How much silver?
Most Roman silver coins were produced from alloys of silver and copper. The Roman mints were able to disguise the copper content of even quite base coins using a process called depletion silvering, where the blanks for the coins were heat treated and then pickled in an acidic solution prior to striking. This process removed much of the copper from the surfaces of the blanks, leaving them with silver-rich surfaces.
This means that the surfaces of the coins are not representative of the alloys from which they were made. Furthermore, the surface enriched zone in fact extends well beneath the surface. The depth of this zone varies from one coin to another and even within an individual coin. The mints had no real interest in controlling the quality of the surface enriched zone other than to ensure that the blanks had a silvery surface, and so the finenesses of these zones is highly variable. It is impossible to obtain an accurate estimate of a coin’s silver content from an analysis of this silver-enriched zone.
However, the alloys used to produce the blanks were generally consistent, and we need to know about these alloys in order to determine the finenesses employed; the ‘recipes’ which the mints used. The only way to achieve this is to reach the ‘heart metal’ of a coin: the interior zone, beneath the surface enriched layer, which remains unchanged since the day the blank was made.