Re-Gluing an Old Hide Glue Joint
New hide glue can re-activate old hide glue. But old and busted joints may present some pesky complexities. December 27, 2008
I know that hide glue will bond to itself, but will liquid hide glue re-bond a 100-year old stool's joints that failed, or do I need to use hot hide glue to get it to bond? If I use alcohol to crystallize the glue in the remaining joints and then take it apart, will hot or liquid hide glue bond to the then crystallized glue?
From contributor G:
You need to clean the joint down to the wood. You cannot expect to make a decent joint by gluing old glue to old glue with new glue. Scrape or sand the old stuff off. You don't need to soak it out of the wood. Hide glue joints are often pre-sized with thinned hide glue to resolve excessive absorption issues, but you will not get satisfactory results incorporating two layers (or even one) of old crystallized glue in the joint.
Hot hide glue can be modified with a small amount of salt, urea, or vinegar to slow the onset of the initial gel, which extends the open time you have to get things together. Warming the joint is better practice, but it can be done by the additives. However, to add enough modifiers to produce a product that can sit for years on the shelf as a liquid produces a product that is vastly inferior to hot hide glue, so I cannot recommend the use of liquid hide glue for anything structural.
From contributor B:
I have had the best success using a mixture of warm water and a bit of vinegar with a toothbrush to remove at least a portion of the old glue. Then I use a different old tooth brush and plain warm water to remove the softened old glue. After that I use fresh hot hide glue in the joint, remove any squeeze out, and clamp for 24 hours. I don't use alcohol or try to clean the joint completely.
From the original questioner:
Thank you. I agreed to try it... So I opened it up and found what I *should* have anticipated but did not allow for: It had already been repaired once with (I'm guessing) white PVA. So I'm now in for more than I'd bargained. I'll try a mild acid and, if/when that fails, I'll go to my scraper.
From contributor O:
Liquid hide glue will not reactivate old hide glue, however, hot hide glue will reactivate old hide glue. Even so, it is advisable to scrape the chunks off, but it is not necessary to scrape to new wood, as is a must when using other glues. Gram strength is an important factor with hide glue granules. I use 251 gram strength but also have 135 gram strength for some applications. Always have urea crystals and/or salt to slow down setting times and have glycerin to add to increase flexibility. Know also that the shear and tensile strengths are higher for hot hide glue than for white or yellow PVA glues.
From contributor P:
I've had great success using a liquid hide glue made by Patrick Edwards, marketed as Old Brown Glue. Good stuff, but yeah, you need good fitting joints.
From contributor S:
I'm working on an old, mission oak dining room set with six chairs and have been wondering about the same issues. I am having considerable success in removing the old hide glue by using a rotary tool and the type of bit that is used for cutting drywall - kind of a rough surface tile cutting bit. This chips out a lot of the old glue and leaves a roughened surface that looks good for re-gluing.
From contributor O:
You are doing a lot of work that is unnecessary although I commend your dedication to thoroughness. There is no need to remove all the old hide glue. The fact that it reactivates is one of the many wonderful properties of the glue. Hopefully you are not causing a loose fit with the grinding.
From contributor S:
Yes. I am aware of this, though I also note that apparently liquid hide glues such as Titebond Liquid Hide Glue don't have this property. Rebonding can only be accomplished with hot melt. More to the point, this furniture is quite old (I'm guessing about 1920) and the old glue in the mortise sockets is chunky, cracked and falling to pieces - not a good candidate for rebonding, as it is not particularly well bonded itself - just enough to make removing it irksome. Add to this the numerous attempts to reglue this piece or that over the years, and the sockets and tenons really require some cleaning.
This particular bit doesn't remove much (any) wood, carefully used. What it does do is allow chipping off some glue and a means of getting down in the bottom of the sockets.