Yes you can enamel on a backed peircework piece of sterling with
a black enamel. Most enamel colors melt between 1450 and 1600F
so use hard solder or IT to join the work. Also carefully scrape
away any solder which creeps out onto the backing, if solder is
left behind it is likely to blow pinholes into the enamel. If
you are giong to work in sterling, get a opaque black, not one of
the japanese semi transparents, as sterling can cause transparent
enamels to cloud over and look putty-like. Moisten your enamels
with some distilled water and pack into the cells wet until the
enamel is level with the top of the cell, then carefully wick the
water away with a paper towel. It is important to use distilled
water as tap water can leave an enamel looking scummy. Be
careful not to get any enamel outside of the wells or you will
have to grind the glass off and then go back and polish your
silver. Do not fire the piece until the water has completely
dried. At this point you can either fire the peice in a kiln
for about 2 minutes at 1500F or use a torch to melt the enamel.
I personally have not used the torch method but have read about
it in some of my enameling books.
If you need a source for mail order enamels, you can go through
Thompsons: 650 Colfax Ave, Belleview,KY 41073 USA or Enamelworks
Supply Co. 1022 N.E. 68th St.,Seattle, WA 98115 USA
(206)525-9271. Both companies will let you order a sample, and
of the two, Enamelworks is the most prompt.
Good luck, I think that once you try enameling you may become
addicted, I iknow I did!