The short answer (in my case) is to avoid firescale when i enamel.
The long answer is below.
To bad we don’t have some scanning electron micrographs (electron
back scatter mode, or better yet a photo of energy dispersed x-rays -
jargon but the pictures are fabulous) of sterling silver around. What
would we see is small lighter areas surrounded by darker ones, in the
ratio of 7.5 to 92.5, the same as the ratio of copper to silver. We
would be looking at a surface and consequently in our mind’s we need
to imagine that light areas are the surfaces of small units of copper
in a matrix of silver.
Metals have differing solubility coefficients in various acids and
bases. For example section B of the CRC Handbook of Chemistry and
Physics lists copper as being very slight soluble in hydrochloric
acid (HCl) and does not list the solubility of silver in HCl, implying
that it is insoluble in this acid. So, if we put a piece of sterling
in HCl and waited an awfully long time the acid would dissolve the
copper on its surface layer and leave the silver (in theory).
To speed things up we can heat the metal to form metal oxides which
are more soluble than are the metals. Common practice has revealed to
us that the oxides of copper are more soluble in acid, specifically
sulphuric acid (guess where i live? ) than are the oxides of
silver. Again common practice has revealed to us that if we heat
sterling lightly (I go to just black to form CUO2), cool, dip in acid
for some time, lightly scrub, rinse and dry, and repeat the preceding
about six to eight times (dare i say seven?) we will end up with a
very thin layer of pure silver on top of our sterling. We can put this
in our kiln and it will not have very much fire scale, if any, at all.
I was taught this method as part of the process for champlev�
enamelling for which one creates pits in the silver with gravers. We
used sterling rather than fine silver because it is easier to engrave.
Similarly one can also deplete gold alloys for the same reason.
I have used depletion with jeweller’s bronze (Cu:Zn, 80:20) to remove
some of the zinc from the surface because of its great propensity to
form scale. Enamelling on this metal is hit or miss but can produce
some interesting effects.
Finally, does any one know of a table listing the solubility
coefficients of metals and oxides in the mineral acids, and where it
can be found?
Hope this helps.