Visualization of vermilion degradation using pump-probe microscopy

Abstract

Here, we demonstrate the use of pump-probe microscopy for high-resolution studies of vermilion degradation. Vermilion (mostly -HgS), an important red pigment used in historical paintings, blackens over time, and metallic Hg and -HgS have been implicated as possible degradation products. Conventional analysis techniques have trouble differentiating - and -HgS with sufficiently high spatial resolution. However, pump-probe microscopy can differentiate metallic mercury, - and -HgS, and map each distribution on the microscopic scale. We studied artificial degradation of -HgS; femtosecond-pulsed laser irradiation induces an irreversible phase shift of - to -HgS, in which the initial presence of -HgS grains can increase the rate of conversion in their vicinity. Continuous ultraviolet exposure instead generates both liquid Hg and -HgS, with a conversion rate that increases with elevated temperatures. Last, we reveal the presence of -HgS as a natural degradation product in discolored vermilion layers in a 14th century Italian painting.