With the thickness of the back carved to nominal values, it is time to tune the plate. We want the plate to vibrate freely but we also want it stiff enough to provide carrying power. These two goals are at cross purposes so we have an optimization problem.
Using the work of Carleen Hutchins, I sprinkle tea leaves on the plate and vibrate it with a signal generator and amplifier. I look for two resonate frequencies, F5 and F3. I examine the pattern formed by the tea leaves. From the pattern and the resonate frequency, I can tell where to remove more wood. I also weigh the plate and calculate a number that is proportional to the stiffness of the wood. Finally, I also look at the range of frequencies at which the leaves still vibrate. I find that as I get closer to the optimal thickness, the leaves will only vibrate over a range of a few Hertz for a given resonate frequency. That is when it is time to stop removing wood.
Here is a picture of the pattern I get for F5. This is also called the ring mode for obvious reasons.
With the outside of the back done, we turn to removing wood from the inside and graduating the plate. The first step is to remove the bulk of the unnecessary wood. I use a gouge and take the entire inside down to a thickness of about 6mm.
Next I layout the graduation pattern. Since this is a Strad model, I use a variant of the system described by Sacconi. For this outline and arching shape, I find that this system gives very reliable results for the tone and carrying power of the instrument.
Again, using the gouge, I carve the graduation pattern to about 0.5 mm above the nominal values.
Now, using finger planes and scrapers, I clean up the gouge marks and take the thickness’s down to nominal values. Now we are ready for tuning the plates.
Now that the purfling is installed, we proceed to finish the outside arching of the back. The arching shape has a profound impact on the final tone of the instrument. Over the years I have settled on a particular arching shape for the Strad model that I find reliably produces an instrument with exceptional tone.
The first step is to rough-in the arching using the chisel.
We now move to finger planes and scrapers to refine the arching.
Finally, we use contour lines and scrapers to clean up irregularities in the arching shape.
There is a .75mm channel cut all around the instrument at the purfling. The Italians call this the “sgusciatura”.
Finally, the arching needs to be brought down a bit and blended into the sgusciatura. Again, contour lines are used to identify irregularities.
Using a low raking light, all irregularities are found and removed resulting in a perfect arching shape.
Now that the aqueous phase of varnishing is complete, it is time to prepare the ground. The purpose of the ground is to provide a smooth base for the colored varnish and to prevent the varnish from penetrating the wood.
The first step is to mix some pumice powder into some clear varnish. This is applied to the violin and then rubbed off. This is done twice with some curing time between coats. This step fills the pores of the wood and seals the surface.
Next a thin coat of clear varnish is applied. After it cures, the violin is rubbed down. The ground is now finished.
Now that the outline is established and the purfling platform is made, it is time to purfle the back. Purfling is one of the more demanding tasks in violin making. The purfling channel is cut by hand and must fit the purfling exactly all around the instrument. The purfling strips must be bent and cut to form a perfect miter at the corners.
The first task is to mark the purfling channel with a purfling tool. This tool scribes two lines 1.3 mm apart and 4mm in from the edge of the back.
Next, with a very sharp knife and a purfling picker, the channel is cut slightly more than 2mm deep.
Finally the strips of purfling are bent and cut to length. The miters are formed in the corners. The pieces are glued into place.
The next step for “The David” is to make the back. We start with some quite nice Bosnian maple.
We need to flatten the under side with a plane and then plane the edges which will be glued together. This joint needs to be perfectly flat along its entire length and square to the under-surface. A very sharp and finely tuned plane is required for this step.
With the joint prepared, hide glue is applied and allowed to sit over night. We now have a joined back and can draw the outline and begin the shaping.
The next step with the rib assembly is to install the linings. The linings provide additional strength to the ribs and also provide a larger gluing surface for the top and back. When completed the rib assembly will be quite flexible yet amazingly strong.
To begin the process, I split pieces of straight-grained spruce and plane the pieces to a dimension of 2mm thick and 8mm wide. The lengths are left oversized. I also cut mortices into the blocks to accept the c-linings. The other linings will just have butt joints to the blocks.
The next step is to bend the linings and trim them to length. Bending is quite easily accomplished using a hot bending iron and a little steam. The shape must exactly fit the outline so that there is little to no tension introduced. The trimming to length has to be exact and is best approached slowly.
The linings are now glued into place. The ribs and linings are glue-sized and then glued into place. The clamps are just clothes-pins re-enforced with rubber bands. The linings are just slightly proud of the ribs. The whole assembly will be flattened on a shooting board.
After developing a beautiful tan, it is time for “The Lyric” to get a bath. This is the aqueous phase of the ground preparation. The first step is to apply a coat of a 3% gelatin solution. I do this to seal the end-grain in order to prevent uneven absorption of the stain color. Next I apply a water based stain to the whole violin. This enhances the contrast between the winter and summer grain a gives the instrument a nice orange/cinnamon color. The result is: