Old Brown Glue

Old Brown Glue is a 'Liquid Hide Glue' as opposed to a 'Hot Hide Glue'. Patrick W Edwards a woodworker of international repute and about 50 years experience in fine furniture making and restoration of high end antique furniture.
Old Brown Glue has an excellent website with a good range of FAQ's and instructional videos.
The summary here is this:
For conservation and repair work on valuable furniture, or making of new fine furniture this glue is the ultimate. The thing is that PVA glues in any form like crosslinking PVA's, UV setting glues, Epoxy, RF setting glue, Polyurethane glue, and CA glue, is that ALL of these are not reversible. These glues are of course tested to the best of anyone's abilities in a lab but the fact of the matter is that nobody really knows what epoxy glue will do in 100 years time, you sure as heck won't be able to warm wet and steam a loose epoxy glued joint to repair it back to new.
If you're making furniture that you want to be around and repairable in 100+ years time then you should be using hide glue, either hot hide glue or liquid hide glue. Even the Egyptians used it in their woodwork.
Old Brown Glue is hide glue in the traditional sense, precisely formulated with the correct amount of water and urea. It's the small amount of urea that allows the OBG to remain gel like consistency at room temp in the bottle. By warming it gently to around 60-70Deg C it will become liquid and is to be used like normal hide glue. The only difference in use between Hot and Liquid hide glue is the liquid hide glue doesn't have an immediate tack due to cooling like the hot hide glue, so it's not suitable for hammer veneering or rubbed joints. Liquid Hide Glue tacks and sets by loss of moisture not by loss of heat.