At work on another Pulcinella* cello (more about this another time) today I am dealing with the top arch.To shape the arching of the plates is for me the most creative as well as the most consequential challenge of the making process. The choice and complex interplay of curves determines sound and stability of the instrument and to some degree the thicknessing and inside vault.
Every three foot piece of spruce will have some run out. While jointing I always try to leave the bottom bout on the bass side with the longest grain, keeping in mind that the continuous fiber will be correspondingly short then carving the bass side top bout.
This instrument calls for Engleman spruce, a lovely tonewood, native to the Rockies. If it is not hybridized with White Spruce, it is usually super light and homogenous, imparting a very quick silvery response. Engleman Spruce transfers vibrations relatively quicker across the grain than does European Alpine spruce, and is not as tough along the grain.
PHASEI: I am using gouges across the grain to rough out the arch and an ordinary block plane to establish the general shape and height of the long arch. Considering that I measured the specific weight (0.36 ) and felt the subtle resistance during gouging I stretch the long arch and allow for a good height (28mm or more). The long arch swings with subtle concavity into bottom and top block. It is a critical balancing act to ensure the plates resilience to the pressure without unnecessary dampening bulk.
Thumb planes used along the grain allow me to blend the established proto arching from centre to edges.
PHASE II: The sculpting of the cross arch is achieved by means of straight scrapers,sharp enough to produce shavings. At this point I don’t want any sharp turns and blend the curves without creating bumps and holes. I want to maintain long arch strength in the central area, the “breast”, while defining cross arches . Luthier Quentin Playfair wrote an interesting essay on curtate cycloids, their history and design.Those are perfectly balanced curves, which embody maximum strength and flexibility in the change from convex to concave.
PHASE III: Positioning, design and integration of the sound holes (“ffs”) reveals the experience and assurance of the instrument maker. Do the “ffs” flow within the arching? Will they allow the instrument to breathe? How strong is the connection to the body? Where does the arch hinge? How well does the plate move as a whole, while giving a healthy resistance? How much do they spread across the grain? How bolt or delicate are they and what will it do the air resonance? The choice of stop length (nicks) proportions the whole instrument. Carving the sound hole is a three dimensional concept and requires reshaping the surrounding cross arches. I will add that I introduce more swing (concavity) for this spruce. Slowing the cross grain velocity warms the sound in Engleman in my opinion.
Yes, the “ffs” are sitting high, a feature grown with my Pulcinella model.