[Jewelry making - Article 00123] - [Orchid] Etching 22K / Sterling bi-metal

[Amy Johnston]

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    I've searched the archives and books for this info, but didn't have
    any luck, so I'm posting my problem for you all. 

    I'm preparing to etch through the 22K layer of Reactive Metal's
    22K/Sterling bi-metal so that I can blacken the silver background,
    giving a gold and black effect. My design is very organic, so I'd
    prefer to etch rather than engrave. As I understand it, the only way
    to etch gold is with aqua regia, which is 1 part nitric acid to 3
    parts hydrocloric acid. I've done a test at my school on a small
    piece of the bi-metal, but the solution ate through the asphaltum in
    30 minutes and didn't etch the gold at all. It's possible that the
    school's acids aren't quite right, so I'm planning to purchase my
    own. Here's where my questions come in: 

    What grade of hydrocloric and nitric acids are the best for mixing
    aqua regia? 

    Is asphaltum a good mordant against aqua regia? If not, what is best?
    (I'm guessing PNP won't stand up to it because it's not so great
    against nitric alone, but please let me know if I'm wrong on this.) 

    I didn't heat up the aqua regia, thinking it was evil enough on its
    own. Should I heat it up? 

Any other advice?
Regards,
Amy Johnston

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