I depends on what flux you mean. Here in UK I use Auflux for precious
metals and Johnson Matthey EasyFlo (or occasionally Tenacity) powder
for other metals. The powder fluxes work OK with precious metals, but
its a bind mixing it up and it tends to bubble up rather more than
the Auflux. Since the Auflux is a slightly luminous green liquid, if
a little dries out I just discard it, but I assume you're actually
talking about the J&M powders - probably EasyFlo. I've found that if
it dries out and goes hard I can just crush it with the handle of a
spoon, or a mortar and pestle, and remix it with a little water.
Despite what I've read, it seems to work just fine. After all, the
water doesn't actually do anything except make a convenient medium
for placing the powder on the joint. The water gets boiled away long
before the flux melts and is activated. The powder must melt in order
to form the protective film that keeps the oxygen away from the
I use the EasyFlo only occasionally, so it always dries out like a
little block of plaster before its needed again. I just crush it as
above, and remix it.
By the way, don't forget to add a drop or two (no more) of
washing-up liquid when you add the water - it helps to degrease and
wet the joint nicely.
Regards, Gary Wooding