Friday, September 20, 2013

Theorbo after Christoph Koch

I finished my first model of the Christoph Koch theorbo, Venezia 1650, Number 3581, Musikinstrumenten Museum -- Staatliches Institut für Musikforschung, Berlin. Earlier this year I travelled to Berlin, examined the instrument and reported on my study in this Blog on April 19, 2013. I spent the summer analyzing the templates that I made of the bowl and turning them into a workable mould. I was surprised by the lack of symmetry and I mentioned this in my earlier post but I wasn't ready for the problems this would cause in constructing a mould. Since I had wanted to replicate the shape of the lute's bowl accurately I was willing to put up with mis-alignments of individual ribs and I intended to include these features.

However, when I assembled the cross-sections and proceeded to check the rib contours with a flexible ruler, I could see that the irregularities would be a major problem. (My procedure for laying out rib lines is described in detail for another mould in two posts on May 8 and 17, 2012). On the original lute bowl the rib widths at any cross-section were approximately the same, but to maintain this uniformity, the edge contours were distorted.

 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.

 I constructed the bowl with East Indian rosewood ribs that I cut from the remains of a log that I have had since 1983. The triple spacers between the ribs are made of holly/ebony/holly.

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
with triple spacers of ivory/kingwood/ivory.

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.

I chose not to use as deep an end clasp as on the original. This doesn't alter the flow of the rib lines. Note that the angle between the fourth and fifth ribs (the first and last ribs are not visible) on each side is significantly tighter than the others. The angle on the bass side of the original is tighter than that on the treble. This feature is, in part, the reason that I had such difficulty in constructing the mould.

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 distance that the bridge and rose are placed from the rear of the face affect the instrument's tone. I think it is important to respect the original location of these elements. Theorbo bridges are often off-set to the bass side as a result of balancing the alignment of the extension. I routinely set my theorbo bridges off center by 5 mm or so. The Koch bridge is off-set much more than this.  Although I chose to not to duplicate this feature, I did roughly follow the design and dimensions of the bridge.

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
by an accumulation of wax. I cut a different but similar design using the same location on the face and the same diameters as the original.

The neck and extension of the original theorbo are veneered with kingwood and elaborately inlaid with ivory arabesques. I used ebony and rosewood thinly cut to veneer the neck and extension. The latter I edged with holly/ebony/holly banding front and back.

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.

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