Colorando Auro: third century colouring surface treatment of fire gildings

Abstract

A few decades ago, heritage scientists and art historians started to study the ancient surface colouring techniques of metal art pieces. Colouring of copper and silver has already been broadly studied but no works on gold have been reported yet. This knowledge can be very important for art and technology history, conservation-restoration and surface sciences. This paper contributes to the investigation of the oldest recipe known so far to colour fire gildings. The chosen model samples, the mixture and the application of the recipe have been summarised by referring to the ancient writings known as the Arab and Syrian Alchemist Collection of M. Berthelot. The main aim of this research is to study the colour change obtained by applying the above mentioned recipe, and to identify the processes involved during this colour change. Digital imaging and scanning electron microscopy in combination with colorimetric measurements and elemental analysis have been performed to characterise the colour change and to identify its origins.

“De Colorando Auro” medieval colour surface treatment of silver gilded statuettes decorating the Holy Lady Shrine of Huy (13th century, Belgium)

“…10 We also presented the impact of the colouring treatment on the morphology of the model sample surface. 11 High resolution Field Emission Scanning Electron Microscopy (FE-SEM) images show that the application of the colouring recipes tested on the model samples results in inhomogeneous chemical etching of the gilded surface.…”

“…In the presence of a chlorinated salt spray: (i) iron K-edges are more homogeneous and seem to indicate the presence of FeCl 3 and FeOOH with variable quantities of Fe 2 O 3 , (ii) copper K-edges are homogeneous and seem to be the same as the Blank signal from the model samples although presenting different copper K-edges when freshly coloured. 10 The aim of this paper is to use the knowledge acquired from previous parts of the research, on historical sources and model samples, 1,5,6,[9][10][11][12] for the study of the historical shrine. This article brings back the research from the theoretical and model sample study to the real historical object, the Holy Lady Shrine of Huy.…”