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Coloring 18k yellow gold


#1

dear list does anyone have a favorite way of raising the color of 18
karat yellow gold to the color of 22 karat?

thanks
bill


#2
    dear list does anyone have a favorite way of raising the color
of 18 karat yellow gold to the color of 22 karat? thanks bill 

Hello Bill; My technique only works for articles that don’t have
stones or enamel in them, as these materials would become subject to
thermal shock. I clean the article, then heat it until it turns
black, which is generally just below a visible red, probably around
700 degrees, farenheit. Then I let it cool to the point where I can
touch it. I then submerge it in a small container of muriatic acid
(hydrochloric acid and water) and suspend the container in the
ultrasonic cleaner. This I do by means of a plastic strainer that
hangs across the top of the sonic and in the solution. After a
minute or so in the acid, the black color is gone. I rinse the
article in clear water, then lighly brush with a brass wire brush and
repeat the entire heat/acid/wire brush process as many times as
needed to achieve the color I desire. The final color tends to be on
the green side when used on a standard 18K yellow alloy, so if you
need to get a more orange hue, you’d need to, I believe, use
alternately immersion in acid then silver nitrate, then acid, then
silver nitrate. You could substitute ferric chloride for the
muriatic acid, but it would be a slower process requiring heating and
pickling a greater number of times.

David L. Huffman


#3

Is 18K gold so different from 14K that it has to be pickled in
something other than Sparex to achieve a higher karat color? I
depletion gild 14K all the time, heating it till it blackens as David
Huffman described, but then dropping it in Sparex pickle as I would
normally – not red hot, but hot. I get the best/fastest results if
I wire brush with baking soda in between heatings but I’m usually too
lazy for this and it works regardless.

Beth


#4

Beth, Basically, what you are doing is removing some of the alloy
metal on the surface by converting it to an oxide, then removing the
oxide. Copper is the metal that most readily oxidizes, so alloys
containing a lot of copper seem to depletion guild best.

14k yellow has less gold than 18k, so you have to remove more
surface alloy that you would with 18k, but it works (as you know).
Alloys with a good bit of silver in the mix, like green golds, tend
to get a bit greener with all that silver left on the surface. I
usually give them a dip in the nitric bath, and that seems to remove
the surface silver. What I have now is a lot of gold on the surface,
with the gold+alloy underneath. If you polish too aggressively,
you’ll remove this surface layer. Brass brushing is a good idea, as
it burnishes the gold surface without removing metal.

If you are using commercially prepared alloys, you can’t always tell
what you’ll get until you experiment a little, since they’re all
different and the refiners don’t like to tell you what’s in their
"special mix." If you are alloying your own, you have a lot more
control. Sparex (sodium bisulfate) will work. I have a pickle that I
use just for depletion guilding. It is a weak solution of sulfuric
acid, with a pinch of sodium dichromate. The sodium dichromate acts
as an inhibitor that allows the pickle to only attack the oxide. I
am also using an 18k alloy with a lot of copper (15%).

I know there’s a lot more science to this, but this is the basics of
how it works.

Doug Zaruba


#5
        Is 18K gold so different from 14K that it has to be
pickled in something other than Sparex to achieve a higher karat
color Beth 

Hi Beth and others; I suppose Sparex would work. I always just
assumed that it was not strong enough to be effective on high karat
golds. It’s certainly safer than acid.

David L. Huffman


#6

Hi Doug,

    I know there's a lot more science to this, but this is the
basics of how it works. 

I know how it works (although that was a very nicely put
explanation); what I don’t understand is the necessity for some of
the steps listed by David Huffman for depletion-gilding 18K yellow
gold which don’t seem to be necessary for 14K yellow when the aim is
to get a surface as yellow as 24K.

In particular, why air cool before quenching (hot quenching is no
problem with 14K)? Why muriatic or other special acids (Sparex works
fine with 14K)? Why suspend in an ultrasonic (hot pickle does the
job just fine)?

I realize that 18K yellow gold contains less copper than 14K but is
that sufficient reason to necessitate such a specialized
depletion-gilding procedure? Either method leaves a surface layer
of 24K doesn’t it? so why go to the extra trouble?

Beth


#7
Either method leaves a surface layer of 24K  doesn't it? 

Heating and then pickling usual karat gold alloys with sparex type
pickles will remove copper oxides only. since surface copper is
oxidized in the heating, yo’ve effectively removed the surface
copper, enriching to gold content at the surface. But most gold
alloys also contain silver. The silver is not removed by this
process. Other acids and proceedures are needed if you want the
surface to really be 24K.

Peter


#8
The silver is not removed by this process.  Other acids and
procedures are needed if you want the surface to really be 24K. 

Thanks, Peter! Now it almost makes sense :-). When you heat the
alloy, copper oxides are formed at the surface and these are removed
by pickle. But … are silver oxides formed at the surface at the
same time? Or do these other acids actually act on the alloy itself?

Beth


#9
    . . . I don't understand is the necessity for some of the steps
listed by David Huffman for depletion-gilding 18K yellow . . 

