Realgar and Light

Light, necessary for viewing a work of art, as it involves the transmission of energy, it can damage the artwork. Nowadays, the most common methods for controlling light are based on the control of the overall lighting intensity. In particular, museum lighting guidelines recommend to limit the time of illumination or to remove wavelengths of light to which human eyes are insensitive. This research aims at determining how lighting systems can be standardized and developed, by studying the interaction between light and the materials constituting cultural heritage. An interesting case-study was considered as starting point: the arsenic sulphides photo-oxidation induced by visible light. In particular, among arsenic sulphides, realgar photo-degradation was studied, as this pigment was used since antiquity. Light transforms realgar (red mineral, As4S4) in arsenolite (white as powder, As2O3) and pararealgar (yellow, As4S4). The process is not completely clarified so far. This study deepens the comprehension of the realgar degradation process and determines the ability of different halogen lamps and a LED lamp, used in museum exhibitions, to induce photodegradation of pigments The study was carried out by means of FT-IR, micro-Raman spectroscopy, XRD and spectra colorimeter analysis.