Hardenable silver is not popular with enamellists in the UK and I
don't believe that its still manufactured here. I have just completed
my 4 year City & Guilds enamelling certification training at John
Cass and naturally have mixed with more enamellers than one would
normally have the opportunity to meet. They all use good quality
sterling which makes an excellent enamelling surface provided there
is no surface oxide or firescale present. The only other silver alloy
I use is Britannia which is 95.8% silver with the rest in copper -it
gives a brighter colours than sterling but is softer.
Most new sterling comes wrapped in plastic and requires minimal
preparation, perhaps a quick polish with fine emery paper or as in my
case champleve engraving to reveal a fresh surface.
If you have heated the sterling silver prior to applying the enamel
you will need to dip the piece in acid to remove the oxide. Oxide
does not prevent the adhesion of the enamel but irregular patterns of
firescale can be seen under the transparents. If I need to clean
oxide from the silver I usually dip it in concentrated nitric acid
-just a few minutes does the job followed by a wash in clean water
but you will probably need to repeat this several times until it
comes out of the acid an off-white colour. If its very oxidised you
can dip the piece in hot water first before the acid dip. Take care
with concentrated nitric.