Graduation is the process of thinning the top and back to their finished thicknesses. This is done after the outside of the top and back have their finished shape. Tops are generally of a fairly even thickness throughout, with a bit more thickness near the soundpost area in the center of the instrument and around the soundholes. Backs are almost twice as thick in the center as at the edges, and how you arrive at this tapering scheme can vary widely.
This is an extremely important aspect of violin-making, as it has such a direct effect on the tone of the instrument. Too thick and the instrument won’t sound, too thin and it could be susceptible to damage or warping and the tone could suffer as well. So then the question follows: What is the right thickness? The challenge for makers is that each piece of wood presents a different answer.
To that end, I try to synthesize all the information I can get about the individual piece of wood that I am working with. Calipers are a first and foremost tool, because they act as signposts for me and let me maintain consistency. I weigh the plate to get an idea of the density. I hold the plate by two fingers in various places and tap on the arch, producing a “tap tone”. I listen to the pitch and quality of the notes produced. I flex the plate with my hands and rely on muscle memory to tell me if it is too stiff or too flexible.
A background in restoration is very helpful in developing that muscle memory. I don’t believe there is any substitute for flexing the plates of old instruments while working on them. After hundreds of instruments you can get a feel for what works and this is the kind of information I simply cannot get by exclusively making new instruments.
I spend a lot of time on my graduations and arches because it is so important.
This picture is of one of the iconic tools of the luthier, a fingerplane. For as long as violins have been made, makers like myself have been using small, (some less than 2 cm long) round-soled planes to carve the curves of the top and back.