By now you probably know that I never simply copy an instrument from a plan. Often the instruments have deformed over time or were never symmetrical to begin with. And there is always a margin of precision while making.
The instruments were originally designed using simple drawing tools; ruler, square, dividers, etc. So to re-construct the design, one must find the original compass points and radii of the arches. But you need something to compare it with, a reference. So the original unit of measurement can be used as a touchstone
For the Sellas it wasn’t easy to find that original unit of measurement. All dimensions for the Venetian Braccio, given in old metrology books, didn’t fit the theorbo at hand (M255 of the Muziekinstrumentenmuseum in Brussels).
So after a couple of fruitless attempts, I decided to try a different approach; re-engineer the unit of measurement from the instrument. So for the first attempt I decided to use the body width (345 mm) as a foot length and break it up in twelve thumbs. Then these thumbs (oncia) were tried in the rest of the instrument. I was a bit sceptic, because it will never work on the first try, right? Ehhhm… Well, it seemed to work… The found thumb was fitting other measurements, luke the body length. And it even held up when I tried it on other instruments by Matteo and his brother Domenico Sellas. Could it really be that simple?
Apparently it could, because no matter what I tried, it always had the same outcome…
So here are the basic measurements fot the “Braccio da Sellas”:
1 Braccio (arm) = 2 Pide = 690 mm
1 Pide (foot) = 12 Oncia = 345 mm
1 Oncia (thumb) = 12 Punto = 28,75 mm
1 Punto (point) = 2,395833333…etc mm
The next step is to make a couple of rulers: one for the complete Braccio and one for a Pide. These will be a reference while making the theorbo. I still rely on my metric steel rulers and trusted Texas Instruments TI-83 calculator. These old calculators have about 30 memory spaces, enough to store all historical measurements I use in the shop.
It’s great to see that this method is catching on amongst other builders. My Oregon-based friend and harpsichord maker Darin Molnar just told me he found the system of measurement Bartholomeo Cristofori used for his oval spinets…