Margery, I have been using Superior 6 flux from H & N Electronics.
Jeffrey Herman, Society of American Silversmiths, suggested this
fluoride free flux for working in sterling hollowware. I love the
stuff. It was a bit difficult to read in the beginning, but just
like soldering, you learn to look for. It makes a very nice skin
around the piece, allowing for repetitive heatings without breaking
down. I think Frei and Borel sells it also, but this could be the
Gesswein product too. I have used Prips on forged pieces, and found
that is also a very good flux. However, the repeated applications
to create a solid buildup is time consuming. For the little
firescale that I get with the Sup 6 flux, a good 3M fiber wheel will
remove it instantly.