Old brown glue is a high quality traditional hide glue or scotch glue, but in a more convenient format that doesn't require a glue pot. Just stand the bottle in hot tap water for a few minutes until the glue becomes runny and then use it as you would any other glue - but with plenty of open time, easy cleanup and no worries about it showing through your finish.
Developed by professional furniture restorer Patrick Edwards, old brown glue is a sideline of his furniture restoration business in San Diego California, it is the same stuff he uses in his workshop and has several advantages over modern PVA based adhesives:
It isn't stronger than the wood itself - so if a joint is going to fail, it will fail on the glue line, leaving the original components intact and therefore easier to repair properly. In tests, the bond strength of old brown glue has proven to be equivalent to traditional hot hide glue - which has been successfully holding furniture together for hundreds of years.
It is reversible; by adding heat and moisture, hide glues can be temporarily reverted to a liquid state, so furniture made with old brown glue can be gently disassembled without damage, days, weeks, years or centuries after it was put together.
Hide glue will bond to itself, so unlike modern glues, original hide glue doesn't need to be completely removed from components before reassembly, adding OBG will reactivate the original hide glue and cure with it.
Because it is water soluble, any excess can be cleaned up with a damp cloth, the residue doesn't affect stains and finishes, so if you do get any in the wrong place it won't spoil the work.
Old brown glue will not deteriorate by being repeatedly warmed, cooled or frozen any number of times, as long as you don't heat it to anywhere near boiling point it will be fine.
Like hot hide glue in it's hydrated form, old brown glue does have a shelf life. Each bottle is printed with a best before date 18 months from when it was manufactured. However, this process can be slowed by refrigeration and paused by freezing (Antique Refinishers Inc make a fresh batch for us every time, and we store it in the fridge here at Workshop Heaven).
Old brown glue cures by moisture loss over time, it dries as hard as hot hide glue but takes longer. For processes that rely on temperature change causing the glue to 'grab' (rub joints and hammer veneering) hot hide glue is a better option, but for everything else old brown glue gives the same result but is much quicker and more convenient.
Read more about Patrick Edwards, the maker of Old Brown Glue, in Issue 10 of Mortise and Tenon magazine.