The following two articles by William M. Fulton on finishing violins first appeared in the May 1972 and July 1997 SCAVM Bulletins, respectively.
Bill Fulton is one of the founding members of our Association and was our president during 1968. He has been a regular contributor to our Bulletin and has made many presentations at our meetings through the years. He is well known for innovative and sometimes controversial ideas such as terpene varnish, plate bending, and the glazing method of finishing.
Bill took up violin making in 1951 while serving with the U.S. Air Force in Germany. He studied with Willibald Raab, first in Mittenwald, then in Erlangen. He retired from General Dynamics in 1980. He is author and publisher of the books Propolis Violin Varnish, Terpene Violin Varnish, and Violin Varnish Formulation Manual. He resides in Idyllwild, California.
There are only two materials required to make terpene (or turpentine) varnish. These are linseed oil and gum turpentine.
The turpentine must be pure gum turpentine. The gum turpentine is distilled from the oleoresin that exudes from the living pine tree. It contains about 60% alpha pinene and 3035% beta pinene. It is the beta pinene that is most reactive and will form the terpene resin. Such products as wood turpentine contain no beta pinene and therefore must be avoided.
Polymerization of the fresh turpentine will occur naturally over an extended period of time. From six to eight months are required during which time the turpentine must be exposed to air and heat. The process is accelerated by the presence of moisture, ultraviolet light, and heavy metal ions. I have found that the process can be reduced to 25-30 days by forcing air continuously through gum turpentine in the presence of manganese napthenate acting as a catalyst. The process should be carried out in direct sunlight if possible so as to receive heat and ultraviolet light. The method I use is as follows:
Place 3 quarts gum turpentine and 15 cc 6% manganese napthenate into a clear glass 1-gallon jug. Locate the jug in a place that will receive direct sunlight during the day. Connect a bubbler so that air is bubbled continuously through the mixture.
After several days (10-15) the mixture will change color from dark brown to light straw. At this time filter the solution into a clear 1-gallon jug and re-connect the bubbler. Continue bubbling until the turpentine is very thick (20-30 days). The yield from this process is less than 50% since a good portion of the alpha pinene is evaporated.
The thick (polymerized) turpentine made by the above process is no different from that produced naturally. The catalyst is soluble in the fresh turpentine; however, it is insoluble once polymerization has started and can therefore be filtered out. It should be removed as completely as possible because excess amounts of manganese in the final varnish can cause defects in the dried film.
The thick polymerized turpentine must now be made into a resin suitable for making varnish. This is the point at which the old Italian varnish makers started since turpentine available to them was “aged” and thick.1
The thick turpentine is a mixture of many complex substances derived from the terpenes. It is only necessary to evaporate the volatile substances in a cooking process to free the desirable terpene resin. Extreme care is required, however, since the mixture contains some hydroperoxides. These substances are very unstable and when heated change to acids with the liberation of considerable heat. The reaction is very exothermic and the fumes are heavy. The reaction begins at about 125 degrees Celsius and ceases when all of the hydroperoxides are expelled. I use the following cooking process:
Place the thick turpentine in a high wall 5 quart pan. Connect an electric hot plate in an open outside area.
Place the pan on the hot plate and gently heat observing the temperature increase on a thermometer. At approximately 125 degrees Celsius bubbling will occur. Use extreme care and as the temperature increases be prepared to remove from the heat and cool by placing the pan in cold water. Do not let the temperature exceed 150 degrees Celsius until after the exothermic reaction has ceased. This can be determined by a change in odor or by observing a more gradual change in temperature. I find there is no danger in cooking the turpentine if one proceeds slowly and if one does it out in an open area. Cooking in an enclosed space or proceeding too fast can cause fire and perhaps explosion with serious personal injury.
The resin must be cooked until it is brittle. The temperature at which it is cooked and the metallic salts with which it is cooked will determine its color.
To make yellow varnish required for a ground coat (one that will not discolor the wood due to differential absorption), cook the thick turpentine in a pan made of aluminum, stainless steel, glass, or enameled ware and do not exceed 180 degrees Celsius. Cook until brittle.
