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Silver porosity

I’m using fine silver to cast my originals because they seem to make
better rubber molds than bronze or even sterling silver. I’m using
an electric furnace/kiln for the burnout and a Kerr Micro-melt for
melting the metal.

The problem I’m having is that there seems to be porosity in the
fine silver castings. They don’t appear to be from shrinkage but
rather they are individual pits in random areas. If I cast the same
way, using the same spruing format but cast in bronze the pieces
come out perfect. If I try to form the cast silver piece, the
surface designs deform quickly, leading me to believe that there are
unseen pits below the surface. And of course when I rubber mold
there are rubber “warts” inside the mold.

Is the problem gasses trying to escape the mold. I am re-using the
silver but it is clean. I have noticed that once there was a bubble
that formed atop the sprue pit. Can I get a de-glassing flux? Or
is the problem something else?

Anyone faced this problem? Is there another metal I could use to
make good clean vulcanized rubber molds? I don’t want to plate my

Thanks for the help