Wednesday, September 28, 2011
Building a Kaiser Theorbo bowl
Assembling lute bowls is demanding and sometimes frustrating work. I took this photo on Sept 11 when I had seven ribs in place. I usually only spend part of my work day on this activity; averaging three ribs a day. Each rib takes 1 1/2 hours unless there are problems. This bowl has 27 ribs. My assembly technique differs little from other makers. For narrow rosewood ribs like these I dampen the rib with hot water and bend them on a very hot iron -- spit jumps off of it. This model of bowl has a moderate flattening in the cross-section which means that the ribs are different shapes. They vary from "spear" shaped where the two edges are mirror images or nearly so, for the ribs in the center, to crescent shaped, for the ribs on the shoulders of the mould, to "S" shaped where the rib twists in opposite directions at each end. It is possible to cut this variation of shape into each rib but it is time consuming and wasteful. However, when assembling bowls with fewer ribs it is often the only choice that is possible.
The technique for planing the rib edge doesn't vary according to the shape. After the rib is bent to the contour of the mould I hold the rib against its neighbour and estimate where it needs to be trimmed and by how much. The excess is removed with a fine blade on a bandsaw, with a broad chisel or on the plane board as in the photo to the left if only a little is required. After the "rough shaping" a final fit is achieved using the plane board with an ever finer setting. This is time consuming and frustrating when a mistake is made. But this is only one edge of the rib. I shape the second edge by measuring the required width as determined by the lines on the mould, transferring those measurements to the new rib, and shaping accordingly. The rib is then glued into place using hot hide glue, push pins and plastic wrapping tape to secure and seal the joint.
When the contour of the mould requires a rib that is crescent shaped or "S" shaped I bend the demanding portion sideways. Only certain woods will accommodate this technique: rosewoods, ebony, snakewood and yew. And only with ribs of a narrow width such as those found on multi-rib bowls. I use a simple technique of damping the part of the rib that is to be bent, using a very hot iron and wearing thick deer skin gloves. While the rib is held in the proper longitudinal contour against the iron with my left hand -- I'm left-handed -- I push or pull slowly, but firmly with the other hand. This is exacting and delicate work. I am always amazed at the properties of wood that make this possible. It is also this property of certain tonewoods that resulted in 16th century lute makers inventing complex bowl designs.
I finished this stage of the bowl several days ago. Since then I have applied the rear apron and papered the inside of the bowl. Today I will glue in the inside apron and report on these in my next post.
Posted by Michael