Identification of pigments in some colours on miniatures from the medieval age and early renaissance

Particle-induced X-ray emission with an external proton beam has been used at the University of Florence to analyze the colours of miniatures on medieval manuscripts. Since the paint layers cause significant attenuation of the low-energy X-rays from elements, such as Na and Mg, which are important for identifying the pigments, a quantitative analysis is often needed for a correct identification of the recipe used in the preparation of the colour. Unfortunately, the colour layers are of unknown intermediate thickness. Consequently, the analysis involves an initial estimate of the thickness for each element, assuming that the target is thin, and then the application of an iterative correction for the estimated total thickness of the layer. This correction is computed from a comparison of the expected yield of X-rays from a target of finite thickness to that from a thin target. The atomic-number ratio for the observed elements, rather than the concentration or element thickness, is computed because it is from these ratios that the actual pigments used for the colours can be deduced.