To make amber varnish, cook as with the yellow, but continue to 300 degrees C.
To make red-brown varnish, cook in an iron pan that has a film of iron acetate in it. This can be made by putting a few cc of strong vinegar (glacial acetic acid) into an iron pan and allowing it to evaporate. When the acid has completely evaporated, add the thick turpentine and cook to 300 degrees C.
After cooking the resin to a brittle state, allow to cool and break into pieces. The resin can be kept in this state until desired for compounding the varnish.
The linseed oil used in this varnish is any good grade of boiled oil. This material will contain a balanced mixture of dryers which will not interfere with the varnish, but may aid in drying.
The turpentine varnish is made by cooking the terpene resin with the linseed oil. The process is the same for all of the different colors except the pale yellow ground coat varnish. To make the pale varnish, cook the resin and oil in a ratio of 150 grams to 100 cc oil to a temperature of 180 degrees C. Hold at this temperature for a firm pill.2 With this varnish one may have to cook for more than an hour or two to achieve a firm pill. If the temperature exceeds 180 degrees C, the varnish will darken.
All other colored varnishes are cooked in the same ratio (150 grams resin to 100 cc oil) allowing 20 minutes to reach a temperature of 300 degrees C. Hold at this temperature until a firm pill, which will take 20-30 minutes. Cool the batch to 230 degrees C and reduce with 250 cc volatile solvent for each 150 grams resin. The yellow varnish is reduced in the same proportion (250 cc solvent to 150 grams resin), however it may be added at 180 degrees C.
In the varnish industry the choice of solvent to use is dictated by economic considerations and is usually a compromise between price and quality. The old Italians had few solvents from which to choose. The most logical was turpentine although most of the available turpentine was oxidized or thickened. The resin-oil mixture requiring a solvent is both polar and nonpolar. It requires a polar-nonpolar solvent; however, the 5-10 day old straw colored turpentine made with the bubbler, the product filtered and not allowed to become thick, is a polar-nonpolar solvent and can be used. Additionally, the following solvents can be used: oil of rosemary, spike lavender oil, pine oil, and pine needle oil. All of these substances were available to the ancients and are polar-nonpolar solvents. For myself I use the thin oxidized (5-10 day) straw colored turpentine.
To summarize the varnish formula we have:
Each of the colored varnishes except yellow and amber contain iron salts. It is the iron that causes the salmon pink fluorescence of the varnish under ultraviolet light. The ground coat exhibits a gray color characteristic of terpene resins. Both of these indications can be observed on the old Italian instruments of the classical period.
The varnish is applied to a violin using a good quality clean bristle varnish brush. There is no need to use the fine sable, etc. brushes. The varnish will flow properly and form an even thin coat. Each coat will appear dry to the touch rather quickly. However, complete drying of the oil requires 2 or more days and preferably in direct sunlight or an ultra-violet drying cabinet. It is wise never to rush varnishing. After all, what are two or three days in the life of a fiddle?
1. Gum turpentine is in great demand by the chemical industry for use in producing synthetic camphor and a wide range of polymerization catalysts for the plastics industry. The price has quadrupled in the past 5 years. It is used too fast to naturally change state to the thick gummy material we desire for turpentine varnish. The reverse was true during the classical Italian period.
2. To test for a firm pill, place a drop of hot varnish on a cold glass plate. Allow to cool and probe with a finger. The varnish should be about the consistency of taffy—that is, firm but not hard. It will form a thread as the finger is pulled away from the pill.
To introduce the subject of the ground I use under violin varnish let me present the following background:
In 1989 I attended the Tiverton Violin Conference in England and heard a presentation by Claire Barlow. It concerned the results of an investigation she had conducted on the ground that was used by the classical makers. She revealed that the ground appeared to contain something she described as “rubble.” Claire was unable to identify what the material was, but she did say, with some certainty, what it was not. Then in 1993 I attended a joint meeting of the Violin Society of America and the Catgut Acoustical Society and heard a presentation by Andrew Dipper where he discussed a mineral ground that he and Geary Baese were investigating in an attempt to reconstruct what Claire Barlow had discovered. That is the background now for my ideas on this mysterious ground.
