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Coloring 18k gold


#1

A few days’ worth of Digests didn’t arrive last week so I’m not sure
if my post thanking everyone made it into the Digest. I guess that’s
what happens when you live on an island.

I wanted to thank everyone for their input and especially Charles
for his in the archives and Niels on Bornholm, Denmark
for the following post:

 Potassium nitrate  KNO2  4 parts per weight
Potassiumaluminumsulphate  KAl(SO4)2, 12 H2O 2 ppw and Sodium
chloride (salt)  2 ppw Add water till a thin slurry is reached.
Fill into a fireproof container (not metallic) and boil. Suspend the
item to be 'surface enriched' from a silver wire into the solution
and let boil for 10 - 20 minutes. Clean and scratch brush. Repeat
as necessary. A warning: Do not let the piece rest on the bottom,
it simply dissolves (I speak from experience :-) 

This procedure is very effective because it doesn’t require that the
alloys in 18k be turned into oxides prior to depletion gilding. The
items that I’m working on will lose all of their structural
stability if they are heated to the point of oxidation, therefore
heating them prior to pickling isn’t an option.

If the item is left in the solution for too long, it starts to form
a brownish-reddish skin that is relatively difficult to remove so
timing is important.

It would be interesting to hear from the chemists in Orchidland as
to what is occuring when the above procedure is used.

Thanks again
Bill


#2

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)

#3

G’day; In my opinion boiling gold in the chemicals mentioned above
wou result in a liquid that closely resembles aqua regia. Thus, gold
and alloys would be surface etched in a kind of 'depletion gilding’
The liquid would turn yellow as gold chloride is formed and
dissolved

Cheers for now,
John Burgess; @John_Burgess2 of Mapua, Nelson NZ