I think the study shows the ancient mints were aware of the copper content, and actively manipulated this to improve the ‘gold’ appearance. As the authors note, there is a pattern to the copper . Relatively high silver coins won’t always have high copper; but where there is high copper, there is high silver.
I went through the tables of all 209 coins in the study and made some calculations.
Of the top 10 lowest copper content coins,
average gold content is 63.92% and
average silver content is 34.53%.
Compare that to the top 10 highest copper content coins, where
average gold content is just 43.43% and
average silver is 46.75%. 8 out of these 10 have more silver than gold, compared to none of the 10 in the lowest copper coins.
The copper effect is well illustrated by Kyzikos staters 95 and 96 in the study. Both have 61% gold, but 95 has 34.6% silver and 2.96% copper compared to 96 which has 37.47% silver and just .19% copper. On XRF analysis 96 is the more precious metal rich coin (and .05 grams heavier), but based on the photos anyway I’d bet the
average ancient grocery shopper would prefer 95: