Alright silver content, indeed bled from the surface with (probably) acetic acid. Both dies appear worn, the obverse especially so. The SALVS side has sustained scratches but still shows facial features and the snake is clear. Its been cleaned harshly and then allowed to tone over many years leaving it a pleasant dark colour.
ID:
Volusian, SALVS AVGG, Salus feeding a snake. Rome mint.

