Vernice bianca is a type of sealer varnish used in violin making. It is mainly prepared with a mix of egg white and gum arabic.
This preparation is highly efficient in strengthening the inside and outside of violins, as well as improving the acoustic of the instruments. (Increased wave propagation speed and decreased dampening of vibration).
According to Simone Fernando Sacconi, this "white varnish" is of significant importance in violin making as it allows for thinner plates, lighter and better sounding instruments. One can apply two layers of this preparation inside as well as outside of the sound box. This white varnish is transparent and is absorbed by the wood. It is absorbed by the spruce top more than by the other maple parts; it has an uniforming effect on the whole soundbox.
It must be noted that this preparation is likely to be a key element for the "secret" of old Cremonese instruments as a classical violin with most of its varnish gone keeps its acoustical qualities, whereas an instrument re-varnished with a wrong varnish is suffocated and loses its acoustical qualities.
One must also insist on the fact that this varnish is not doing any miracles and won't turn a bad instrument into a good one. The soundbox prior to the white varnish application must be fine tuned (plate tuning). The genius of Stradivarius was, according to Sacconi, that he was able to anticipate the modification and the potential improvement of the Vernice Bianca application and tune the unfinished instrument accordingly.