This paper deals with the microstructural and morphological characterization of some ancient bronze coins excavated in the Al-Fustat archaeological site. Al-Fustat is the first capital of Egypt in the Islamic civilization. It was built by Amr Ibn Al-As in the year 641 AD on the east bank of the Nile and replacing ancient Memphis to the west.
In the ancient Egypt, two main manufacturing methods have been generally used for coins’ production: casting into moulds or the hammering technique generally used to imprint an image on the coin surface. Different manufacturing techniques lead to different metallurgical features of the bronze coins, which together with the chemical composition and the physical properties of the soil in which the artefacts have been buried for long time, can significantly affect their conservation state.
Five coins which have dimensions in the range of 1.23 to 2 cm, and weight in the range of 1.27 to 4.02 g, have been characterised by means of optical microscopy (OM), X-ray diffraction (XRD) and field emission scanning electron microscopy equipped and energy dispersive X-ray spectroscopy (FESEM+EDS). The micro-chemical, micro-morphological and microstructural analyses have been performed to investigate the manufacturing technique and to study the corrosion mechanisms affecting the bronze coins, with the final aim of developing tailored conservation procedures for these important artefacts of the Egyptian history.
As an example the microstructure of a bronze coin etched in FeCl3 solution is shown in fig. 1. The metallographic observations indicate the absence of a texture in the polycrystalline alloy and the presence of equi-axed grains and twinned. These results allow concluding that the bronze coins have been produced by hammering in mould.
The FESEM observations of the coin cross-sections put in evidence that these artefacts are coated by quite thick corrosion products layers (300-500 μm) with a complex stratified microstructure composed by Cu oxides and oxy-chlorides. Furthermore, in some cases, the presence of copper chloride corrosion products, directly in contact with the metallic core, which have a detrimental effect on the long-time preservation of the artefact, has been detected.