Hide glue, old brown glue, liquid hide glue, etc., these names are for the glue that used to be used exclusively for furniture, bookbinding, musical instruments, and it still is. Hot hide glue is made out of old horse hides, cow hides, and hooves. These are the parts of animals not edible.
The hot hide glue comes in dry granules or pellets and have to be soaked in water overnight and then heated in a glue pot made for the purpose. The only thing I use this type of glue for is making roll tops for desk, etc., and for hammer veneering. (Subject for future blog)
This glue is reversible. Steam or hot water will soften it for disassembly. Possibly why it is used in making guitars, violins, and wooden stringed instruments. I have found that very old antique furniture is glue with this type glue.
Now if you were re-gluing an antique that was glued with hide glue, you wouldn’t have to clean off all the old glue if you use new liquid hide glue to re-glue the piece. But using any other glue type you would have to clean the old glue off with hot water and then let it dry out completely.
There have been tests of different glue types for strength. Hide glue when dry in a good joint is stronger than the wood itself. Just as strong as common aliphatic resin glue, such as white or yellow carpenters glue (Titebond, Elmers, etc.)
This was something I was very glad to learn because hide glue sets up slowly allowing you plenty of time for complex glue ups and letting you get all clamps tight before the glue seizes.
In my classes we use liquid high glue for just that reason. It takes most of the stress out of glue ups. The only downside is that you must leave the clamps on for about eight hours minimum or overnight. We also use other glues, but for complex glue ups its liquid high glue every time.
One last note on this glue. It has a shelf life of about 1 year or so. Recently I used some and it was still liquid after drying overnite. I checked the date on the bottle and it was 3 years old. So now I am careful to use fresher glue.