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Chemistry


#1

Perhaps you chemistry buffs can shed some light on today’s strange
happening:

I wanted to blanche/depletion guild a gold and silver piece. Had
available my little shot glass of strong sulphuric acid pickle (half
acid, half water) with hydrogen peroxide added which I have been
using with great results on nickel white gold. As soon as I gently
immersed the piece (with brass tweezers), it let off a smokey, white
substance and the entire piece got what appeared to be a beautiful
(very even) pure silver plate! This would have been very nice,
except for the fact that the gold parts are supposed to be yellow! I
removed the plate abrasively on the flat, accessible gold parts, but
was wondering what the story is here and if there’s a way to
chemically remove such a silver plate to re-expose the gold. I was
amazed at the extent to which any hint of gold or copper was removed
from the original alloys, leaving such a complete silver coating
which, by the way, was quite resistent to abrasive treatment.

Janet in Jerusalem


#2

I use suphuric to pickle my silver, and I have had similarly
diconcerting results when working with sterling and 18K doublee. In
my case, not as much silver was re-deposited, but enough to turn the
gold a weird, silver-green color. Maybe this phenomenon has some
limited useful application, say, in conjunction with patinas?

Lee Einer


#3

Hi A modern alchemist ???

Anyhow if you had used the same pickle for silver earlier, then some
silver may have gone into solution and then replated out onto gold
(Nice little lab demo of electrochemistry and the reason why they say
don’t use a commercial ‘silver dip’ type cleaning solution for silver
then gold jewelry.)

Are you sure it was silver - not nickel, perhaps, dissolved from the
white gold and then replated out from the pickling solution?

Best etc
Jack Ogden
www.osmiridium.com


#4

Janet, I’ll take a stab at it, but without guarantee… (tricky
doing chemistry at arms length!).

You’ve got the formation of an electric cell, from gold and silver,
joined, immersed in acid. There’s also the brass tweezers. If they
were in the pickle too then they could form a cell with both the
silver and the gold. So I start to think that you’ve electroplated
dissolved silver (or maybe nickel if I read your previous use right,
or did you just mean that you’ve used this formula but not this
actual batch of pickle with the white gold?). The problem is that
however I turn it around in my mind I can’t get the polarity correct.
Silver would plate out at the negative electrode and the only way I
can figure that is if there were some localised currents flowing so
that you have a silver/gold cell formed in one part which then makes
another part of the gold into a negative electrode. Now, this sort
of thing happens with less pure metals (corrosion of iron and steel),
but I would have thought it was less likely here. Someone else chip
in and show me the obvious bit that I’m missing!

If the gold contains appreciable quantities of zinc then dissolved
silver would plate out, with the zinc going into solution.

As to the white smokey stuff…who knows? Zinc sulphate is
soluble, so it’s not that. Copper sulphate is blue, so rule that
out. Silver sulphate is white, but I don’t know its solubility (have
a feeling though that it should be soluble). Nickel sulphate is
green…

Putting that to one side, I’m a little puzzled by your depletion
gilding nickel white gold… What effect were you aiming for,
indeed, what did you get (you said it went well). Depletion gilding
normally raises the gold content of the surface so on the face of it
this should make your white gold yellow… unless there was a
similar plating action going on and you got a plating of silver. All
a bit of mystery! Isn’t this fun! –

Kevin  (NW England, UK)

#5

Hi Jack!

Are you sure it was silver - not nickel, perhaps, dissolved from
the white gold and then replated out from the pickling solution? 

Well, it certainly could be a silver plate (I hope not!)—but it
has that real nice white so characteristic of (only) silver. I can
test by seeing if it stays nice and white upon heating, but I’m not
keen on risking what would be involved if it is nickel…

But would the electrochemistry here call for a nickel-plating of
gold and silver? A silver plating? I guess that was my question.
Remember, no iron was introduced into the pickle. I’ve been
blanching/gilding gold-and-silver pieces for years, and never saw
anything like this. The not-usual elements for me here were 1) very
highly concentrated pickle, 2) the addition of hydrogen peroxide, 3)
a pickle which probably had a good amount of nickel in it.

Janet in Jerusalem


#6

Hi Kevin, I think perhaps you missed the original post:

  1. I was depletion gilding/blanching a piece made from sterling
    silver and 18K yellow gold.

  2. The only way white gold and nickel is involved here is very
    indirectly: the pickle is one I had previously been using for nickel
    white gold. This pickle works well cleaning the stains on white gold
    after soldering. Normally for depletion gilding and blanching yellow
    gold and silver respectively I use a dilute sulphuric acid solution
    (no hyd perox).

