Ancient Mercury-Based Plating Methods: Combined Use of Surface Analytical Techniques for the Study of Manufacturing Process and Degradation Phenomena

Abstract

Fire gilding and silvering are age-old mercury-based processes used to coat thesurface of less precious substrates with thin layers of gold or silver. In ancient times, these methods were used to produce and decorate different types of artefacts, such as jewels, statues, amulets, and commonly-used objects. Gilders performed these processes not only to decorate objects but also to simulate the appearance of gold or silver, sometimes fraudulently. From a technological point of view, the aim of these workmen over 2000 years ago was to make the precious metal coatings as thin and adherent as possible. This was in order to save expensive metals and to improve the resistance to the wear caused by continued use and circulation. Without knowledge about the chemical-physical processes, the ancient crafts-men systematically manipulated these metals to create functional and decorative artistic objects. The mercury-based methods were also fraudulently used in ancient times to produce objects such as jewels and coins that looked like they were made of silver or gold but actually had a less precious core. These coins were minted by counterfeiters but also by the official issuing authorities. The latter was probably because of a lack of precious metals, reflecting periods of severe economic conditions. In this Account, we discuss some representative cases of gold- and silver-coatedobjects, focusing on unique and valuable Roman and Dark Ages period works of art, such as the St. Ambrogio's altar (825 AD), and commonly used objects. We carried out the investigations using surface analytical methods, such as selected area X-ray photoelectron spectroscopy and scanning electron microscopy combined with energy-dispersive spectroscopy. We used these methods to investigate the surface and subsurface chemical features of these important examples of art and technology, interpreting some aspects of the manufacturing methods and of disclosing degradation agents and mechanisms. These findings may contribute to cultural heritage preservation, thus extending the applicability of the surface analytical techniques.

Techniques and Challenges for Characterizing Metal Thin Films with Applications in Photonics

“…Well before the age of dedicated scientific inquiry and technological innovation, metal-smiths of the ancient world made use of metal thin films to gild less-precious materials in thin layers of gold and silver [1]. Although these craftsmen were likely ignorant of the physicochemical processes involved in their plating procedures, their contributions to metal film deposition techniques have inspired centuries of thin film innovation for applications that extend far beyond their mere artistic origins.…”

Surface investigation of naturally corroded gilded copper-based objects

“…By using these amalgam-based methods different objects such as jewels, statues, works of art, weapons, armours, ornaments or commonly used objects such as coins and clasps could be coated not only to decorate them but also to simulate solid gold or silver objects, sometimes fraudulently [1]. The amalgam was prepared by mixing Au or Ag grains with mercury and to obtaining a paste with about roughly the consistency of butter.…”

“…A gGentle thermal treatment was then carried out at aboutpproximately 250-350 °C, below the boiling point of mercury (357 °C), to drive off remove a large fraction percentage of the original mercury. The thermal process leaves behind produced a well-bonded layer of gold or silver having with a variable amount of residual mercury, and whose thickness could ranges from a few tenths of micron to about one micron [1]. Because of its unevenness and porosity, the final finishing was carried out mechanically by means of a bone or agate burnisher.…”

“…Because of its unevenness and porosity, the final finishing was carried out mechanically by means of a bone or agate burnisher. This to delicately compressed and compacted the porous structure of the mercury-enriched Au or Ag layer, in order to createing a smooth, and shiny surface, similar to that of solid gold or silver [1]. The gilded metal-works artefacts that have come down to us from many years ago, have likely been then subjected for a long time likely to different degradation phenomena caused by the interaction with the surrounding context environment, either underground or in outdoor or indoor atmospheres, which have almost certainly changed sometimes buried or exposed to indoor or outdoor atmospheres and therefore probably changing their surface chemical composition and morphology [2][3][4][5].…”

“…This is due to the fact that because the combined action of different degradation phenomena could have caused relevant chemical and structural modifications over time. Furthermore, it is worth pointing out that despite many authors haveing described the corrosion processes occurring in archaeological bronzes, only very few works about regarding the patinas naturally grown on gilded objects have been published [1][2].…”