Indeed. But I would appear 'haggish' if I posted any of my 'moderns' here. Carry on gentlemen........
I believe around 35-40% silver. Under Decius and Gallus the coins were debased further, and again by Valerian and Gallienus, who struck silver-appearing coins early in their reigns, but eventually struck just AE antoniniani with the silver wash. Things didn't improve until the monetary reforms of Diocletian when the fine silver argenteus was introduced, but this was only issued for a few years.
Nice OP coin, and others. I will stick to Felicitas with two more Philippus I, Antoninianus Rome mint, AD 247 IMP M IVL PHILIPPVS AVG, radiate, draped and cuirassed bust right P M TR P IIII COS II P P , Felicitas standing left, holding caduceus and cornucopiae 5.16 gr 20/23 mm Ref : RIC IV, part 3 #3 Philippus I, Sestertius Rome mint 248 AD IMP M IVL PHILIPPVS AVG, Draped and laureate bust of Philippus right P M TRP V COS III PP, Felicitas standing left, holding cornucopiae and caduceus 18.71 gr Ref : Cohen #149, RCV #9008 Q
Whatever the percentages, it is usual to see Philip in good looking silver, frequent in Decius, occasional in Gallus and rare after the first years of Valerian/Gallienus joint reign. Some coins benefited from surface enrichment but there is a point in the alloy where the silver fights to look gray and loses. Postumus issued a few coins that looked decent in the ~20% range but it was straight downhill after that. Our terms for alloys of precious metals allows some fudging. Is 10k gold really gold with a lot of alloy or alloy with a bit of gold? If we applied the rules required in food labels, 10/24ths would not make gold the first ingredient but we are willing to accept 10/24 silver as 'decent'. I consider Decius the end of 'decent' but everyone has an opinion.