Argentium is not usually a suitable metal for enameling, although
there is a lot of experiementation going on right now with different
argentium alloys, and some people are succeeding.
The chemical composition of the enamels (which is glass combined
with various oxides to give the desired colors) reacts differently
to the chemical composition of various metals.
Generally, enameling is best done on copper, fine silver, pure gold,
sterling silver with the fine silver raised by repeated pickling,
certain alloys of gold, steel and glass (with enamels developed to
work with the co-efficient of expansion for the glass).
Probably the non-silver part of the argentium alloy is reacting
negatively with the enamel. You might try an undercoat of high fire
clear enamel, or an opaque white as a base to separate the blue
transparents from the argentium. I don’t know if that will work, but
it might provide a barrier against reaction from the enamels to the
argentium. You might try facing the argentium with a thin layer of
fine silver where you want to enamel.
I have never tried raising the fine silver in argentium, but you
might want to give that a try. With sterling, you need to heat with
a torch, pickle, rinse and repeat about 5 times or more. You will
see a thin layer like a shell of fine silver on the metal.
Some colors are generally more sensitive when enameling, but blues
tend to be stable.
VP, The Enamelist Society