Historical linseed oil/colophony varnishes formulations: Study of their molecular composition with micro-chemical chromatographic techniques

Résumé En

Mixtures of siccative oil and Pinaceae resin were among the most widespread formulations used for varnishing in the 16th–18th centuries. The aim of this research is to study the molecular composition of different formulations of varnishes based on linseed oil and colophony. The mixtures were prepared by cooking both raw materials in different proportions, following the outlines of historical recipes gathered in treatises and manuscripts from that period. Size exclusion chromatography (SEC) evidenced the presence of numerous oligomeric compounds from linseed oil and colophony. Analysis using Electrospray-Quadrupole-Time of Flight mass spectrometer (ESI-Q-ToF) allowed the characterization of oligomeric compounds from colophony. In particular, structures of dimeric diterpenic acid units attached by ester, ether, anhydride or keto linkage are proposed. Concerning linseed oil, oxidized and chain-shortened triglycerides were characterized with High Performance Liquid Chromatography coupled with Electrospray-Quadrupole-Time of Flight mass spectrometer (HPLC–ESI-Q-ToF). SEC quantitation of compounds allowed us to assess the influence of the oil to colophony ratio on the molecular composition of the varnishes: oil polymerization appears to be minimized in favor of its hydrolysis in presence of colophony, whereas linseed oil seems to inhibit the formation of diterpenic dimers and trimers. In addition, analyses evidence the reaction of diterpenic acids with linseed oil compounds. In particular, we identified hybrid molecules formed by a diglyceride and a diterpenic acid. Addition products of a of a triglyceride and a diterpenic acid may also be present in the mixtures. These results bring new information about the molecular composition of varnishes made from linseed oil and colophony, and will improve the knowledge of historical varnishes. To this purpose, all analytical protocols were implemented on micro-samples so that they could be applied later on ancient artifacts.