[Jewelry making - Article 00071] - Re: [Orchid] Coloring 18k gold

[Kevin]

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    Bill, the potassium nitrate supplies nitrate ions while the
    potassium aluminium sulphate supplies hydrogen ions, all in aqueous
    solution. This is equivalent to having dilute nitric acid, so I
    think the action is primarily one of dissolving the base metals
    straight into nitric acid (making copper nitrate etc).  This
    contrasts with the more usually described process where the copper is
    first oxidised, so that it can be dissolved in normal sulphuric acid
    pickle (potassium hydrogen sulphate in solution is equivalent to
    sulphuric acid).  As you mention, this could be useful in cases where
    heating to high temperastures are not appropriate. 

    What concerns me though is the inclusion of sodium chloride, which
    supplies chloride ions. Now, chloride ions plus nitrate ions plus
    hydrogen ions, in water, is equivalent to a mixture of nitric and
    hydrochloric acids, also known as aqua regia (when concentrated),
    which dissolves gold.  Now, we have here a much weaker effect than
    aqua regia proper, since we're not mixing concentrated acids, but
    it's still functional.  Indeed the first message carries a warning to
    this effect. 

    Overall of course the base metals are dissolved much faster than the
    gold, but it's something to be aware of, particularly if you use it
    on delicate pieces such as filigree. 

	Kevin  (NW England, UK)




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