Compositional and microstructural study of joining methods in archaeological gold objects

Abstract

This work arises in response to the need to understand the production techniques of pre-roman gold jewels, starting from the systematic characterization of the different methods of metal joining used in antiquity to produce these objects.To meet this requirement, different procedures used in antiquity were reproduced by means of experimental archaeology. These models correspond to three metal-joining methods reported by ancient recipes: solid-state diffusion with copper salt, brazing and autogenous welding. These techniques were studied by different analytical methods in order to characterize them at both compositional and microstructural level (SEM-EDS, metallography, μ-XRF and μ-PIXE). The results obtained show that the solid-state diffusion bonding with copper salt presents a crystal hexagonal structure without phase changes in the joining point, where only a variation in the grain size is observed. At the compositional level, this soldering method is characterized by a local increase in copper concentration and a small decrease in gold content. In contrast, brazing shows a dendritic-porous structure due to the superposition of the soldering alloy and the crystal structure of the original foils. Besides, the compositional analysis revealed an increment of silver and copper concentrations and a simultaneous reduction of gold concentration. Finally, in the autogenous welding, no changes at the microstructural and compositional levels were observed.The results obtained have also demonstrated the usefulness of a portable μ-XRF system for the in-situ characterization of ancient jewels.

Archaeometric studies on early medieval silver jewellery from Central and Eastern Europe

“…SEM-EDX analysis, described in detail in Goldstein et al (2007), is a popular non-destructive technique used in studies of archaeological finds (e.g. Scrivano et al 2017a, b;Ontalba Salamanca et al 1998;Ashkenazi et al 2017Ashkenazi et al , 2018Linke and Schreiner 2000, Linke et al 2003Ingo et al 2004;Miśta et al 2017;Gójska and Miśta 2016). Twenty-six artefacts were tested altogether.…”

“…Published results of contemporary studies on ornaments made with the use of granulation and filigree techniques are mainly focused on gold artefacts (e.g. Scrivano et al 2013Scrivano et al , 2017aOntalba Salamanca et al 1998;Šmit et al 2000, Šmit andŠemrov 2006).…”

Technological characterization of gold jewellery from the Sogdian tomb of Shi Jun (d. 579 CE) in Xi’an, Shaanxi Province

“…The elemental analysis shows that the granules of the two roundels are composed of a similar percentage of Au and Ag to crescent and axle (Table 1) whilst a marked difference is notable at the joints of each roundel. The EDS analysis reflects that the content of Au (96.3-98.2 wt%) and Ag (1.8-1.9 wt%) is very similar to that of the surrounding beads (Au: 97.8-98.8 wt%, Ag: 1.2-1.3 wt%) of roundel-1, indicating that the tiny grains were bonded by autogenous welding 12 where no solder was used. And interestingly a considerably higher amount of Ag (8.3-11.2 wt%) and Cu (4.7-5.1 wt%) were tested at the joint (Cu and the high content of Ag in area 10 deriving from the solder during the bonding process, Fig.…”

“…The grains of roundel-1 were joined together by autogenous welding. Autogenous welding is a kind of liquidus phase bonding process, that is, the elements are welded together by heat alone, resulting in no composition change between the joining zone and the metals 12,15 . This technique creates narrow joints 16 as no solder was used, and an external force was executed to press the grains during the welding process for better bonding.…”

In-situ non-destructive analysis of Etruscan gold jewels with the micro-XRF transportable spectrometer from CNA

“…Indeed, when brazing was used, the joining areas show an increase in Ag and Cu contents; the solid state diffusion bonding is characterized by an increase of Cu content in the soldering area and fusion welding is identified by an increase of Au content (Demortier, 1996;Scrivano et al, 2013). However, a fourth method has been cataloged recently: autogenous welding, that does not show compositional changes and is only detectable by visual inspection (Ferro et al, 2008;Scrivano et al, 2013Scrivano et al, , 2017.…”