  3. The brass tweezers was in only momentarily to put the piece in.
    But since the pickle was so strong, perhaps zinc did get inroduced
    into the pickle from the brass even from such brief contact.

Janet in Jerusalem


#7
Hi Kevin, I think perhaps you missed the original post: 
  1. I was depletion gilding/blanching a piece made from sterling
    silver and 18K yellow gold.
  1. The only way white gold and nickel is involved here is very
    indirectly: the pickle is one I had previously been using for nickel
    white gold.

Hi Janet. No, I saw your original posting. Looks as though the
only way you’ll get to the bottom of this is by doing some
experiments. As I said, doing chemistry at a distance is tricky at
best! Good luck, and please keep us posted on what you find. You
may just have stumbled on something useful!!

Kevin (NW England, UK)


#8

The unexpected result reported by Janet in Jerusalem is very
interesting. I believe the explanation involves the same kind of
chemistry as is operative in the plating of copper on a noble metal
from an iron contaminated pickle solution.

In order to keep this as brief as possible, a lot of detail and a
couple of major caveats will not be discussed at this time. I will
include some equations and the corresponding half-cell/cell voltages
as a help for those not very familiar with electro chemistry.
Questions and comments can be addressed in future posts. A question
mark following a word/statement indicates an assumption has been
made.

The introduction of ferrous ions (Fe plus 2) (by acid reacting with
steel or iron) into the pickle containing cupric ions (Cu plus 2)
produces a redox (oxidation-reduction) reaction when a metal more
noble than copper is put into the solution. The reaction is:

Cu plus 2 + 2Fe plus 2 yields 2Fe plus 3 + Cu E cell is 0.78
volts

This reaction occurs at the cathodic surface (the more noble metal
in the solution) with copper (Cu) being deposited on the cathodic
surface. The reaction is over quickly because of the limited supply
of ferrous ions (Fe plus 2) at the cathode. In most cases, only a
thin coat (“flashing”) is deposited on the work.

The only thing that makes sense to me in Janet’s case of silver(?)
plating out on gold and sterling is a modified process where copper
is anodic to silver just as iron is anodic to copper as described
above.

The peroxide-acid system will oxidize metals up thru gold. What is
meant by that statement is that the cell voltage from the two
half-cell reactions involved (reduction of the peroxide to water and
the oxidation of the metal to the metal cation) is positive and the
reaction is, therefore, spontaneous at standard state (a major
caveat here). The metals of greatest interest in Janet’s report are
copper, silver and gold. The half-cell potentials (they are
negative) for the oxidation of these metals aRe: copper (Cu) to
cupric ion (Cu plus 2) -1.43 volts, copper to cuprous ion (Cu plus

  1. -1.25 volts, silver (Ag) to argenous ion (Ag plus 1) -0.97 volts
    and gold (Au) to auric ion (Au plus 3) -0.27 volts. The potential
    for the reduction of peroxide to water under acidic conditions is
    +1.77 volts. To get the cell voltage for the redox reaction, the
    reduction potential is added to a selected oxidation potential.
    When several reactions could occur the one with the largest positive
    cell voltage will occur first, then the second highest cell voltage,
    etc. When the cell voltages differ by a large amount the reaction
    with the more positive cell voltage can occur almost exclusively to
    the other. When the cell voltage differences are small, as is the
    case here with a difference of only 0.18 volts between the oxidation
    of copper to cupric or cuprous ion, both reactions can be expected
    to occur with the more positive exceeding the other by some factor.
    That is fortunate in this case because cuprous ions are needed for
    the proposed process to proceed. Trying to find a way to produce
    cuprous ions by reduction of cupric ions has not been fruitful. The
    supply of copper is limited to the supply at the surface and is
    available as the surface of silver and gold is removed with the
    removal of silver being favored. The amount of gold oxidized is not
    germane to to the process being examined.

The solubility of silver sulfate (Ag2SO4) is 0.8 g per 100 g of
water at 20 deg. C. The common ion effect will reduce the
solubility to 0.0002 g per 100 g of the 9 molar sulfuric acid if my
calculation is correct. Hence the appearance of the “smokey white
substance.” Reduction of silver sulfate to silver metal has a
half-cell potential of +0.65 volts. Cuprous ion, with an oxidation
to cupric ion half-cell voltage of -0.15 volts, is the only
candidate available for the oxidation half of the redox reaction .
The reaction and the cell potential is as follows:

Ag2SO4 + 2Cu plus 1 yields 2Ag + SO4 minus 2 + 2Cu plus 2 E
cell is +0.50 volts

The bonding of the 18KY gold to the sterling by soldering(?) should
result in a cathode with an potential between -1.50 volts for gold
and -0.80 volts for silver(?). This would make the sterling part of
the jewelry piece immersed more noble than the silver and silver
would plate out on the sterling as well as on the gold. The
situation cries out for experiments. The suggestions made here
regarding the possible chemistry would provide a basis for the
design of a number of experiments.