I believe this ground was based on propolis, a byproduct of the beekeeping industry.
Back in the days of the old master violin makers, 1550–1730, people who kept bees didn't have the hives we have today. They kept their bees in skeps, baskets woven of straw. Each year, in preparation for the honey run, the beekeeper would clean the hive by leaching it in lye, the result of leaching wood ashes with water. (Today we scrape the propolis from the hive.) The lye would digest the propolis making the hive ready for the bees. This digesting process created a liquid soap from the propolis. It is my belief that this liquid soap was the source of the mysterious ground the violin makers used as an undercoat. (It was also probably used by artists of the time to make a smooth, sealed surface for paintings). Someone, be it the violin maker, artist, or perhaps a person who was engaged in dying cloth (it gives a beautiful gold color), would add alum (other metal salts can be used) to the lye-propolis mixture, precipitating an aluminum propolis soap. The resulting very bright yellow material, when washed, would be a very fine slurry ready to use as a ground.
How did the violin makers use this propolis soap? Here are my thoughts. The violin makers would scrape the surfaces of the violin smooth with scrapers. (You don't have to worry about the small imperfections caused by the curl of the maple picking out or small imperfections such as tool marks, etc., because they will be filled.) Then the wet slurry would be rubbed into the wood leaving a coating of this bright yellow soap on the surface. It is allowed to thoroughly dry and then it is rubbed, with the hand, to a very smooth, almost silk-like, surface. The material will fill all tool marks, all imperfections, gaps in the purfling, but the grain of the wood will be completely hidden. Next, a coating of clear varnish is applied and, if the index of refraction of the varnish is correct, the wood grain suddenly appears and the propolis soap will disappear. When dry, if there are any imperfections in the surface they are repaired by rubbing with more propolis soap and sealing with more varnish. This is continued until the surface is acceptable. Then more varnish is applied, a color glaze, and the instrument is finished.
This bright yellow aluminum soap actually seals the surface of the wood. The varnish is absorbed by the material but the varnish does not penetrate into the wood. When it is applied to the scraped surface, the top appears like corduroy, but the material fills the grooves of the corduroy, and the surface becomes smooth. After varnishing the growth lines in the top are accentuated, look darker, due to varnish being absorbed by the propolis soap.
Originally, when the skeps were leached in the lye, some organic matter other than the propolis was digested, such as dead bees, beeswax, etc. This gives a greenish-yellow color to the aluminum precipitate. To get a bright yellow color the liquid soap was allowed to stand until the organic matter formed a scum on top which was subsequently removed (It actually rots to form the scum.). Then a bright yellow color is produced when the alum is added. Of course today we do it differently but the results are the same.
The reason I believe this to be the ground is because it is, first of all, a very bright yellow, the color seen under most old varnishes, and, second, it is a product that was readily available to the violin maker.
Instructions on making propolis ground follow:
Fill a one quart glass jar half full of raw propolis as scraped from the bee hive. Add enough acetone or denatured alcohol to the jar to cover the propolis at least one inch. Allow the jar to stand until the propolis resin has dissolved, approximately 2 weeks. Filter the solution into a glass drying pan and place in a warm location to allow the acetone or alcohol to evaporate. (Some residual solvent will not affect the process). Collect the propolis resin.
Place 100 grams of propolis resin in a jar and add 300 cc of water. Add 3 grams of potassium hydroxide and heat the mixture, below the boiling point, to dissolve the resin; then filter the liquid into a clean jar (There should be an excess of propolis resin meaning all of the potassium hydroxide has been exhausted by the reaction).
Make a solution of alum (aluminum potassium sulfate) and water. A 5% solution is about right. Add this solution to the propolis resin solution causing a bright yellow precipitate to form. The reaction is complete when, upon standing, the precipitate settles to the bottom and the liquid above is clear. Pour off the clear liquid, add water and shake. Allow the precipitate to settle, then pour off the liquid and add more water. Do this several times to get rid of excess alum. Pour most of the water from the precipitate and filter. Collect the wet yellow paste into a jar and cap. This is the yellow mud that will be used as a ground.
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