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Depletion gilding leaving brown stain on 18k


#1

First question:

My attempts at depletion gilding using weak solutions of aqua regia
leave a brownish color on the surface of the item. This can be
removed with rouge but I’m having trouble getting a “true” fine gold
color. Any ideas as to what this brownish stain is and how to
prevent it?

Second question:

The color that I’ve always thought of as “18 karat yellow” was
actually the result of repeated heating and pickling of the item.
It’s the creamy yellow color most easily seen on a matte finish.

From reading the orchid archives and the very informative posts
pertaining to coloring gold, I’ve learned that heating the gold item
causes the copper alloy to oxidize, thus making it removable by the
pickling procedure.

I’ve been trying to produce that color of yellow by using various
chemical combinations without the use of a torch; the idea being
that some chemical treatments will remove the copper alloy without
the intermediate step of first oxidizing the copper with high heat.
Haven’t succeeded yet. Also tried oxydizing the copper with
concentrated hydrogen peroxide–didn’t work. Any advice?

Thanks everyone!
Bill


#2
My attempts at depletion gilding using weak solutions of aqua
regia leave a brownish color on the surface of the item. 

Bill, if you are really using aqua regia then the answer is simple.
Aqua regia (mixture of nitric and hydrochloric acids) will dissolve
gold. The brown stain is probably gold chloride. For depletion
gilding don’t use aqua regia, use normal pickle, after heating to
oxidise the base metals. If you’re not using mixed nitric and
hydrochloric then maybe the stain is due to copper plating out onto
the gold. There used to be an excellent article all about depletion
gilding on the Ganoksin website, it’s probably still there. –

[Edit Ton: Link to the article -
http://www.ganoksin.com/borisat/nenam/depletio.htm ]

Kevin  (NW England, UK)

#3

Thanks, Kevin for your Re: aqua regia and brown
staining.

I’m still trying to achieve depletion gilding without the use of
heat as a means of oxidizing the alloys prior to immersion in the
pickle.

I’ve used the following formula and procedure posted by Niels of
Bornholm but getting the color just right is taking some practice:

     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 :-) 

Kevin, the following is a copy of one of your recent posts that,
along with a post by John Burgess, has helped fill in some blanks in
my understanding of the above-mentioned procedure.

            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. 

From the I’ve found in the Orchid Archives, ferric
chloride seems to be a much more benign approach to achieve
depletion gilding without heat and with a minimum of the noxious
fumes and risks involved with nitric acid.

My question now is:

Has anyone had experience using ferric chloride to achieve depletion
gilding, without the preliminary heating of the item to oxidize the
alloy?

Thanks again to everyone for your input and for your patience;
chemistry wasn’t my strongest subject in school!

Best
Bill