<i>Colorando Auro</i>: third century colouring surface treatment of fire gildings

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.