I’m in a pickle…perhaps someone out there can help.
I am using an 18k gold alloy that has a high percentage of copper.
When cast, this has a very pink tint. I like this alloy because it
will oxidize when I solder it. When I put it in the pickle, the
oxide is removed and a layer of fine gold remains on the surface.
Depletion guilding. I am using sodium bisulfate for the pickle,
occasionally using a hot mixture of 10% sulfuric acid with a "pinch"
of sodium dichromate.
If I wanted the metal to be pink, I simply removed the fine gold
layer by using abrasives. I am working on a new design series that
has a verydetailed texture done in the wax. I want to retain the
original pink color of the metal, but cannot use abrasives to remove
the surface. Dropping this alloy into the pickle results in a fine
gold surface. Electrostripping did not work. It will turn an
otherwise pink surface into a fine gold surface, and a fine gold
surface will simply get richer. I have been laser welding the alloy
into place. I need to move beyond this.
Can anyone suggest an electro/chemical means of stripping the fine
gold from the surface? Perhaps a coating that can be applied to the
surface prior to soldering that won’t allow it to oxidize and not be
removed by the pickle?
Perhaps you could use Lee Epperson’s trick for oxide-free castings?
[Lee, correct me if I have this wrong] I believe he sets the hot
investment flask into dry leaves immediately after the pour. The
hot flask causes the leaves to burn. The smoldering leaves create a
reducing atmosphere which prevents the development of oxides on the
casting. If it works for sterling, it should work for your pink gold
also. With no oxides, you wouldn’t need to pickle, and should be
able to preserve the pink gold surface.
Can anyone suggest an electro/chemical means of stripping the
fine gold from the surface?
Have you tried aqua regia? (possibly quite dilute, though you’d have
to experiment) that will also remove the copper, but it should first
have to go through the fine gold to get there.
Perhaps a coating that can be applied to the surface prior to
soldering that won't allow it to oxidize and not be removed by the
pickle?
Whats wrong with prips flux? This should prevent the oxidation in
the first place. Without the oxidation, you shouldn’t have much
depletion guilding, at least not with traditional pickle solutions.
Did I miss something here in your process that would preclude these
fairly standard approaches to the questions?
If I wanted the metal to be pink, I simply removed the fine
gold layer by using abrasives. <snip> Dropping this alloy into the
pickle results in a fine gold surface. Electrostripping did not
work.
Doug,
Did you try stripping? Everytime that I have stripped, all the
surface metal was removed. No fine gold. White golds always become
white and Hamilton colors return to Hamilton. I have heard many
people state that electrostripping leaves a fine gold finish, but
that has never been my experience. I am really puzzled by your
statement.
My next suggestion is to bomb it. Same thing.
As a matterr of fact, I try bombing some very pink alloy that I made
up last week. I’ll let you know what happens. I’ll be surprised if it
doesn’t work.
Perhaps I should have mentioned that I am casting rough diamonds in
place in the 18k red gold. Normally, I would just cut the oxygen
the flame on the work, and slide it into water. No oxide. The
diamonds, however, do not appreciate this bath!
I have tried Prips flux, borax flux, even Black flux for steel.
They work…almost. When I do get a void in the coating, I get an
Oxide on the surface, and I find myself in a pickle. Aqua regia
works, sorta. Sandblasting works best, but removes too much surface
detail.
I am hoping to find either a coating that will not produce
occasional voids, or some non-abrasive means to remove it.
The process Lee Einer mentioned prevents oxygen from combining with
the copper in the metal to form cuprous oxide and cupric oxide
during the vacuum casting process.
I am not sure this is what you are looking for.
The process is very simple. I take the hot flask from the vacuum
table and place it on a solder pad covered with wax shavings. (Note:
the leaves mentioned by Lee were what lead me to the process.) I
then place a flask that is the next size larger than the casting
flask around the casting flask. I then place about a table spoon of
wax shavings on the sprue then place a solder pad on the cover
flask. The wax inside the cover flask tries to burn and will absorb
all the oxygen from around the casting flask. I no longer pickle my
sterling silver castings. If the wax is very clean I can skip the
pickle and tripoli and polish the project with jewelers rouge.
I have a copy of a magazine article written on the process and have
put together a paper on the subject and would be glad to sent the
info to anyone interested. All I need is a snail mail address.
After years of firescale and black castings I am continually amazed
at how clean my castings come out.
