From time to time, when I remove a piece of sterling from my
pickle, either citric acid or sparex, there is a dark gray/ black
discoloration on the surface that's really hard to remove.
I'm afraid I can't tell you what that discoloration is --I simply
don't know-- but I can tell you a REALLY smooth way to get rid of
it. I use a medium solution of hydrochloric acid (HCl, approx 20%).
If your HCl isn't too old that black smut will be gone in seconds,
literally. I pickle briefly in fresh Sparex, rinse, then pickle in
the HCl solution and wash. Less than two minutes and your workpiece
is clean as a whistle. I've found that keeping the HCl solution fresh
saves a lot of soaking time. When the HCl is getting old you'll know
it: it gets an intense orangy-yellow color, so much so that it looks
like a urine sample (insert many jokes and jibes from wife here).
I don't know what it's like in your part of the world but here,
Europe, this HCl solution is sold in any hardware store. It comes in
one litre squeeze bottles for about US$1.50. It is used as lime and
scale remover, largely to battle the mineral stains left by hard
water which is very common here.
Peter often advocates that we "Understand whatever you use" and I
wish I could say that I do ... but I don't, at least not in this
case. The chemistry of that pesky black smut --almost greasy, no?,
why the HCl works so well on it when so few other things do, what
exactly the chemistry involved is, and why the HCl turns that intense
"urine sample" color when it's getting tired are all a mystery to me.
I can say though that you'll want good ventilation if you decide to
use the HCl in your work. The fumes can be pretty strong and
choking: the MSDS on dilute HCl says "Inhalation of vapour is
harmful". And those fumes will do a right proper job of rusting any
tools you are careless enough to leave nearby. Does all of this sound
intrusive? My experience has been that it is not. One simply develops
a healthy habit of care and cleanliness, thoroughly washing anything
that comes out of the HCl, and keeping the HCl solution covered as
much as possible. The benefits are that that black smut is bested
quickly and easily. I've found the tradeoff very much worthwhile.