MIMF

Hi Folks,

Does anyone have experience with adding urea to hide glue to lengthen its working time? I'm looking for information on how much to add and what its effects might be.

Thanks,
Ray Frank

Hi Ray and welcome! I seem to recall this question popping up before, so hopefully someone with experience will be along soon to help.

Old Brown Glue is HHG with enough urea added to keep it liquid above 72* F. I have tried it, and it never really dries to a hard crystalline condition, like HHG without urea. It was so flexible when dry, I was scared of it. It was a bit like rubber. It would bend and stretch even after drying for a week. It probably takes a fair amount of urea to keep it liquid at that low a temp, and smaller amounts might be less modifying to the properties of the glue.

Urea stretches working time, and is a commonly used adulterant in traditional woodworking, but is not well thought of for lutherie. Something to try out, expecting it to dissapoint.

I've used it a couple of times when I really needed a lot of open time. From what I've read, up to 5% by weight doesn't particularly affect the strength of the joint, but it should be freshly added. When I've used it, I just dropped a small amount of urea "pellets" (they're really not crystals) into the glue and stirred until they dissolved, then made the joint. Afterward, I threw the glue out. Someone once explained to me what happens (chemically) if the glue is kept liquid or reheated with urea in it, but I didn't really pay much attention to them so I don't remember. I figured it was enough to know not to keep glue with urea in it for later use because it apparently breaks down for whatever reason.
Anyway, I've had no joint failures in the two or three joints I've done with urea in hide glue, so there's my contribution to a representative sample!

Try Fish Glue. It has similar properties to hide glue, but has a long open time and clamping time.

Ron Belanger wrote:Try Fish Glue. It has similar properties to hide glue, but has a long open time and clamping time.

How is fish glue to take apart?

Not easy. it takes higher heat and requires some moisture in the joint. It can be done, but it's tenacious stuff once dry.