Studying the elemental components of archeological coins can help archeologists shed light on their origin by identification of the ore deposit from which they were derived. This is because the compositions of ores in a particular area are often unique. The analysis of coins can also help determine the trade and political relationships of a region during the period.
In addition, a coin buried over a length of time may form a coating or patina or be corroded as a result of its interactions with the environment. A study of coin patina or corroded layer can also help archeologists identify in which types of environments the coins were buried – for example low humidity, acidic or basic soil conditions or rich subterranean water. This information can also be used by museums to understand how to better preserve such archeological finds.
X-ray fluorescence (XRF) analysis is a nondestructive, straightforward and fast method to determine the elemental composition of ancient coins. However, the features in coins may require analysis with a much smaller spot size in addition to traditional bulk XRF techniques. This study shows the capability of Zetium to map the elemental distribution on a selected area of a coin.
Instrumentation and sample preparation
The measurements were performed using a Zetium XRF spectrometer equipped with a 4 kW, Rh-anode SST R-mAX tube, a high-performance ED core, and state-of-the-art SuperQ software.
In this study, a 1768 copper coin (duit) from Overijssel (today a province of the Netherlands) was analyzed. After being buried for a long time, the coin was excavated around 2004 from an area close to the IJssel river. The coin was found as a small lump of material (sand + clay) with some organic remains and cleaned with a soft brush and water. Figure 1 shows the image of the archeological coin. A patina appeared to have formed due to contact with the environment. In the picture one can see that the material around the lion is partially altered by corrosion. The yellowish coloration is probably caused by this corrosion and was therefore, together with the darker and lighter brown area around it, selected for measurement. The lighter part represents the original metal of the coin.
The coin was mounted on a special small spot sample holder and imaged using a high-resolution camera. A 10 mm x 3.75 mm measurement area was defined in the instrument SuperQ software based on the captured image. The measurement spot size used was 500 µm with a step size of 250 µm resulting in a total of 600 spots measured. This was made possible by using a high-precision turret mechanics for sample positioning between different spots. The total measurement time for mapping the sample was about 16 hours.
Figure 1. Images showing the front (a) and reverse (b) of the coin. The diameter of the coin is 19 mm.
Qualitative intensity analysis was performed for the elemental distribution mapping of the coin. Figure 2 shows the sample and the sample area analyzed during the measurements. The selected sample area contains both the patina and the original metal of the coin. Table 1 shows the measurement conditions used in the analysis of the sample.
Figure 2. Measurement area defined on the coin
Table 1. Conditions for the qualitative analysis
Results and discussion
A total of 15 elements were simultaneously identified with just a short measurement time of 80 seconds per spot without the need for calibration setup. 2D intensity maps are shown in Figure 3 with 3D contour maps shown in Figure 4 for three of the elements. Results show that elements found as majors in the soil (Al, Si, K and Fe) have high concentrations in the area surrounding the yellowish band. At the same time one can see that the elements typical for the Cu alloy (Cu, Zn and Ni) having high concentrations in the yellowish band.
This indicates that the general brownish patina is formed by a coating from majors from the soil where the sample was exposed. The yellowish areas indicate whether the coating was never formed, or removed by oxidation, resulting in the actual alloy being exposed. Another clear observation is the presence of heavy elements such as As, Ag, Sb and Pb in the original metal, which gives clues to either where the ore deposit was mined or how it was processed.
Figure 3. 2D intensity mapping of the elements composing the coin
Figure 4. 3D intensity contour maps of Cu, Ni, and Fe
This study shows that the analyzed coin clearly exhibits a patina (the outer layer) enriched with Fe and a core enriched with a Cu-Ni alloy. This clearly indicates an interaction between the host environment and the coin. The presence of heavy elements and their elemental distribution can help archeologists better understand the origin of the metal used to manufacture the coin and the events it has undergone. This easy, rapid, and nondestructive approach using a Zetium equipped with an ED core and small spot mapping functionality is an excellent solution in archeological investigations such as the delicate coin in this example.