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Is there a benefit to one versus the other when making a liquid hide glue or extending the open time of HHG? Obviously salt is right here in my kitchen so I don't have to run out to the hardware store and see if they even have urea so that is one benefit of using salt. But my guess is that the one that is harder to get (urea) is better for some reason???

Does the NaCl break apart and become part of the chemistry of the glue or does it stay NaCl which is hygroscopic and could present a moisture problem? Does urea not do that for example?

I'm tired of rushing bridge glue jobs and dinging the edge of the sound hole with clamps so I want some longer open time.

Regards.

JF--

I don't know enough about the chemistry to answer the question. I order urea online (Homestead Finishing), so availability is not an issue for me. I use urea because it is the additive I first started using to extend gel time, and it works for me. I have not had a need to change.

I can offer this, though: Take the urea/salt measurements for turning hot hide glue into liquid hide glue, and simply scale back the amount in order to extend, but not eliminate, the gel state. Add half or a third of what it takes to make liquid hide glue, and you get extended gel time without affecting the strength of the glue to any appreciable amount.

Wish I could address your main question; sorry.

A few words from Frank Ford:
"Urea will extend the working time of the glue, and will not weaken the adhesion if it is mixed with the glue just before use. More than 5-10% urea (by dry weight) may increase the flexibility of the dried glue; it may allow glue joints to “creep” more, especially in heat stress. Most instrument builders and repairers try to avoid having to use urea."

The complete article:
http://www.frets.com/FretsPages/Luthier ... eglue.html

For gluing bridges I try not to modify the glue at all, and use a fresh batch(not hard to do as I use small squeeze bottles and only make about an ounce at a time).
I try to place the clamps in the soundhole and near their final position, and in some cases will do a dry run if there is something out of the ordinary. Warming the bridge gives a little more open time and having the location well marked out (I tape around all four sides to locate it and help with squeeze out) allows me to place it quickly.
Wrapping tape to "pad" areas of the clamp that could contact the soundhole might also let you work a little quicker and feel less rushed.

When I need time to clamp a joint with HHG, I heat the parts with a heat gun before applying the glue to extend my working time. I’ve recently made a thermostatically controlled oven to heat the larger stuff before gluing, but I’m still working out the kinks on that process. But, the short answer is the solution is heat. Warm the parts and/or warm the room.

Except for gluing the plates to the rims, I usually don’t use heat for my 192g joints, but for the 315g HHG joints (bridge to top, heel block to heel plank), I warm before gluing. (For the neck scarf joint and the rim blocks and linings, I use LMI’s PVA glue.)

I know that warming the parts is a common method to increase the open time and I bought a heat gun specifically for that when I started using HHG some years ago now. I also have a heat lamp. It works well an all but that's not really what I am asking. I am simply wondering if salt or urea are truly interchangeable or if one is considered 'better' then the other and why? In any case I plan on using salt or urea to extend my open time to fit my needs. Or at least to give it a try and see how I like it.

Hi John,
Sorry I couldn't give you good advice on what you asked for, but I did want to make you aware of some of the effects of adding urea to hide glue.
Someone gave me an old (1850's) square grand piano and in researching information I ran into an article on using hide glue (and adding urea) you may be interested in. One thing I wasn't aware of is that it reduces the number of times you can reheat the urea glue without degradation. In the comments section it mentioned the possibility of adding vinegar to extend the working time (something I've never heard before, but have used to soften and remove old glue)

http://www.squarepianotech.com/?page_id=223

Good to know. And it won't be a problem for me since I make very small batches. The stuff is cheap enough to just make a small batch for a specific process and toss it too.

Interesting to hear what he had to say about yellow glue I'm not sure I would agree that anyone who uses it is an idiot though

Nobody should call anybody an idiot. Having said that, I think context is important. If you are restoring old pianos, I can see a PVA glue being a bad choice.

I'm trying to only use hide glue in the construction of guitars, and I mean for everything. It is actually not that hard. The biggest brain teaser I am dealing with right now is figuring out the right mixture of ingredients for the glue that holds custom purflings together. I have used hot hide glue with urea, but that only extends the open time. The glue still winds up crystalline and brittle, which is great for most things, but not great for purflings that I want to bend later. I added some glycerine to the most recent experiment, but I might have added too much. This batch of glue is very, very flexible, and is acting like a rubber cement. Of course, that might be ideal, if the pieces stay together. I need to cut up the log and see what's what.

People seem to have strong opinions about glues. His problem was that he used the yellow glue in places he shouldn't have. I try to keep in mind what properties a glue needs for the joints I am glueing with it. For bridges and soundboard bracing I use hot hide glue, but for glueing the rims to the front and back I will use yellow glue. For laminating sides I use epoxy. I'm not a purest when it comes to glues, but I try to use something that will work and not cause repair people trouble down the road.
I use small squeeze bottles for hide glue, so like you, I don't have to worry about multiple reheatings of the same glue. I found the statement about the source of the urea (horse stable) to be somewhat humorous, but probably true. I've heard that people used horse urine to brown rifle barrels in the past.