There are a large variety of glues available in the modern world, but for string instruments, the one that has been used for hundreds of years and continues to be used, is hide glue. As the name suggests, this glue is made from boiling up animal hides to produce a gelatinous glue. It is applied by making a warm solution in water, brushing it on, then letting it cool. So not surprisingly, if your instrument gets too hot, hide glue starts to lose its strength and may fail structurally. So that’s a really good reason not to leave your instrument in a hot car, or other hot location, as well as the damage that heat can cause to the wood or varnish.
One of the main reasons the hide glue is still used today is that it is a reversible glue. Using enough force will cause a hide glue bond to break, usually without damage to the wood. Sometimes warm water is used to weaken the glue. The top of a string instruments is attached using a weak hide glue, and if a violin maker needs to remove the top of an instrument they can do this by inserting a knife into the join to the ribs and breaking the glue join.
Modern glues may stick as well or better than hide glue, but they are usually not reversible. If you have a crack or an open seam in your instrument never repair it with anything but hide glue, which probably means taking it to a violin maker. A repair with epoxy glue (eg araldite) is the least reversible, while white wood glue (PVA) is somewhat reversible, but seeps into the wood and is difficult to get out when future repairs are needed. Once a surface has been glued with PVA glue, then residual glue in the wood will prevent a future repair with hide glue from forming a strong bond with the wood.
Hide glue is quite hard when set, allowing good transmission of sound vibrations through glue joins. By contrast the softer PVA tends to damp sound vibrations which may adversely affect the sound of the instrument. The rubbery nature of PVA glue can also cause PVA joints to slowly creep, or move over time.
So the two simple things to remember about glues:
1. Never let your instrument get too hot otherwise the hide glue may fail
2. Never do home repairs to your instrument with anything other than hide glue. The safest bet is to visit a violin maker.
For those who want the full story – yes there may be a non-reversible glue that was used by the old Italian masters. Repairers of these old Italian instruments sometimes encounter glue joints which stubbornly resist removal by force or with any type of solvent. Violin maker Roger Hargrave suggests that the glue used maybe a casein based glue, made from milk proteins from cheese.