For the vaulted back I prepared rib stock of macassar ebony about 1.1 mm thick. I also made 2mm wide white/black/white spacers by laminating 20 mm wide holly and ebony stock and then slicing off individual spacers. The ribs were bent to the necessary contour and shaped on an upside down jointer as seen in the top of the adjacent photo. I secured the center rib with positioning pins at each end and then applied one rib after another, alternating sides.
Each newly shaped rib was held in place with clear plastic tape and push pins. Throughout the assembly procedure the vaulting always felt fragile and as I neared completing the vault I left the plastic tape in place.
The work went quickly, in part, because Checchucci designed and constructed the vault in such a way that the shaping of each rib was simplified.
|MFA Boston Restoration photo|
I laid out the ribs lines on my mould assuming that every rib conformed to my hypothesis. Having now assembled the vaulted back and side ribs I don't think I was completely correct.
I was to be disappointed. When glue dries on a papered joint it not only pulls the two sides of the joint more tightly together but it also pulls the joint down toward the inside of the instrument's body. On a arched body like a lute bowl the curvature of the ribs counteracts the downward pull and the ribs find an equilibrium.
The interesting point is that the original guitar suffered the same fate. In the photo it appears as a very slight deviation. If you follow the line of the joint between the third and fourth rib and the joint between the second and third ribs in the lower bout you can pick out the collapsed section just to the right of the glare.
I noticed this feature during my study of the guitar in Boston but I assumed that it was the result of distortion taking place over the long life of the guitar, not an inherit design flaw. The amount of distortion on both my guitar and the original Checchucci was only 1 mm, but it was a noticeable blemish.
Fixing the problem proved to be troublesome. I removed the three ribs that were affected by the collapsed joint and began to rebuild the area. By trial and error, in attempting to fit a new rib in place, I found that I needed to prop up the adjoining rib as seen in the photo, by about 1mm. This was possible because of the flexibility of the vault and side ribs. The procedure not only increased the curvature of the vault slightly but also changed the shape of the new rib from one with a straight edge to one with a concave edge. This created a stronger joint that would resist the downward pull after papering over the inside of the joint. I was able to accommodate the last two ribs by filing down the side rib by a millimeter. This allowed each rib a little space to cant over, creating a slight sideways arch to an otherwise nearly flat slope.
So is this a faulty design? I think it is a design that leaves little room for error. In instrument making there is a fine line between being in balance and being out of balance and success often depends on a finding the elusive balance.
Building replicas of historical models adds another dimension. One that is seems to be measurable, ought to be, but isn't quite.