Etching 22K / Sterling bi-metal

Jewelry Discussion
1

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

2 
   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.

Aqua regia is an extremely aggressive acid mixture it will attack
many resists I am not sure of any type that will stand up to long
exposure in it. I did find on the web some references to using a
mixture of beeswax asphaltum as a resist for aqua regia so you might
try to mix the asphaltum with beeswax. The second problem you have
is that the silver in the 22k alloy it will form a film of silver
chloride which will act as a resist and stop the etching of the
gold. This is probably why you did not see much etching of the gold.
One way to deal with this is to alternate between the aqua regia and
a solution of aqua ammonia which will dissolve the silver chloride.
It will be a slow process.

Jim Binnion

James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160

@James_Binnion
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