Most of the metals we, as metal smiths work with, are combinations
of metals (alloys, mixtures, intermetalic compounds, solutions…).
They act together sometimes like when the melting point lowers, the
liquidous temperature spreads, the color changes (blue gold
intermetalic compounds, white, yellow, pink, and ?green gold alloys)
or the surface oxidation is affected (deox and argentium silver).
Sometimes they act independently, like mixtures of copper and tin or
zinc (brass/bronze). When you hit it with an acid, most acids will
will first attack the most vulnerable atoms, those higher in the
electromotive series like the zinc. So, when you pickle, (usually
with a -SO4 ion) the acid selectively attacks the zinc, leaving a
copper rich surface. Much like ‘depletion’ gilding’, where dissolving
the copper oxide from the surface of sterling leaves a silver rich
surface. The copper in sterling is not attacked unless you already
have a copper oxide or you add an oxidizer like H2O2, hydrogen
peroxide (great way to lose the copper flash on sterling). Note that
there are very active acids like bright dips that are mixtures of
acids like HCl, HNO3, and H2SO4, throw in a bit of HF and they seem
to eat almost anything.
marlin, back from Burning Man and trying to adjust