Characterisation of varnishes used in violins by pyrolysis‐gas chromatography/mass spectrometry

Abstract

The correct characterisation and a detailed knowledge of the materials originally used in violin varnishes, like natural resins, is crucial for the conservation in museums and for a suitable restoration technique. The study presented here reports on the potential of pyrolysis (Py) coupled with gas chromatography (GC) and mass spectrometry (MS) for the identification of chemical markers of each resin; this tecnique is very sensitive and selective, it needs a small quantity of sample and does not require chemical treatments. To improve the chromatographic behaviour of polar compounds the derivatising agent tetramethylammonium hydroxide (TMAH) in combination with pyrolysis has been used, in the so-called TMAH thermochemolysis or thermally assisted hydrolysis and methylation (THM), or more simply pyrolysis-methylation. The natural resins studied were colophony, sandarac, manila copal, elemi, amber and benzoin, mainly composed of terpenic compounds, with the exception of the latter, composed of aromatic compounds. Many compounds were identified; in particular, methyl esters of resinous acids that, individually or in a group, can be used as chemical markers. However, through this technique it was not possible to distinguish between the sandarac and manila copal resins because their chromatographic behaviour is very similar. Finally, the procedure applied has been employed in the characterisation of original varnish samples.

Recent applications in analytical thermochemolysis

“…Chiavari et al [68] applied both analytical pyrolysis and thermochemolysis-TMAH for the analysis of resins in varnishes used on violins. The presence of benzoin and terpenoid compounds including colophony, sandarac, manila copal, elemi and amber was presented.…”

The analysis of European lacquer: optimization of thermochemolysis temperature of natural resins

“…Table 1 summarizes the total of 27 molecules selected, most of them were (partially) known markers [4,[7][8][9]24,41,44,46,49], one is an unidentified peak that appears when Congo copal is submitted to high temperature; it was selected for this characteristic. A THM-GC/MS chromatogram of each resin at 480°C is given in figure 1, with the position of markers followed indicated.…”

Characterization of Materials and Artistic Techniques on Two 17th‐Century Neapolitan Wood Sculptures

“…Also gas chromatography, combined with mass spectrometry and pyrolysis (Py‐GC–MS), is fast, reliable and microdestructive, and permits us to obtain the fingerprint of the organic materials in paint samples (Chiavari et al . ; Colombini et al . ; De Benedetto et al .…”

“…Among the instrumental techniques, μ-Raman spectroscopy has proven to be highly efficient, so it is widely used for the identification of constituents of works of art (Bersani et al 2006;De Benedetto et al 2011;Fico et al 2016;Freitas et al 2016;Stanzani et al 2016). Also gas chromatography, combined with mass spectrometry and pyrolysis (Py-GC-MS), is fast, reliable and microdestructive, and permits us to obtain the fingerprint of the organic materials in paint samples (Chiavari et al 2008;Colombini et al 2010;De Benedetto et al 2013).…”