Nondestructive analysis of Portuguese 'dinheiros' using XRF: overcoming patina constraints

摘要

"Dinheiros" are the first Portuguese coins, minted with a billon alloy (majority-based copper alloyed with silver). In this work, a set of "dinheiros" from D. Fernando of Portugal was analyzed and the composition of the alloy was compared with other "dinheiros" from previous reigns. Although the coins were in good state of conservation and no active corrosion was macroscopically observable, they still presented a corrosion layer of unknown thickness that would impair the XRF quantitative determinations. In order to overcome this hindrance, the silver K/L intensity ratios were determined and compared for the analyzed samples in order to choose "clean" spots for quantitative analysis. The results show a clear decrease in the Ag content: from 7-9 % in the previous reigns to 0.2-0.3 % in the coins attributed to D. Fernando. The silver content determined is very comparable to the silver content determined in other copper or bronze artifacts analyzed, leading us to believe that this low amount of silver was not - intentionally introduced to create a billon alloy but relates to impurities present in the original mineral specimen.