Hi Beth;

My method is just my personal solution which I worked out while
removing soft solder from gold coins. I was experimenting with
several variations and noticed some changed the color of the coins.
I like muriatic simply because I can get it at the local hardware
store and avoid hazardous shipping charges, etc. It’s just
hydrochloric acid. I may be imagining it, and a chemist could tell
you, but it seemed to remove the copper better than Sparex or other
sodium bisulfate products. I let the article cool before quenching
for two reasons. One: I’m less likely to get spattered with hot
acid. Two: it seems when I dump a hot article in the acid, it works
on some areas better than others, and it also seems that the steam
blows off some of the remaining layer of gold, making it take longer
to build up. I sometimes will actually burnish the article after
rinsing with a polished carbide burnisher instead of a brass brush.
I think I get a more even surface effect by immersing a cool article
in the acid, and besides, dipping a hot article in acid eventually
heats the acid up and hot acid makes me more nervous still. Finally,
the ultrasonic, I find, helps lift off the black smut of oxide, or
makes the acid penetrate the article better. I wouldn’t claim that
my technique is necessarily an improvement over the typical depletion
method but it seems to work nicely on the small articles I work on,
especially the intricate filigree work.

David L. Huffman


#10

heating the items prior to pickling, in order to oxidize the copper
component of 18k, takes out the work-hardening that the very thin
items need to retain structural stability. would plating with fine
gold be the best solution? does plating require a lot of equipment,
etc.?

thanks
bill


#11
       Thanks, Peter!  Now it almost makes sense :-).  When you
heat the alloy, copper oxides are formed at the surface and these
are removed by pickle. But ... are silver oxides formed at the
surface at the same time?  Or do these other acids actually act on
the alloy itself? Beth 

Silver doesn’t oxidize much just with heating, and the slight oxide
that does form is very easily converted back to silver, since it’s
not really stable at normal pressure and temperature. So in an acid,
even if the acid affects silver oxide at the surface, (silver oxide
won’t be affected by sulphuric much in any case, I don’t think.) the
result won’t be any significant lessening of the percentage of the
silver at the surface.

Instead, try a nitric acid or nitric acid salt, like silver nitrate.
this will attack both silver and copper, but not gold. Be careful
when you mix the diluted acid to NOT use tap water. It contains
chlorine, even if only in small amounts. Even traces of chlorine in
your nitric bath would let it start to also affect gold a bit,
lessening your bath’s ability to enrich the gold color of the
surface. Mix the acid for this with distilled or properly deionized
water, so that it has no traces of chlorine (or salt, which also puts
chlorine ions in the water).

Peter


#12

Hi David, Thanks very much for the explanation. Now I see where
you’re coming from. It’s not, or at least not primarily, that 18K and
14K yellow gold react so differently to depletion gilding, but rather
just a different approach to the process.

A couple of comments:

I let the article cool before quenching for two reasons.  One: I'm
less likely to get spattered with hot acid. 

I took a workshop with Charles Lewton Brain a few years ago on fold
forming. We needed a whole bunch of annealed squares of copper and
when Charles saw how we were quenching the pieces, he gave us a quick
refresher.

His advice was to quench fast and quench deep. In other words don’t
toss the piece into the pickle ( whether hot or cold); instead hold
it in tweezers and plunge it quickly and deeply into the liquid.
When you do it that way, you get a more even result and without
splash. (Charles, if I’m remembering this incorrectly, please
comment!)

... dipping a hot article in acid eventually heats the acid up and
hot acid makes me more nervous still. 

One of the advantages of Sparex and similar pickles is that they’re
mild enough that you can dip a finger in them without ill effect.
Not that you’d want to do that, of course, and you’d certainly want
to rinse in water, but it happens at times. I’m sure I wouldn’t be
so sanguine if I used stronger acids!

I wouldn't claim that my technique is necessarily an improvement
over the typical depletion method but it seems to work nicely on
the small articles I work on, especially the intricate filigree
work. 

As you said in the beginning, whatever works!

Beth


#13

A large compilation of recipes and discussion of 'coloring the gold’
is published at the Ganoksin project:

Charles Charles Lewton-Brain/Brain Press Box 1624, Ste M, Calgary,
Alberta, T2P 2L7, Canada Tel: 403-263-3955 Fax: 403-283-9053 Email:
@Charles_Lewton-Brai1 Metals info download web site:
http://www.ganoksin.com/borisat/index.htm Book and Video
descriptions: http://www.ganoksin.com/brain/gallery.htm
Gallery page at: http://www.ganoksin.com/brain/gallery.htm


#14

Hi David

Two: it seems when I dump a hot article in the acid, it works on
some areas better than others, 

You may be right here. The thicker areas will hold the heat longer
so that the effect is somewhat varied. This is easily observed when
using a weak liver of sulfur solution to develop patinas on silver.

Pam Chott
Song of the Phoenix


#15

I wonder why nobody else referred to this method, - therefore this
post: Use the following:

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 :slight_smile:

I think I owe this recipe from a previous posting here on Orchid,
but could not find it in the archives. However, it has worked for me.

Regards from Niels on Bornholm, Denmark, where snow has poured down
all day and extinguished our hopes for spring (at least for a few
days)