De Colorando Auro: Medieval colouring techniques researched using modern analytical techniques

The visual appearance of gold and gilding can be influenced in many ways, such as by changing the composition of the gold alloy or the nature of the gilded substrate. A less known medieval technique, reported in historical treatises, is the chemical treatment of the gilded surface itself, after application and burnishing of the gilding. We reporte here results regarding the study of the Holy Lady Shrine of Huy (13th c AD Mosan, Belgium) on which a possible artificial colouration of the gildings was detected. This led to many questions regarding applicable conservation-restoration treatments. Here is an overview of the results obtained, along with a discussion of the methodology that was developed to study this colouration process, also highlighting the necessary complementarity between laboratory and synchrotron-based analytical approaches. The 3-steps methodology proposed in this paper is generic for most cultural heritage problems where the application of ancient surface modification recipes is suspected but where the scarcity of the historical samples does not allow a direct study of such samples.