The graduation, or adjusting the thickness, of the plates is considered by most makers to be the key step in determining the voice and playing qualities to a violin. In actuality these playing qualities are a cumulative result of the nature of the wood, the shape of the arch, and the graduations. The way the wood is sealed and the varnish applied later on further modify the acoustic qualities of the plates. Graduation of the plates is the step where the maker feels the most control of the process, making adjustments for earlier decisions and bringing the whole together.
The term “graduation” refers to the fact that the plates are not a uniform thickness but graduate from thick areas of greater stiffness and mass, to thinner areas of greater flexibility. Patterns of graduation vary from maker to maker and from instrument to instrument and are the subject of endless argument and personal pride among makers. My own practices derive from the mix of all I have observed and been taught, and my experience of what works for me. Like all of my making practices it is more an evolution of what seems to give a better end result than a design based on preconceived principles.
In general the back is twice as thick in the center than it is around the edges. The center of the back must support the pressure of the strings transmitted via the soundpost. This requires enough thickness and strength in the center to resist cracking under the force of the post, and enough stiffness to distribute the vibrations from the post to the back as a whole. The rest of the back needs to balance lightness to consume less energy with stiffness to transmit vibrations to as much of the back as possible.
As I carve away wood from the inside of the back I pay attention to the relationship of thickness in various areas by measuring with a caliper. I weigh the back as I carve with a target weight in mind. I find that weight is a good way to adjust the actual thickness to woods of different densities and the related stiffness of the wood. To feel the stiffness directly I flex the plates with my fingers, aiming for that magic balance of freedom and resilience. Tapping the back gives a resonant frequency that goes down in pitch as the plate thins and becomes less stiff and can also be used as a graduation guide. I listen for a ring that is free and clear as a sign that this back has reached its ideal thickness.
As with most violinmaking processes, this is a subtractive process; you can take wood off, but you can’t put it back. It thus has a brinksmanship quality where you want to take off as much as possible, but not too much, leaving wood where it needs to be, but remove everything superfluous.
The tools used for this process are fingerplanes, scrapers, and finishing sandpaper; the same as used for carving the arch.
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