An investigation of “faded” metalpoint drawings by Leonardo da Vinci in the Royal Collection

Abstract

A number of metalpoint drawings by Leonardo da Vinci in the Royal Collection appear to have become partially or wholly invisible over time, but can be rendered visible under ultraviolet illumination. A selection of these drawings, including both well-preserved and ‘faded’ examples, have been investigated under the microscope, in raking and transmitted light and using infrared reflectography, X-ray fluorescence (XRF) and Raman spectroscopy in an attempt to account for the disappearance of the metalpoint lines and to learn more about Leonardo’s metalpoint technique. Examination of the surface of the drawings showed that very little material is present in the ‘invisible’ lines, seeming to indicate a physical loss of material has occurred rather than a chemical alteration. It is not certain why such loss has occurred but the evidence suggests that a stylus of different composition may have been used for those sketches which are now no longer visible. XRF analysis found silver, copper and mercury were present in the visible lines, indicating silverpoint; while copper, usually with zinc and mercury, but not silver, was present in the ‘invisible’ lines. However difficulties in detecting silver by XRF means that the use of silverpoint here too cannot be ruled out. The pigments used to colour the grounds on seven drawings were also identified using Raman spectroscopy and bone white, indigo, hematite, red lead and vermilion were found.