La trasformazione del realgar come sistema per lo studio della migliore illuminazione applicata ai beni culturali

In a museum all objects are subjected to decay caused by many contributing factors: temperature, humidity, pollutants. Among these environmental risks, light remains unique because it can neither be eliminated nor completely controlled. Light is necessary for viewing a work of art, but, as it involves the transmission of energy, it can damage the artwork. Nowadays, the most common methods for controlling light are based on the spectrum of the sources and 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. The most important parameters are illuminance and time of exposure. According to CIE 157:2004, cultural heritage objects are classified according to their sensibility to light exposure in no sensibility, low sensibility, medium sensibility and high sensibility, but no guidelines directly consider the photo-chemical interaction between artwork and light. This survey aims at determining how lighting systems can be standardized and developed, by studying the interaction between light and the materials constituting cultural property. To develop this research, 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 if powered, As2O3) and pararealgar (yellow, As4S4), a compound with the same formula of realgar but characterized by a different disposition of sulphur and arsenic atoms. The process isn't still completely clarified. This study deepens realgar degradation and determines the capacity of inducing degradation by different halogen lamps and a LED lamp, used in museum exhibitions. The survey was carried out by means of FT-IR, micro-RAMAN spectroscopy, SEM/EDX, XRD, XPS and spectra colorimeter analysis. Because vision is generated by the brain interactions with energy and matter, the capacity of eyes to evidence the realgar transformation has been studied. This aspect allows to know the best lighting source in terms of lighting reproduction and quality. The results partly confirm literature studies about the realgar degradation induced by light, but also suggest that some mechanisms are still not completely clarified. The LED lamp produces a less degradation, keeping a high quality of lighting.