[greg c]

Hi all, I searched sterling silver without any significant findings, so I'm going to post my question here.  If it's in the wrong place or it has been asked before, please let me know!

I was looking for a way to blacken a piece of sterling silver jewelry and tried some gun blue on it.  Actually, that's not true- I tried two types of gun blue.  One worked and the other did not, and it got me curious as to what was really going on.  Can anybody analyze this one for me?

Gun Blue the First contains: Phosphoric and selenious acid, cupric and nickel sulfate.  Result: No change in the silver finish.

Gun Blue the Second contains: Hydrochloric and selenous (yes, spelled differently than the above) acid, and copper sulfate. This one blackened the silver very nicely.

Can a kind soul explain the reaction here?  Also- if I created anything toxic please let me know!

Thanks so much.

-G

[UnintentionalChaos]

This is just random speculation, so feel free to correct me if I screw up. The sulfates can be ignored in both cases since sterling silver is silver with some copper mixed in and silver is below copper on the electromotive series, nickel being above both of these. So, it isn't a film of copper or nickel that was deposited. In the bluing with phosphoric acid, you likely made a thin layer of silver phosphate immediately, which refused to dissolve further since it is insoluble (I think, most phosphates are and there seems to be very little info on this stuff) and phosphoric is a non-complexing acid. In the other bluing, silver chloride was formed and can dissolve as [AgCl₂]^- since HCl can form complexes. From what I can find, selenous/selenious (same compound) acid (analagous to sulfurous acid) is a decent oxidizer and probably oxidizes the silver, itself being reduced. This: http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1953/75/i18/f-pdf/f_ja01114a005.pdf?sessid=6006l3
seems to indicate that silver selenide is indeed formed and will "darken" a silver surface.