Toward a Green and Sustainable Silver Conservation: Development and Validation of Chitosan-Based Protective Coatings

When exposed to air, silver artifacts undergo an unpleasant darkening and shiny loss, commonly known as tarnishing. At the present, the development of protective coatings by using eco-friendly and biocompatible materials, able to ensure high transparency and to hinder the degradation of silver objects, remains a huge challenge. In this study, chitosan was used for the first time to realize sustainable coatings for silver protection. Both pure and benzotriazole-containing chitosan coatings were prepared and applied on sterling silver disks. A commercial product based on acrylic resin was used as a reference. The aesthetic features and protective properties of these coatings were evaluated by performing two different types of aging treatments. In particular, the assessment of the protective efficacy was carried out by reproducing both highly aggressive polluted environments and real-like museums' storage conditions. In the first case, chitosan-based coatings with benzotriazole performed better, whereas in storage conditions all the chitosan films showed comparable efficacy. Compositional, morphological and structural analyses were used to evaluate the protective properties of the coatings and to detect any physical or chemical modifications after the aging treatments. Our findings reveal that the two different testing methods provide complementary information. Moreover, chitosan coatings can achieve protective efficacy comparable with that of the commercial product but using non-toxic solvents and a renewable biopolymer. Chitosan coatings, designed for cultural heritage conservation, are thus promising for the protection of common sterling silver objects.

Physicochemical approaches to gold and silver work, an overview: Searching for technologies, tracing routes, attempting to preserve

Gold alloys and silver alloys have always been widely employed in the production of significant objects. With high reflectivity, precious metals are perceived as both materials and colours, and can be skilfully combined to produce metallic polychrome effects. Because their structure and composition contain information on their manufacture, use, disclaim and degradation, items in gold and in silver enclose major information on the technologies employed by past societies and on exchange networks. This information can be acquired using appropriate analytical protocols, established according to the nature of the query and the characteristics of the objects.

By using physicochemical techniques, it is possible to identify the technologies, materials and tools used by the artisan and, in particular cases, to situate the sources of raw materials and the workshops producing the objects, as well as to follow the trade routes. The aim of this work is to outline major achievements in the study of goldwork and silverwork based on the different physicochemical methods that are available, and to refer the analytical difficulties that have to be faced when studying objects made from precious metals. Based on several examples, three topics are addressed. The first concerns the major role of the techniques of exam when describing shaping, decorating, assembling and finishing; the second considers the search for metallic polychrome effects in some cultural areas; and the third discusses the challenging question of fingerprinting. A fourth section is dedicated to a short reflection on the difficulties related to the identification of the atmospheric corrosion mechanisms of precious metals.

Metallurgical investigation on fourth century BCE silver jewellery of two hoards from Samaria

A fourth century BCE silver jewellery collection, which is part of two hoards of Samarian coins (the Samaria and Nablus Hoards), was studied by non-destructive analyses. The collection, which consists of pendants, rings, beads and earrings, had been examined by visual testing, multi-focal microscopy and SEM-EDS analysis. In order to enhance our knowledge of past technologies of silver jewellery production, we developed a metallurgical methodology based on the chemical composition of the joints and bulk. The results show that all artefacts are made of silver containing a small percentage of copper. Higher copper concentrations were measured in the joining regions. Our research indicates that the manufacturing of the jewellery from both hoards involved similar techniques, including casting, cutting, hammering, bending, granulating and joining methods, indicating that the artefacts were made by trained silversmiths. Although the burial date of the Samaria Hoard – 352 BCE – is some 21 years earlier than that of the Nablus Hoard – circa 331 BCE, a noted continuity in the local production technology is apparent in the analysed items. This information provides better understanding of the technological abilities in the late Persian-period province of Samaria and bears implications on the local silver coins produced in the region.