But virtually no 'silver' coinage of the third century was really silver. Both denarii and antoniniani were at or below 50% from the very beginning.
This chart has done the rounds on the internet for a couple years now, and is based on lab testing actual coins. Between 193 and 235, the emperors steadily debased the
denarius from a
standard of 78.5% to 50%
fine or less from 241 onward.
So in essence, for at least 1/3rd of the third century, the coins were over half silver and generally are not thought of as
billon. From 241 until about 274, many coins
still test above 50%, and it appears the official "target" for the mints was around 48%, though from about midway through the
Gallienus' reign, you can see this
standard trailing off quickly from year to year. Until you start seeing coins with obvious copper content under
gallienus, many numismatists
still refer to these coins as silver, not
billon - though there seems to be a lack of real consensus on where silver ends and
billon begins. These coins in the roughly half-silver range
still visually preset as silver and tarnish like silver. In some cases, you see pebbly surfaces where copper corroded out of the
fabric matrix - this is called selective phase corrosion in chemistry circles.