To take unseen: direct gel-supported liquid extraction for multi-technique characterization of organic colourants in paint layers

Abstract

The study of organic colourants employed in artworks is a fundamental topic for history and conservation research; however, the most critical aspect in such matter arises from the necessity of sampling. Instead of binding media and inorganic components, natural organic colourants often exhibit weak signals during non-invasive analyses, which can be easily masked in complex mixtures, such as paint layers. To overcome this problem, the preferred approach is to take a sample, extract the colourants and analyse them using specific analytical techniques. The proposed method of direct agar gel-supported liquid extraction, performed using a Pasteur pipette tip, offers a promising solution to this challenge. By extracting colourants without the need for paint sample removal, it minimises visible impact on the artwork while still providing sufficient material for spectroscopic and mass spectrometric characterization. The experimental approach, which involved mock-up of mixing different organic and inorganic pigments with different binders and subjecting them to artificial ageing, simulates real-world complex matrices and assesses the effectiveness of the extraction method. Photomicrographs were used to document the procedure's visible impact. Mass spectrometry and surface-enhanced Raman scattering spectroscopy, the techniques of choice for invasive analysis of organic colourants, were used on the analytes extracted from the gel. The method application on paint patches from historical catalogues, housed in the Stiftung Preußischer Kulturbesitz collection, adds practical relevance to the study and demonstrates the applicability of the technique to authentic artworks. The procedure enabled the identification of natural and synthetic colourants without compromising the integrity of the paint systems.