You might think that painting a violin or a string instrument generally concerns only aesthetics and conservation as well as making it pleasant to touch and sight. All those issues would be worked out by a good coat of "copal" or a little more. It is so in most cases of wooden handworks. On the other hand, if the artefact should become, for instance, a valuable piece of furniture, the cabinetmaker would keep busy to get nice color effects and light reflections from the treated surfaces.
In case of a high quality violin making, the matter gets a little more complicated because the artefact, in addition to show the above mentioned features, has to perform a much more difficult task: playing. And it must do it very well.
In a string instrument, the vibrations originated by the strings spread through the bridge across the soundboard to the back through the sound post causing a resonance within the sound box coming into the surroundings through the typical f-shaped slots called "harmonic holes". The technique and the kind of substances used to paint the surfaces affect the way the wood reacts to vibrations.
The famous violins by Antonio Stradivari, legend has it, hid their excellence just in the painting technique but legends, you know, are destined to remain so.
Marco Cioni studied hard the books of the ancient masters and experimented their painting techniques and preparation of bases and pigments for years. His words reveal some scepticism about secrets and legends. He declares:
"There is a lot of talk of Stradivari's paint and secret but there are no secrets in violin making. The result depends on the luthier's hands. It's only this, along with the ability to comply with certain basic parameters, that makes the difference".
Therefore a clear stance supported by Marco, as he points out, by the fact that, if Stradivari had kept any secrets, he would have certainly passed on two sons of his who followed the art of violin making but who never reached the ability of their illustrious father. Furthermore it is known that in the early eighteenth century, in Cremona, there was a workshop at least. All the luthiers used to get there the same resins and pigments.
However, we can say that, besides a great experience in dosages and application techniques, each artisan eventually keeps his "secrets".

In violin making paints mainly contain ethyl alcohol (spirits) in which particular and precious natural resins are melted. They are also called gum resins like Benzoe, Sandarac, Damar, Copal, Mastic in Tears, together with some coloring agents, such as the root of Turmeric, sandalwood and Caliatour.
Alternatively, some boiled linseed oil is used to prepare the paint based on fossilized Amber, a substance a little more difficult to treat than the gum resins.
However, just with the fossilized Amber from mine or from the Baltic sea shore, Marco achieved his best results. "It was no easy to find out the procedure to get the paint of the ancient violin makers", he says.
In fact the Amber, also called "succino" or "succinite" because it contains succinic acid, is the result of Copal fossilization, a resin emitted by some conifers, which solidifies in the course of 3 - 4 million years thus becoming very hard. It can vary in color, from yellow to red, brown or green and can be dissolved in chloroform, benzene or other similar solvents. The latter ones, certainly not available at the time of the ancient violin makers.
Then, how did they manage to use it ?
Marco says: "After a period of research I found out that the only way to make the Amber malleable they had at that time was to heat it to about 350 C° in iron containers called "flasks". This process frees the succinic acid from inside and the substance becomes doughy and soft".
However, before melting the Amber, it is necessary to thoroughly wash it with sodium hydroxide, commonly known as caustic soda, in order to remove impurities from the surface. Then, meanwhile you proceed with the Amber, it is also necessary to heat the boiled linseed oil up to the same temperature of the resin. Mixing the two ingredients, when the temperature begins to drop, some turpentine must be added. This is when you can choose to maintain the natural color of the preparation or to get a more brownish paint. To do so you need to treat turpentine in advance with the dye it only accepts, that is, the "bitumen of Judea".
Once cooled the mixture is not ready for use yet because it shows some suspended impurities to be removed. Some of these, being heavier, may settle in a short time but the others, mainly slag, keep suspended so that filtering is needed. In this regard let's go back to Marco's notebook clicking the image on the side.
"The first time I performed the procedure at home", says Marco, "the smell of acid didn't vanish for a week! Since I discovered the Amber, let’s say, I mostly use the oil-base paint for my instruments although, compared to alcohol-base paints, it needs much longer time to dry. But the result is excellent for both aesthetics and sound".
Read from Marco's notebook

Finally look at the pictures below showing two painted violins. One (on the left) with alcohol-base paint and the other (on the right) with oil-base paint.