Okawbow,
The thing about how old violins were finished is largely nonsense. Back in the early 1980's a professor from a Texas college claimed to have found the secret to Stradavri violins. He looked at the wood and the finish on one using a scanning electron microscope. His first info release claimed that the wood was stained with a mixture of animal blood and chicken droppings. Now he had no experience in old ways of making stains or finishes. That description of the coloring matter would be consistent with the nitrate of iron stain.
Later he published a lengthy piece in a chemical industry magazine where he went into his version of what the finish had consisted of. With no chemical background and no background on varnish preparation of that time nor finishing techniques. His finding of "semi-precious" stones was simply finding garnet particles in the finish. Now Italy was long the world's largest producer of garnet. Large crystals were indeed used for semi-precious stones in jewelry. The very small non-precious garnet was commonly used as an abrasive. Thirty years ago I could still buy garnet sandpaper at the local corner hardware store.
He described a "lattice structure" in the finish. Now crystalline materials will form what would look like a lattice structure under high magnification. But what he was really looking at was how a thin coat of a boiled oil based natural resin finish film behaves. The film of liquid oil will begin to dry by oxygen from the air above the film. The boiled oil begins to form minute beads commonly called micells. These are squashed together so are not perfectly round. As they dry, by oxygen, the spent dryer metal, at that time lead, becomes insoluble in the oil and is pushed to the surface of each minute bead of oil. The natural resin used in the varnish was soluble in the oil before it started to dry. but as the oil beads dry the resin becomes insoluble and it to is pushed to the surface of the beads where it form minute solid particles that fill in between the minute beads of drying oil. This filling in process greatly reduces the permeability of the oil film thus cutting way back on how much moisture the finish will transfer in and out of the wood through the finish.
What am relating here is not theory. I spent hours over powerful microscopes watching oil and varinsh films dry on wood and glass test panels. The lab I ran did industrial coatings based on various PVC resins.
Regarding shellac. Shellac, as a natural "resin" was not known in Europe until very late in the 1600s. In India they did not use it as a solvent based finish. It was used only on articles made in a lathe where they pressed sticks of cooled lac onto the rotating wood pieces. It was melted onto the wood by the friction involved. You don't see lac being used as an alcohol based finish until you get into the early 1700s. It then began to see use as a sealer coat under oil based varnishes. A shellac finish will not stand up to liquid water on it's surface. Swells and turns white and looses adhesion. But under a good boiled oil varnish it forms the ideal sealer. Boiled oil varnishes will transmit a good deal of moisture through their films. But the shellac filler in the wood will swell with any moisture and that slows or stops any additional transfer of moisture into or out of the wood. This shellac film regulation of moisture changes within the wood helps to limit stresses in the wood that would cause it to split.
Bill K.
