Composition of the six coins analysed in this study according to SEM-EDX (wt%—weight percent, n = number of analysis points averaged, s.d. = standard deviation).

Note that because a value of 0 wt% is automatically returned for analyses below the practical detection limit in the range 0.0–0.5 wt%, such results were assigned a value of 0.25 wt% for the purposes of plotting and the calculation of averages.