BI is an abbreviation for billon, a silver-copper alloy containing less than 50 percent silver.
DICTIONARY OF ROMAN COINS
Please add updates or make corrections to the NumisWiki text version as appropriate.Billon. This term is applied to coins of silver mixed with much copper alloy, or to copper with a small alloy of silver. From the reign of Gallienus to that of Claudius II
(ad 253 to 270), almost all the coins were made of billon. Some of them
were first struck on copper alone, and afterwards covered with a thin
silver coating; and in that case they are called saucees or washed coins; others have a leaf of silver struck upon the copper, and these are fourrees or plated coins.
On this subject M. Hennin makes the following remarks: From and after the reign of Claudius II, coinages of billon are no longer found. The standard
of silver having been successively lowered, the money, which replaced
that of this metal, proves under the above mentioned emperor, to be of silvered
copper. In almost all such pieces, the effects of friction, and of
time, have removed this covering, which appears only on those coins in
the best state of preservation. The coins of Claudius II, and of the reigns, as far as Diocletian,
which have been published as of billon, are but pieces of washed
copper. Those of the same reigns described as being of silver are false.
Manuel - Nomenclature, ii 440. See the word Potin.