The role of the under-layer in the coloured perception of gildings in mediaeval mural paintings

A study of the gildings used in mural paintings from the 12th to 16th century showed rather limited types of application techniques (essentially mixtion techniques, but also in rare examples distemper techniques) and a higher variety of employed metals. Samples have been taken off from mural paintings in the southwest of France as in the Bordeaux Cathedral, Meyrals Castle, Ste-Marie Church of Audignon. Samples analyses have been carried out by scanning electron microscopy coupled with an energy dispersive X-ray analysis system, Raman spectroscopy and Fourier transform infrared spectroscopy. Depending on the metal used for the gilding, the under-layer could be different in terms of material and colour. Under the gold leaves, the layer is usually red or yellow. That can be explained by the absorption properties of light by the gold leaf. Under tin leaves, which are thicker and opaque, the usefulness of the under-layer is less obvious. An experimental study illustrates the role of the coloured under-layer.