In this extract from the October 2021 issue, Balthazar Soulier, Stefan Zumbühl and Christophe Zindel introduce their study of minerals historical varnishes

Figure 3 for reference

Cross-sections of a sample from a c.1700 Gofriller cello (left) and from a 1724 Stradivari violin (right) under UV light

The following extract is from The Strad’s October 2021 issue  feature on historical varnishes ‘Beneath the Surface’. To read it in full, click here  to subscribe and login. The October 2021 digital magazine   and print edition   are on sale now

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.

One of the most intriguing findings of a large number of studies has been the discovery of a significant content of inorganic elements (mostly calcium, silicium, sodium and potassium) within classical Italian varnishes. This inorganic content was ascribed mostly to silicates and calcium salts by Michelman and Condax as early as in the 1950s, and to a volcanic rock (pozzolana) in the 1980s by Barlow and Woodhouse. Although the distribution of the mineral compounds within the layers were not established, the researchers related them almost solely to the composition of the ground layer. These analytical results had a considerable influence on luthiers, notably through Simone Sacconi in the US and David Rubio in the UK, who both developed different types of ‘mineral grounds’ and gave rise to a wide variety of speculations about the origin and role of mineral content in violin varnishes.

Thus, many fundamental aspects of the composition and elaboration of historical varnishes remain to be discovered. In particular, matters such as the origin and role of the mineral compounds; the precise type of resins and their relative proportions; the different types of pigments and dyes; and the effects of the cooking process still need to be studied in depth.

Read: Historical varnishes: Beneath the surface

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Read: Stradivari’s varnish: a chemical analysis