I pondered this difficulty for some time considering whether it was possible to remove the irregularities from the rib lines while maintaining the uniformity in width without altering the shape of the bowl. I decided that it wasn't. My solution was to start over.
Most of the irregularities are found on the bass side of the bowl. The treble side is accurately assembled. I decided to make the bass side a mirror image of the treble side. If the lute bowl had been symmetrical in the first place this would have been the case. I assembled a new mould using the treble side contours. I felt this would result in a bowl that, while not a replica of the original, was representative of it.
The low profile of the bowl is obvious in this view. I was pleased that the contour lines of the ribs of my model are a close match with the original.
The Koch bowl is constructed with kingwood ribs
This photo of my Koch shows the flow of the ribs over the rear of the bowl. When I examine an original lute I carefully measure and plot the position of each rib as it disappears under the end clasp. This is an important aid in reproducing this difficult area of the bowl shape.
Note too, that the center rib is narrower than the others. These factors lead me to believe that the original bowl was not assembled on a mould but assembled from templates and not starting from the center and working outward, one rib after another, as is the method today but from each edge inward.
When I constructed a symmetrical mould by altering the bass side it affected the line of the ribs, not the contour of the face which was retained. By comparing the two photos you can see the slight different in the curving line through the middle third of the face and that in the lower quarter of the face from one side to the other.
I used 10 year old Italian spruce for the top, thinning it to just under 2 millimeters on the treble side, a little thinner on the bass and down to 1.1 millimeter around the rose area. I do not know the barring arrangement in the Koch theorbo so I assumed it was similar to that found in his archlute, E.546 Musée de la musique, Paris which is a standard design and similar to what I described in my post on the Kaiser theorbo from November 7, 2011. Plans for the Koch archlute are available from the museum.
The tips are missing from the wings of the original bridge so I finished my bridge in a design that could have been a continuation of the original.
Notice that the top front edge of the Koch bridge is smoothly canted back. I believe this was done to accommodate the metal diapasons that risked breaking if bent over a rounded but more abrupt edge. I liberally rounded the front edge of my bridge.
The rose is beautifully cut but the detail is somewhat obscured
The fretted string length of my Koch model is 84.6 centimeters with diapasons at 167.5 centimeters in a disposition of 7+7.
All photos by the author.