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
piece.
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.
I think students are taught incorrectly when they are taught to rely
on pickle exclusively to clean their metal. Nickel pickle is
stronger than the regular sodium bisulphate, it needs to be for
nickel.
Believe it or not when I learned metalsmithing many years ago we
never used pickle at all, I was only vaguely aware of it, instead my
teacher sent us back to the bench to sand everything clean, maybe it
was just busywork (high school) but it was a good habit to learn,
especially with metals like nickel which don’t always get clean
enough in pickle.
There are times when pickle is a wonderful thing, but it is not a
requirement for soldering and there are times when it can cause
problems.
Believe it or not when I learned metalsmithing many years ago we
never used pickle at all, I was only vaguely aware of it, instead
my teacher sent us back to the bench to sand everything clean, maybe
it was just busywork (high school) but it was a good habit to
learn, especially with metals like nickel which don't always get
clean enough in pickle.
When I first started self-teaching silver work, I was sanding off
everything until I got back to the lovely white silver colour. After
a short while I figured it was a bit stupid because I was removing
far too much material and that the pickle was supposed to clean up
the silver. I am now more patient and pickle multiple times, doing
something else whilst waiting for it to do its job. That way I save
more metal and have less clean up in the way of sanding to do later.