Identification of organic materials in historical stringed instruments by off-line analytical pyrolysis solid-phase microextraction with on-fiber silylation and gas chromatography-mass spectrometry

Historical stringed musical instruments are a unique class of cultural heritage objects. Crafted during the 16th-18th centuries, these instruments remain somewhat mysterious. Due to the absence of written historical documents of traditional varnish recipes, their chemical characterization is the only way to recover the lost ‘secret’ of the Cremonese stringed instrument maker Antonio Stradivari and his contemporaries, whose traditions had been lost by 1800. Pyrolysis combined with gas chromatography-mass spectrometry (GC–MS) using solid-phase microextraction (SPME) followed by on-fiber silylation (Py SPME Syl GC–MS) was applied to a large collection of standards to evaluate its validity for compositional analysis and material identification of historical stringed instrument varnishes. The standards included pure resins (colophony, amber, mastic, sandarac, elemi), different types of shellac, as well as colorants or additives such as madder, dragon's blood, and Aloe vera powder, and other materials (propolis, beeswax). The aim was to obtain an extensive collection of GC–MS data of silylated pyrolysis products useful for the characterization of the finishes of these unique historical objects. A set of distinct pyrolysis patterns and products were chosen as specific molecular markers for each standard. The method was applied to instrument varnish samples from Stainer, Baioni, Amati, Guarneri, Gasparo da Salò, Maggini, and Guadagnini instruments. The method was shown to be effective for the identification of oxidized lignin, drying oils, and diterpenic resins. In the case of dragon's blood, madder, shellac, and propolis new markers were identified, suggesting their presence in some instrument varnishes. The identification of proteins and madder appeared most challenging.