If the black flux you are referring to is what I think it is, it is
like one sold by Handy Flux as “Type B-1”, a black paste flux designed
for soldering stainless steel and nickel silver (and I would assume,
nickel also). It probably wouldn’t be my first choice for gold or
silver work. I prefer that radioactive green colored liquid stuff
called “battern’s flux”. Both these fluxes contain fluorides, which
have been associated with health risks and require adequate
ventilation. Before I’d rush out and get the black flux, I’d call the
tech reps at Rio Grande. If you don’t have an account with them, go
to their web site and get their phone number and set one up. They
have an extensive selection of tools, findings, packaging, etc. And.
they have a broad selection of fluxes for many different applications.
Handy Flux (white) is great for silver (but contains fluorides).
Grifflux #1 flux is a past flux without fluorides, as is Solder Fast
liquid flux (for high temperature gold and silver work).
David L. Huffman