Identification of lapis-lazuli pigments in paint layers by PIGE measurements

Lapis-lazuli is a semi-precious stone used in the past to produce a blue pigment. Its main component is lazurite, 3Na2O!3Al2O3!6SiO2!2Na2S. The possibility of using PIXE to identify this pigment in canvas and wood painting is severely limited by the strong absorption of low-energy X-rays in the protective varnish and – when, as typical, the pigment is mixed with lead white – by the overlapping of Pb M lines with S Ka. In this work we discuss the possibility of identifying lapis-lazuli by detecting sodium with PIGE. PIXE and PIGE measurements have been performed on samples containing lapis-lazuli mixed to lead white in different percentage, covered with polymeric foils to simulate the presence of varnish. At a percentage of lapis-lazuli below 50%, Na X-rays are hardly detectable even with the thinner foil; on the contrary the characteristic gamma-rays are clearly detected down to about 1%. A first application has been successfully performed on the ‘‘Madonna dei fusi’’ by Leonardo da Vinci, in the framework of an extensive scientific investigation on the painting techniques used by the Renaissance genius.