Look but don't touch: Non-invasive chemical analysis of organic paint binders - A review.

Diagnostics on historical art samples are decisive for assessing degradation and understanding the chemical composition of supports and polychromies. These investigations help us in uncovering the artist's style and techniques and provide invaluable information for restoration, preservation and conservation. In paint formulation, the binder, also known as medium, disperses insoluble pigments and creates a homogeneous, adhesive mixture. Various analytical techniques, often used in combination, are usually employed to characterize binders with infrared (IR) and Raman spectroscopies being the most common choices. Recently, mass spectrometry (MS) has gained prominence for its ability to allow detailed structural characterization and identification, thanks to soft ionization sources such as matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI). However, MS typically requires micro-sampling, which is often prohibited for highly valuable artworks. This limitation has driven research toward the development of innovative minimally invasive sampling strategies like enzyme-functionalized gels applied to polychromies for in-situ protein digestion and peptide extraction. These quasi-non-invasive methods offer powerful solutions for extracting and characterizing organic painting binders, unlocking valuable insights into these elusive materials. This review aims to explore both the most common non-invasive analytical techniques used to characterize ancient and contemporary painting binders, and the most recent advancements in minimally invasive sampling strategies, which represent convenient and interesting approaches to enable the use of invasive analytical approaches while preserving the integrity of precious artworks.