Hello Everyone,
I am repairing a platinum mounting with white gold solder on it.
Does anyone know of a chemical way to dissolve the white gold solder
completely so that I can do laser repair on the mounting.
Thanks
Shan Maloy
Maloy’s Jewelry Workshop
Portland, OR
If it is truly white gold solder (there’s a chance it could be
silver, if it’s antique - nitric will get that) then aqua regia is
the only thing. That will also dissolve the platinum, though, but
more slowly. I would suggest mechanical removal, even though you
could
use acid. But then you’ll have to walk a tightrope of getting the
gold before the Pt. goes to far, because it will dissolve much
slower. And it will do something to the Pt. even then.
If it is truly white gold solder (there's a chance it could be silver, if it's antique - nitric will get that) then aqua regia is the only thing. That will also dissolve the platinum, though, but more slowly. I would suggest mechanical removal, even though you could use acid. But then you'll have to walk a tightrope of getting the gold before the Pt. goes to far, because it will dissolve much slower. And it will do something to the Pt. even then.
Platinum, at least the common 10 percent iridium platinum, does NOT
appreciably dissolve in aqua regia IF the acid is cold. Warmed up or
hot, it attackes it strongly. Cold, it does not, at least not without
very extended immersion (days, not hours). Alloys with other than
iridium, I don’t know about. But I’ve routinely used aqua regia as an
echant on pieces made of platinum over which I’ve flowed 24K or other
generally high karat alloys of gold into recesses. After the “inlay”
is in place, the piece gets a full final polish. The acid etch then
creates a nice pattern showing the crystal structure of the gold, a
rather sparkly crystaline look when it works right. The platinum is
mostly unaffected, and retains it’s polish well, perhaps needing just
the lightest of rouge polishings afterward to fully brighten it
again. If left too long, the gold inlay can be totally removed, still
without significantly affecting the platinum. Platinum SOLDERS, at
least the old style which often didn’t have much, if any, platinum,
are strongly etched. Very occasionally, a casting with a serious
porosity problem will experience some pitting from the acid, but it
doesn’t seem common. Also, I find there is a greater chance of the
acid affecting the platinum if the acid has been somewhat diluted
with a bit of water. Water, if not so much as to make the acid very
weak, seems to make it MORE aggresive, not less. What with the
ability to do really good restorations of old platinum work with a
laser welder, I’ve often had situations where doing the best job
required removal of prior repair work done with gold solders. The
aqua regia etch has never yet disapointed me, other than the fact
that it will also damage the existing old style platinum solders
which I might have preferred to remain. But since I can fix those
parts again also with the laser, the key thing is the removal of the
gold solder, while not damaging the platinum itself, and in this,
I’ve had fine results with aqua regia. Usually, I mix up a small
batch of fresh acid (3 parts HCL, 1 part Nitric, both reagent grade),
and let it sit for a half hour. It’s color will change from clear to
amber. At that point, in goes the platinum. Etch times to remove old
solder are generally in the range from ten minutes to a half hour or
so. Remove periodically, and clean with ultrasonic and steam, since
if you’re trying to remove a solder with a high silver content, it
will form a silver chloride skin that stops efficient etching.
Cleaning it removes any build up of residue that might impede
continued etching. If the piece has platinum solder you wish to
retain, you can sometimes mask it off first, though i’ve had mixed
results with trying to mask it with things like the usual nail polish
or wax that I might have around. Presumably a more proper etch resist
like asphaltum would work better, if you have it around.
Peter Rowe
A question: Since you seem to have a grasp of the chemistry involved
in metallurgy have you ever used or observed the reactivity of a
solution of:
Sodium Bisulphite (7631-90-5), Ammonium sulfite (10196-04-0),Acetic
Acid (64-18-7) Boric Acid(10043-35-3) in a base that has the
additional components; Water, (7732-18-5), Ammonium
thiosulfate(7783-18-8) and Sodium acetate (127-09-3) as a
pickle/stripping solution.
I found an archaic recipe for this for use with non-ferrous metals
and am wondering if anyone else has found it to be superior to the
simples like: Sodium bisulfite and water, or nitric acid and water,
or citirc acid and water, or hydrogen peroxide and water…or any
combination of those more common compounds.