As a musician, you probably never think about what kind of glue holds an instrument together. You should, though, precisely because it is the glue - and only the glue - which keeps any fortepiano from collapsing into a pile of sticks! A poor choice in glue will eventually ruin an otherwise fine instrument.
Not all glues are ideal for this job, and many instruments are made today with a kind of glue which is not designed to resist continuous stress. Therefore, if you have not yet bought an instrument, it is highly recommended that you ask each builder you are considering what kind of glue he uses. Taking this simple step now may save you a lot of grief and money in the future.
Caution: the price of an instrument is NOT an indication of the type of glue used. While low-priced instruments are almost inevitably made with the wrong glue (PVA - "Titebond" - see below), the reverse is not always true, that is, an expensive instrument is not necessarily made with proper glue. There is only one way to know: ask the builder.
Glues used in woodworking today are of two types:
Natural glues have always been preferred by the world's finest makers of all kinds of wooden musical instruments, because they are stronger, more reliable, more practical, and far easier to repair than modern glues. Natural glues have proven their durability over very long periods of time; furniture found in Egyptian tombs is still in perfect condition after 3000 years!
The natural glues most often used in instrument making are derived from animals and fish. Each type of glue has its unique properties which make it ideal for a certain application. Different kinds of glues can be combined to tailor the working qualities and the final strength to specific applications.
Natural glues usually come in a hard, dry form, either granules, powder, or thin sheets. They are dissolved in water and then heated to about 60° C (140° F). The glue must be both hot and fluid when used. As the glue cools, is turns into a rubbery gel, which eventually becomes hard when it dries. As long as this gel is wet, however, applying heat will liquify the glue again instantly. This is why natural glues are ideal for complicated assemblies, like soundboards or cello top and back plates. The builder can assembly the object slowly, and once the clamps are on, he can then apply heat to liquify the glue inside the joint. This is but one of the many reasons why no respectable Luthier would ever use anything but natural glues.
The natural glues I use most: (1) a high-strength hide glue; (2) a medium strength hide glue; (3) a very-high strength fish glue; (4) my normal mix of equal parts 1 and 2, hot and ready for gluing; (5) a cold liquid bookbinder's fish glue, excellent for leather and cloth; (6) the hardest natural glue in existence, "eisenglass", made from fish bladders and stronger than epoxy!
When natural glues are dry they are incredibly hard and rigid. This not only gives them superior strength to resist string tension, but it also allows them to transfer sound between the various parts of the instrument with maximum efficiency and minimum energy loss.
Finally, a natural glue joint can be disassembled at any time with a combination of moisture and heat. This is very important for musical instruments, since damages resulting from transport or extremes of weather are common. When natural glues have been used, it is very easy to take the instrument apart and put it back together again without undue effort or damaging the wood itself. It is not necessary to clean away before any old glue before closing a joint again; the application of fresh hot glue reliquifies and combines with any of remaining old glue.
For more information on natural glue, click here.
The most commonly type of modern glue which is misused in instrument making is Polyvinyl Acetate (PVA), commonly called "Titebond" after the most popular brand. Titebond has numerous qualities that make it unsuited for instrument making.
The most dangerous aspect to using Titebond in piano construction is that it never really hardens completely. When a PVA glue joint is under constant tension or pressure - as in musical instruments - the glue can move slowly or, in the worst cases, fail completely. Adhesive chemists call this "cold flow" or "creep". This is why Titebond and similar PVA glues are not allowed in the architectural construction or other applications where failure could cause life-threatening structural collapse. Here is what the manufacturer of Titebond says about their own product:
The wooden framework of a piano must certainly be classified as a "structural application", especially when we consider the fact that these structures must constantly support a string tension between 1,500 kg (5 octave Walter) and 5,500 kg (6 1/2 octave Graf)! Despite this warning from the manufacturer, a number of fortepiano makers today persist in using this kind of glue. Such instruments may last for a while, but eventual structural failure is almost inevitable.
Frank Ford of Gryphon Stringed Instruments (guitars, banjos, mandolins, etc.) is but one of the many master instrument makers who feel that Titebond has no place in instrument making. He has conducted some very interesting tests comparing Titebond to natural hide glue, showing how an instrument assembled with Titebond can collapse completely if left in a closed car (or an uninsulated moving van) on a hot summer day. You can read about his experiments here.
Another problem with Titebond is that it is very difficult to repair. While it can also can be released with heat and water, unlike natural glues, the softened glue does not dissolve; it turns into a gooey mess that resembles bubble gum on a sidewalk. Even worse, it will never reharden after it cools and dries again. Before the joint can be reglued, the wood must be completely cleaned, a difficult process often requiring harsh chemical solvents like MEK.
Finally, because PVA glues are always slightly rubbery, even when they are technically "hard", it's like putting a thin piece of sound absorbing rubber between all the pieces of the instrument. Many guitar, violin, and lute makers testify that using Titebond kills the sound of even an otherwise good instrument.