I'm posting this here, but I'm not really a student, so if it's inappropriate please feel free to move it elsewhere.
Quick bit of background:
I'm not a chemist - but I do have a physics degree. Feel free to science at me.
I'm enamelling on to sterling silver (92.5%Ag, balance Cu). In order to avoid adverse colour reactions, I need to 'depletion silver' the surface of my sterling. This is generally done by heating the sterling to 760C, at which point surface copper oxidises, then cooling and washing in a dilute sulphuric acid soln (which jewellers call 'pickling'). This dissolves the CuO and leaves purer silver on the surface. This process is done several times until no further CuO can be seen.
Now, the protocol I'm following comes from a superb enameller, but I don't think he's a chemist, hence this post. Here's an extract from the book:
"pickle (dilute sulphuric acid)...will not remove the fire stain (CuO), it is more a bleaching process, and the copper oxide is still there as a fine layer over the silver surface. The aim is to remove the oxide, and this is done using commercial standard nitric acid ,which is graded at around 70%..."
Now, I can't get nitric acid without a licence from the home office.
So, the questions:
- Does anyone recognise this bleaching effect? Is it true that sulphuric won't fully dissolve CuO?
- Is the another reagent, or combination of reagents, that will fully dissolve surface CuO without attacking the silver?
Thank you for your kind attention, I'll try to answer questions...