De Colorando Auro: Experiments and literature study of medieval colouring recipes on gilded plates

In the restoration-conservation practice of medieval art masterpieces, such as the Holy Lady Shrine from Huy, knowledge about the techniques used to create and shape them is of major importance, especially when they concern the surface finishing treatments. This influences the selection of methods employed for the conservation-restoration process. This paper discusses the method and the materials that might have been employed to modify the colour of gilding on silver. Recipes for chemically modifying the colour of the gilded surface can be found in medieval texts and might have been used on the masterpiece discussed in this paper. This article gives an overview of technical instructions related to this topic and presents a method to interpret their contents in order to be able to implement them. It reports observations carried out during the implementation tests, such as information about the temperature, timing, and other important elements. It concludes with the visual and the colorimetric results on the samples after application of the tested recipes.