your completly correct Jin (as always)!
A nice way of using a solution with Borax and water is to mix borax
with hot water, shake well and let it cool of. The obundant borax will
cristalize on the bottem of the jar. Poor the liquid solution of and
you've a perfect inexpensive solution against firescale.
After watching many jewellers and novices, I've seen that people are
using a way to much of flux! Some of them are using it in the way to
learn a metal swimm in flux I quess. The proper way of using a flux
is to preheat your metal first then aply flux (with a brush) where
the flux immediatly evaporates. A thin white layer of powdered flux
should be visable and that's it. No more no less.
I still experience that many goldsmiths are pooring there flux on
their workpiece, heat the metal "cooking" their flux and then place
their solder on top of it. Some of them like to see their solder dance
by laying their solder on top of the wet flux. One whould be amazed
how often this happends! Other once apply a tick layer of powdered
boric acid (about 2mm and I'm serious know!).
By all means, I'm not correcting anyone or shooting arrows but there
is a better way of using flux and this way can be one of them saving
you a few bottles of flux a year. Feel free to perform you
solderoperation the way it works for you.
Enjoy and have fun,