Douglas, I have used fine pumice powder before with good results. I
use either a brass brush or a steel brush with water or, depending
on the detail you have to work with you may even be able use a
scotchbrite pad or steel wool with it.
I believe he sets the hot investment flask into dry leaves
immediately after the pour. The hot flask causes the leaves to
burn. The smoldering leaves create a""
I wouldn’t recommend this … Red golds , esp. 18k gold/copper
binary alloys need to be quenched while still very hot , they should
be quenched from Red Heat. They can harden so much , if cooled too
slowly that you can have castings break !
Keeping red golds red is something of a challenge . Fire coat very
well [ try using dry boric acid , repeatably heating and dunking
into the powder to build up a good coating ] don’t pickle remove
firecoating with hot water and your ultrasonic.
Could you "reverse"plate [or strip] in gold/plating solution? Use
the piece as the anode and plate the gold onto some scrap? I
haven’t tried this as I’ve always been able to sand/buff the rose
gold to a satisfactory finish.
I wouldn't recommend this ..... Red golds , esp. 18k
gold/copper binary alloys need to be quenched while still very hot
, they should be quenched from Red Heat. They can harden so much ,
if cooled too slowly that you can have castings break !
Mark,
That’s hotter than you need, and can cause cracking just from
thermal stress, especially in castings which may have a rather coarse
crystal structure, and are thus weaker. Red golds get brittle from
formation of an ordered array structure upon slow cooling, but this
structure doesn’t start to form at your higher/red hot quench temps.
You can safely cool the red golds to around 800F or so with no
hardening taking place yet. Then quench, preferably in alcohol, or
perhaps boiling water, so as to not cause stress cracking. Either
alcohol or boiling water will still chill the metal fast enough to
prevent that hardening…
If you look up a phase diagram for copper and gold binary alloys,
you’ll see two inverted parabolic narrow zones , one centered on
around 18K corresponding to a formula of AuCU, which forms at temps
below about 760F, and another a bit to the left of that,
corresponding to a formula of either AuCu2 or AuCu3 (I forget which,
right off the top of my head…) which starts to fom at a slightly
lower temperature. Those are the stable zones for the ordered
array structure.
By the way, just for fun, any of you who have copies of Diego
Pinton’s “Jewelry Technology”, take a look at the diagram of the
copper/gold phase diagram on page 327, figure 380. Then tell me,
“what’s wrong with that picture?”. Unless I’m missing something
essential, that diagram has a couple fundamental errors, which mess
up a bunch of interpretations of the thing. Anyone else see what
I’m seeing there? (grin) If you’re not sure, check another source
for that diagram, such as T.K. Rose, “metalurgy of gold”. (If
anyone wants me to email them those diagrams, let me know. I can
scan them and send you the scans. Probably best with a higher speed
connection…).
From the discussion thus far it sounds like the general consensus is
that red gold can often be problematic. Here are a few of my
observations about this metal:
I use 18kt red gold quite frequently, and so far I find it actually
quite nice to work with. I am utilizing it in fabrication projects
and I haven’t experienced any difficulty in either amalgamating and
pouring ingots, or in milling and annealing the metal. The
malleability and general working properties are very similar to 18kt
white and yellow alloys. Soldering hasn’t proven much different in my
applications, although I have heard other goldsmiths say they felt it
was much more difficult to solder red gold.
Due to the high copper content it is slightly more ‘sticky’ than
18kt yellow or palladium white gold, and when crocheting with this
metal there does seem to be a little more drag or friction on the
wire. The annealing and finishing processes for my crochet chains are
basically the same as with the other colors. This issue of
stickiness might be more of a factor for pave’ setting and raising
beads, but for flush or burnish setting it hasn’t been an issue.
I have not cast anything with this alloy, but I have heard from a
colleague who recently had very disappointing results with having
some of her rings cast in 18kt red gold. The caster attempted the
castings twice and apparently the problems of cracking were equally
as prominent on both batches of rings. As others have mentioned, it
might be due to the cooling and quenching of the flasks, and quite
possibly the firm involved has not had much experience casting red
gold. I’m not sure if they will be very anxious to try it again.
Red gold is a somewhat relative term, like" expensive." If you are
using 18k that is 25% copper, you will have a lot more problems that
if you use an alloy that is 20% copper. The 20% alloy is still
considered red by most people, but you can see a slightly yellower
tone if you compare it directly to the 25% alloy. Even a 15% alloy
will still look “peachy,” but it works just like most 18k yellow
alloys.