G’day; I wrote something on these lines a few years ago in Orchid,
but here we go again. again; Question: why is borax a flux? What
actually does a flux do anyway? (same question)
A flux is a substance that cleans the surfaces of metals which are
to be heated, and as a consequence of the heat, will combine with
oxygen, sulphur, and other materials present in the atmosphere or
metal. thus keeping the surface fresh and clean. Fluxes do this
because they have a special affinity for metallic oxides and other
compounds. Borax has such an affinity for metal oxides, etc, and
converts them into acid soluble borates which are iin fact glasses
and can be washed off heated work and dissolved in acids such as
sodium bisulphate, citric acid, 10% sulphuric acid and even lemon
juice. after heating is finished. Borax is not the only material used
as a flux. Plumbers used to use tallow when jointing lead for
pipes and tanks. Electronics technicians use a special resin as a
flux for tin/lead solders, as any acid left after soldering could
cause electrical faults. Borax used in some higher temperature work
with uncompromising metals, often contain ammonium or sodium
fluorides, which help with the dissolving of metal oxides. Brazing,
using brass alloy rods with some bronzes, iron, nickel, and so on
work better with the presence of fluorides in the flux, and even
silver and gold join more easily with fluoride fluxes. But
fluorides are poisonous, and care must be taken not to inhale the
fumes. But don’t go overboard about fluorides; just be careful,
but remember that your natural teeth are coated with a fluoride
complex. (Heat a tooth with sulphuric acid and the resultant vapour
will etch glass)
Now, given all this, what might one expect to happen if gemstones
were to be coated with boric acid or borax and heated to red heat?
As you now know that boron compounds have an affinity for hot metal
oxides and other metal compounds, you should expect it to etch
gemstones like ruby, sapphire, tourmalines, agates, and so on… I
am well aware that some books reccommend this, but now you know why
people like me say you should be careful to keep fluxes away from
precious and semi precious stones.
In other times before instrumental chemistry became possible. one
had to get the unknown chemical into water solution to enable
subsequent analysis, and this was accomplished by heating the
unknown strongly with borax, then dissolving the result in
aciidified water so that other reagents could react and provide
clues as to what the unknown contained. Any person over the age of
50 will tell you this form of analysis was used to help teach
chemistry in the sixth form. (Don’t really know why they stopped that
method of teaching)
Here endeth the lesson.
Cheers for now,
John Burgess; @John_Burgess2 of Mapua, Nelson NZn fact, glasses,