Historical varnishes: Beneath the surface
The inclusion of minerals in Italian varnishes from the 16th to mid-18th centuries has long been a source of speculation. Balthazar Soulier, Stefan Zumbühl and Christophe Zindel present the first results of a long-term study showing that key answers can be found in early German recipes
Since the 1950s, researchers have applied modern analytical tools to elucidate the composition of ‘classical Italian varnishes’, in particular those of Antonio Stradivari. Despite the advanced technologies used over the past few decades, the investigations conducted by research groups across the globe have provided results that are only partial and often contradictory, at least in their interpretation. In most studies, the varnishes of only a small number of instruments were examined, using complementary analytical techniques. Most of them focused only on Stradivari’s instruments, which allows no contextualisation.
Nevertheless, some significant and consistent findings have been obtained. In particular, the varnish layer of instruments made before the mid-18th century has been unanimously identified as a mixture of drying oil and resins of the pine family. Only in a few cases have other resins been detected, such as mastic, sandarac and shellac.
The main divergences concerned the nature of the ground layer and the interpretation of the layering (stratigraphy). While the most comprehensive studies on Italian varnishes have reported a bi-layered structure based on oil–resin mixtures, other researchers have considered more complex systems involving protein and mineral particulate sub-layers, as well as special wood treatments…