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Interesting Facts
This month lets talk about fire scale.
What
is it
Fire scale is basically the copper that is contained in
the sterling silver alloy that is oxidised, it appears as a grey shadow on
the silver.
That in itself is not really much of a problem
except that the copper oxide is not just found on the surface of the metal,
it penetrates the surface and depending on how bad the firescale is means
how deeply into the surface of the metal the copper oxide can be found.
Pickling removes the surface oxide because it etches the
copper from the surface of the silver leaving a layer of basically fine
silver, but this is only quite a thin layer so once the surface is polished
the firescale will return as the surface layer is polished away.
The only real way to remove firescale is to emery or
polish off the surface layer until the firescale is no longer visible.
How
To Avoid It
The best way to avoid it is to heat your metal in a
reduced atmoshere where there is little or no oxygen present. This is the
way we anneal silver when we manufacture it. Of course this is not possible
in most workshop situations so there are really only two ways to avoid it.
One is to apply a protective layer to the surface of the
metal so that the oxygen does not come in contact with the metal, this can
be done by using a thin layer of flux over the whole surface of the metal
which is then cleaned off afterwards.
The other way is to ensure that you do not heat the
metal to the point where firescale occurs. Firescale is basically a
function of time and temperature so a lower temperature for a long enough
time will still result in firescale, it is a matter of achieving the
desired temperature for the shortest time possible.
For example the normal annealing temperature for
sterling silver is 650 degrees C. But if you do this in air you will
probably get some firescale but if you anneal at 560-570 degress C.
then firescale usually does not occur and provided you maintain this
temperature for about 10 minutes you will achieve a properly
annealed state. So when you are torch annealing you should make sure
that you are doing it in a dark area way from any direct light so that you
can see what colour the metal is actually going. You have reached the
correct annealing temperature when the metal just starts to glow slightly
pink. If it is red you have gone too far and will probably get firescale on
the surface of your metal.
The same thing applies to soldering, the temperature you
can reach is somewhat higher as you are not leaving it there for any length
of time and the piece you are soldering is somewhat protected by the flux
but you should not overheat the metal, and when solding a large item you
should make sure that you do so in a space that is surrounded by fire
bricks to hold the heat and keep the whole piece at an elevated temperature.
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