Summary of results
Results show that all objects display the same type of corrosion (Figure 7). The original surface is located between the inner layer and the outer layer. This localization of the original surface has already been reported by MacLeod and North [3]. They also observed that cleavage between these layers was good, but failed to explain this phenomenon. This is in fact due to the variation in porosity between the layers. The outer layer is porous and is mainly composed of silver chlorides (AgCl) together with sediments (soil markers). The inner layer is denser and incorporates inclusions such as Cu. It generally consists of a silver chloride (AgCl) mixture and other phases containing oxygen. These oxygen-containing phases were not clearly identified as they are always associated with other elements (Ca, Si or Cu). Calcite (CaCO3) was detected in the dark phase of sample S2. Silicates and copper oxides may also be found, although the presence of silver oxides has never been proved. In sample S8, the inner layer is composed of silver chlorides only. This variation between S8 and the other samples can be explained by the difference in burial environments: objects S2, S3, S4, S5, S6 and S7 come from Saint Martin’s Church, whereas object S8 comes from the Gallo-Roman site of Rezé. MacLeod and North [3] have already referred to the connection between the formation of the corrosion products mixture (AgCl and Ag2S) and variations in the environment (anaerobic/aerobic). In our case, we did not detect Ag2S but a mixture of AgCl and another, difficult to identify, oxygen-rich phase. A better knowledge of this phase is essential to confirm the absence of silver oxides. Since AgCl formation requires the absence of silver oxides [23], it would seem unlikely that this dark phase contains silver oxides. This phase becomes more important when the object is completely corroded. Figure 6b shows a schematic diagram of corrosion with a metal core, but it can also be extended to completely corroded objects: in some objects (S2, S3 and S4), the inner layer has developed to the extent that the metal core has completely disappeared. Object S3 presents both profiles: the first sample taken from the object is completely mineralized (Figure 5a); whereas in another sample, the object still displays a metallic core (Figure 5b). When the metallic core is completely mineralized, the outer layer is much thicker: in Figure 5a, the outer layer measures about 250 μm; on Figure 5b, the outer layer is about 30-50μμm. We also observe that the oxygen-rich dark phase (with Ca, Si and Cu) is detected only in the mineralized core. Copper comes from inclusions in the metal and copper oxides are formed by galvanic coupling. Si and Ca elements were not detected in the metallic core of sample S3 in contrast to the mineralized core. This difference shows that the origin of these elements is not due to the manufacturing process but to the burial environment.
Proposal for the corrosion mechanism
The formation of AgCl has been investigated by many scientific papers, but in other media and under different conditions. The formation of AgCl on silver electrodes has been specifically studied in chloride media [24–26]. Formation of the first layer follows an adsorption-desorption mechanism, then a thin film of AgCl grows due to diffusion. As the AgCl film is discontinuous, ion transport takes place via spaces between grains of AgCl. As the film thickens, the spaces between the AgCl grains close up and the ion transport occurs mainly via micro-channels running through the AgCl grains [24]. In these studies [24–26], the thickness of the AgCl layer is of the order of tens of μm, and Ag+ ions are produced by the anodic potential. The formation of AgCl on archaeological silver objects occurs under different conditions. Accelerated corrosion tests were conducted to study the corrosion film observed on archaeological objects [14, 23], but these standard test samples also had a fine layer of corrosion products. Removing the protective oxide film is necessary to support the formation of silver chloride. In our analysis, we did not identify the presence of Ag2O. Jaro M. [27] suggested that in burial conditions (e.g. in crypts or graves), ammonia dissolves silver oxides, and silver ions react to form silver chloride, chlorargyrite (AgCl). Organic decomposition products (ammonia, nitrates, acetic acid, H2S) favour the initial corrosion process [6]. Silver chloride does not create a protective layer, thus the metal can be completely transformed into silver chloride.
Horn silver has bilayer films that correspond to the corrosion process as identified by Payer et al. [28]. A generalized model for this corrosion system is presented for metals in chloride solutions. Corrosion products form 2 layers: a porous outer and a compact inner layer. According to Payer’s model, corrosion is initiated by the protective layer becoming non-protective. In the case of horn silver we examine, it is thought that ammonia may have destroyed the silver oxides in the burial environment [27]. This new non-protective layer then becomes an inner layer. A thicker outer layer then develops through the precipitation of soluble species (e.g. sediments) and further reaction of corrosion products (AgCl). The porous outer layer can permit the penetration of water and the diffusion of Cl. Cations are produced by the oxidation of metal at the metal/inner layer interface. High ion transport in the inner layer allows for the growth of corrosion films: Ag+ cations diffuse to the outer layer, and Cl- anions diffuse to the inner layer.
Copper inclusions do not seem to modify the corrosion mechanisms of horn silver. When copper is an alloying element (Sample S5), copper corrosion products are found in the corrosion layers. Otherwise copper inclusions are found only in the inner layer (and in the dark phase). Copper may play a local protective role through galvanic coupling. However, pure silver samples, such as S8, or samples with 2-5at% copper are subject to the same corrosion system: a compact inner layer and a porous outer layer of AgCl.