Etching in silicon bronze

I have been etching designs in brass and bronze for sometime now.
Lately I attempted to etch in silicon bronze using RioGrande’s
copper and brass etching mordant. The asphultum resist lifted off,
radiating out from the edges of the open areas, leaving a residue
behind. I’m guessing that the silicon is the leftover residue. I’m
sure the metal was clean before applying the resist. My goal is to
achieve a relatively deeply etched design with sharp edges. Do you
have any suggestions?

Eric Joseph