A network for historic guitars and vihuelas
In the Flageolett-Schule (Franz Bathioli - Vienna, 1832) Bathioli praises
the work of Stauffer and Enzensperger. Especially their use of spirit varnish for
the deck of the guitar. Before Stauffer and Enzensperger guitar decks where not
varnished, which was a big mistake according to Bathioli (!). A not varnished deck will
be influenced by moisture and dirt which destroys the tone over time, while a
varnished deck will become better in time just like violins. Stauffer and
Enzensperger used a special spirit varnish which they developed themselve.
Has anybody experience with the difference between a varnished and a
not varnished deck?
I was told to use a tiny amount of white beeswax, or egg white, to close
the un-varnished deck.
I originally ( between forty and thirty years ago) varnished the bellies of lutes, 4 course guitars and citterns using a spirit varnish. Indubitably the lutes especially lost brightness, and I stopped using it in the mid-seventies. However, two lutes which I kept until about eight years ago, one much-played (Mediaeval Banquets!), the other not, seemed to have regained brightness and to sound better than when new.
Both spirit and oil varnishes add weight to the belly making it less capable of vibration? Especially the upper partials? Spirit varnishes dry mostly by evaporation, but oil dries by the addition of oxygen causing it to weigh even more as it dries.
Dirt and grease will also add weight of course. Currently I use a (fairly) protective coat of glue size for bellies. It is immediately available in the workshop and can be made to be insoluble by various hardeners. (Look for gelatin + hardener on the internet.) I use alum from the local chemists because, like 16th c. makers, I have it in the workshop to use in making oil varnish. If afraid of undoing glued joints, cold glue size can be put on as a jelly with a sponge. With the alum omitted it makes a good undercoat for oil varnish. (Apparently used by Gasparo da Salo amongst others.)
As always, experiment first.
Use something. A Spruce top will absorb dirt, grime, grease pretty quickly - especially where the hand or fingers make contact. I doubt that an unvarnished Top will destroy the tone (unless it has been really abused) but it will look unsightly.
I've tried many thin 'finishes' on period instruments - egg white, glue, beeswax, both Oil and Spirit varnish. I've settled on a coat of very thin Shellac (Spirit Varnish that acts as a sealer), followed by a thin wiped on oil Varnish - perhaps 2 or 3 coats.
There is really very little finish on there, just enough to prevent the obvious discolouration. It will show it eventually though. Fortunately a wipe on oil varnish is extremely easy to renew.
I think the ground glass was just a theory, along with numerous others, especially when it comes to Violin varnish.
I would be very careful of making assumptions such as Varnish improving the tone of instruments. Most of that idea comes from a violin making tradition and an obsession with the 'secret of Stradivari'. You will find hundreds of theories on the Varnish composition of the Cremonese. A simple Pine resin and Linseed Oil varnish according to our best scientific analysis.
It's hardly a straight forward subject. Mostly anecdotal. One of the few scientific approaches were the tests conducted by the Violin maker Martin Schleske. It's a long time since I read the results but he observed things like linseed Oil based Varnishes as actually dampening the sound. Probably not what is required on our low tension plucked instruments. The Violin is a very different beast though. . . so much more energy available in comparison to our plucked instruments.
I believe that ground glass, washed sand and pulverised brick all appear in 16th and 17th century recipes for oil varnish. Their purpose was to clarify the varnish by attracting impurities which could then be filtered out. The alum found in most recipes partly has the same effect.
Oil paints and varnishes are always applied in layers to speed drying. If the upper surface dries more than the layers below cracking occurs, as can be seen in many (or most!) paintings from previous centuries. A top layer which does not dry would obviate this but be sticky to the touch, so on paintings a comparatively soft varnish layer is used which can be removed and replaced if necessary. Traditionally furniture has a beeswax layer, which we also apply to our cars although their comparatively short life makes it not really necessary. (And the paint is baked). So oil-varnished instruments could also be waxed, whilst bearing in mind that they are to be played rather than sat on!
Some time ago, I made a number of simple dulcimers. Their sound-boards were finished by wiping over with a first coat of turpentine substitute, immediately followed by a second coat of around three parts turps subs to one of (polyurethane) varnish. The first coat of turps subs is to prevent the second coat from sinking in very far. When dry, rubbed down with very fine paper and the varnish coat repeated. This gave a very good final surface. I have always hesitated to use the technique on a serious instrument but it could perhaps be considered, perhaps using different materials.
There was discussion of belly finishes on Wayne Cripp's list some years ago - see his archives.
I don't know who told you that Linseed Oil is not used - obviously not plain Linseed Oil but I mentioned using Stand Oil for which there is some historical justification (see Michelson's work). Stand Oil is Linseed Oil allowed to stand so as to partly oxidise. It ican be very viscous and does not sink into the wood but forms a thin satin type finish sufficient to protect the surface. Provided it is sufficiently oxidised it hardens to a buffable surface in a couple of days.
Note that modern Stand Oil is just Linseed Oil with chemical additives, Just but Linseed Oil, pour into an open dish and allow to stand for a few weeks/months (depending on viscosity required).
Try it on an offcut..
I think that Shellac was available in Europe by the 17 th century, if not before. I think the Marciana manuscript has recipes for both Oil and Spirit varnishes but being 16 th century, I doubt that it includes Shellac. Certainly by the late 18 th century I would expect a widespread use of Spirit (Shellac?) varnishes, even for musical instruments. They are harder to apply than oil, quicker drying but have much less problems with dust.
I'm at a loss as to why Shellac gets a bad press from some quarters. I expect most of the criticism comes from the traditional Violin makers who constantly look back at the oil varnishes of Strad. Probably the right approach is to keep the finish very thin - as do makers of modern Guitars with French polishing. Some use a drying oil (Walnut or Linseed) whilst French polishing, perhaps resulting in a hybrid varnish.
Here's a very rough time line for the use of shellac. It's first appearance goes back at least 3000 years, not necessarily as a finish material. It seems to have appeared in Spain and Italy in the early 13th century as an element of the painters tool kit. By the late 14th/early 15th it became quite common as a finish ingredient for furniture and possibly musical instruments.
It was in the 18th century that the technique of "French polishing" (French Polish is technically not a substance/varnish/finish but a technique for applying a finish) became widely used as a means to finish wood. It's unclear when this technique of applying shellac was developed as padding was a very common means of applying finishes throughout history. Probably, like most things, it evolved over time and was perfected by during the late 17th early 18th centuries.
As a finishing technique French Polish fell out of favor by the early to mid 20th century, largely because of the time involved to apply a finish in this manner and the availability of synthetic easily applied finishes.