Thordahl states that when the furniture industry used hide glue they would add 20% urea/salt to create liquid hide glue.
The 5% slat made a big difference to me.

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Jim Watts
http://jameswattsguitars.com

Interesting. How much longer would you say the open time is?

Similar to Titebond, but things still depend on shop temperature and part temp.
My shop is fairly cold good chunk of the year.

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Jim Watts
http://jameswattsguitars.com

With the 10%, I didn't do a real "edge of failure" type test; I wanted to stay well within the gel time. I preheated the rims a bit, but not excessively. After turning off the heat gun, I figure I got the glue spread, the top on the rims, the caul in place, and 70 go bars sprung, in less than 10 minutes. All was fine afterward. It dried overnight just fine, and the drips are hard as a rock. 10 minutes is way too long for untreated hot hide glue, so I'm glad I added the urea.

Thanks for the info...

Don, are you using a brush, squeeze bottle or .... other to put the glue on. I have a good method with a brush for braces and it’s become low stress, it I would worry about a brush when putting a back or top on. Too much time dipping?


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Brad—

I have used bottles in the past, but I use brushes now. I can get the glue on pretty fast. It’s a bit sloppy, but it works. The brushes fit in better with my overall glue process.

I've been on a violin making site for years, and the have talked about HHG a lot; but I haven't read much about urea or salt. Don't think I ever heard of salt. I use rubbed joints for backs and bellies, that would be hard to do with the thing backs and bellies of guitars! And gluing the head on a baroque with a V joint is quick and easy. I've never been a fan of loosening clamps, gluing, tightening; loosening more, glueing, tightening. I can't wait to try it. HHG on the violin site is like religion.

Does the glue lose its stick? If it does, it would be easy to just add a little salt while doing operations that are more time consuming.

How do you measure out 5% salt if you are only mixing up 5 grams or so at a time?

My latest favorite for applying glue is a cheap foam brush. The 1 inch ones. They hold a ton of glue, and spread it fast and evenly. When done, you squeeze the glue back into the pot, and rinse them a few times in the hot water; squeezing them a few times until they are clean. I've found that they last a very long time. I had a hard time getting brushes clean, and they don't carry anywhere near the amount of glue; leading to starved joints.

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Why be normal?

I use a gram scale to measure the percentage by weight. I cannot say how long it extends the open time but it's long enough for me to comfortably glue on linings and even bridges without sweating it. Just enough to take the edge off.

Speaking of HHG it's come to my attention that gram strength has nothing to do with actual strength but has to do with how fast it gels.

I use canning slat becasue supposedly it has nothing added to it. I'm not sure that Iodine would have any negative affects to the bonding properties butt still...

Same here. I only mix and use small amounts of glue, anyway. The maximum I mix at any given time is 1 oz. of dry glue. So, the gram scale (in ounces mode) allows me to add in just the right amount of urea.

The glue I have now is the Bjorn 192 high clarity, the lowest they have in high clarity. I like it. You can go down to 135 in regular clarity. I haven't had a problem with mold that I was getting with some others. I even had some that would turn to liquid in the jar after use. Very weird.

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Why be normal?

I pretty much only use 192 gram strength these days. I have some glue in a higher gram strength (251, I think) sitting unused in a cabinet. I have a good idea how the 192 acts, and that predictability is important to me.

On the whole bottles versus brushes thing, I will give more detail about my process and gear:

It is perfectly reasonable to make a decent sized batch of glue, separate it into small bottles, freeze the bottles, and just reheat the bottles as needed. I did it for a while. But nowadays, I really prefer a different process. I only make a small amount at a time (1 ounce of dry glue or less). That way, I can vary the water content a bit to get either runnier glue or gooey glue, whichever is best suited to the task at hand. I can also put in urea, not put in urea, vary how much I put in, etc. Every day's batch can be different, but different by design.

With such a small amount of glue, the water soaks in really fast. By the time I have the water bath warm enough to be useful, the glue has sucked up the water and it is good to be heated. I can have hot hide glue ready in 15 minutes, probably. I use a double boiler from MusiCaravan, but instead of the coffee cup warmer they provide, I use classroom laboratory hot plates. They hold a constant temperature really well, and they fit the double boiler just perfectly. I use whatever brushes work for the task. My current favorites are the small handle (not thick handle) acid brushes. The bristles work well, and the tiny handles fit in the hole in the lid of my double boiler. I've fallen out of love with silicone brushes. I have them, but they don't seem to hang onto the glue very well. For urea, I started out with a little jar from Homestead Finishing, but after I ran out of that, I just ordered some plastic jars on eBay. The pills are a bit larger, but it is pure urea, so no real difference to me. I buy glue wherever I can get 1 pound bags of 192 gram strength. It all comes from Milligan & Higgins, so I just shop on price. I use a regular hardware store heat gun to preheat parts, if I preheat parts. I built a nice little stand for the heat gun, and use it the way I would use a stand for a soldering iron. It's just a holster. I use an electronic scale to measure dry glue, distilled water (not sure distilled is necessary, but I have it anyway, so I use it) and urea. It works great. I have an electronic thermometer that I only use every once in a while to make sure that the marks I have on my hot plate dial are still the right settings for the temperature I want.

Anyway, that's my stuff and process.