Thanks, Kevin for your Re: aqua regia and brown
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
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
Thanks again to everyone for your input and for your patience;
chemistry wasn’t my strongest subject in school!