To clean up from soldering we use nickel pickle. The problem is that after multiple soldering, the metal is pitted, and has layers etched away
Helen, when I solder nickel-silver and/or brass (=Nugold), I use
Ph-Down (the sodium bisulphate swimming pool additive) for pickling
both. It’s important not to leave the metal in the pickle too long;
after some time (depending on the strength of the pickle solution) it
can start to etch the metal. Left overnight, it can perforate the
The pickle gets rid of the black cupric oxide. After rinsing and
wiping the the piece dry, I bright-dip the piece to get rid of the
red cuprous oxide. Personally, I use dilute nitric acid for
bright-dipping, but with students a less dangerous mixture is better
– use another solution of Ph-Down with some hydrogen peroxide (from
the drug store) mixed in it. You can watch the red disappear (except
on copper, which is basically the same color as its cuprous oxide).
Again, don’t let the metal soak too long.
This will clean off the oxides so that the next soldering job should
go well. I think your students are leaving their metal in the pickle
too long, or else the pickle is stronger than it needs to be. Gentle
heating speeds both pickling and bright-dipping, but do not EVER let
the solutions boil!
There are several discussions on this issue in the Orchid Archives.
All the best,