I thank Janet for a very interesting report. Her description of a
beautiful, very even, pure silver plate intrigued me. Perhaps Janet
discovered a simple way to get a superior silver plating on small
items. I am going to let this rest for awhile. My wife tells me if
I don’t get more work out of the shop we will have to hit the
streets with a tin cup! As Kevin in the UK said, “isn’t this fun!”

Captain Blood
"Marlinespike Seamanship in Precious Metals"
@Alden_Glenda_Blood


#9

Alden, that was a fascinating post! But a couple of questions:

  1. You say “The peroxide-acid system will oxidize metals up thru
    gold.” The peroxide-acid system is recommended by many Orchidians to
    remove firescale, which would be mostly copper oxides and ( to a
    lesser degree) silver oxides. Are you saying that if this
    de-staining is used on an object which includes yellow gold as well,
    it would be oxidizing and then reducing the gold too, thereby
    decreasing the yellowness (3D a very undesired result)?

  2. Do I understand correctly that your explanation does not require
    the introduction of iron but rather ‘gets the (plating) job done’ as
    a result of the effects of the peroxide?

  3. I think I would take issue with your statement "The metals of
    greatest interest in Janet’s report are copper, silver and gold."
    Once you say this (and from the jist of the rest of the post), your
    discussion would then be about the usual gold-silver-copper items in
    the usual peroxide-acid system. But this silver(?)plating-effect has
    not usually occurred in my experience and I don’t believe other
    Orchidians who often use peroxide-acid have either reported a
    similar experience or confirmed my experience. In fact, since I
    don’t usually use the peroxide in the acid, I tested to check
    (tried to reproduce the experience) by heating a piece of 18K yellow
    & sterling (to oxidize it) and putting it in the perox-acid, but it
    did not get the odd silver(?) plate on the gold. (It was in fact
    mechanically joined, not soldered, and I gather from your post this
    may make a significant difference). Did it several times and got the
    usual yellow gold from depletion and white silver from blanching as
    desired. Thus, I thought the significant (not usual) metals in my
    report were rather the nickel (in the acid solution as a result of
    having used it with white gold), and the zinc (from the brass
    tweezers). If this plating effect ought to happen with the usual
    yellow-gold/silver/copper pieces in a perox-acid as you suggest, we
    probably would have heard of this in Orchid before.

  4. Is this an error: “This would make the sterling part of the
    jewelry piece immersed more noble than the silver” ?

General comment: Perhaps I might mention that in principle I am
actually not an advocate of doing a lot of experiments to solve
these sorts of questions—there are always way too many variables
in a metalsmith’s studio! I tend to favor a theoretical answer
(usually arrived at with the help of a ‘real’ a scientist) and then
to work in accord—it always comes out successful that way and
saves a great deal of time, effort, and valuable material :o)…!
[This approach enabled me to reconstruct ancient techniques with
100% successful results at a time (in the very early 70’s) when they
were not being taught anywhere—at least certainly not with
consistently successful results!] I think a scientist would be able
to explain the above ‘mystery’ with little difficulty, no…:o)?

Are there Orchidians out there who regularly use hydrogen peroxide
in their sulfuric acid pickle for items made of yellow gold and
silver? It would be very helpful here to know if their collective
experience makes this a good pickle for yellowgold&silver
items—that it does not make the yellow gold white, as happened to
me. And it would of course be good data to help assess Alden’s
theory.

Bye for now,
Janet in Jerusalem


#10

I have been trying to figure out how to reply to your penetrating
questions and comments without ending up with a huge post. I
decided to answer each one of your points briefly here and then
respond with an e-mail to any questions that would involve a rather
lengthy reply. There are, I believe, some statements here that will
raise some eyebrows.

    1) You say "The peroxide-acid system will oxidize metals up
thru gold." . Are you saying that if this de-staining is used on an
object which includes yellow gold as well, it would be oxidizing
and then reducing the gold too, thereby decreasing the yellowness
(3D a very undesired result)? 

Gold and silver are both oxidized in the peroxide-acid systems.
Anyone who uses this type of system to any extent should always
check the spent solution for values before discarding. I am
proposing that reduction of silver ions would occur as a second
step, presumably after most of the peroxide was expended, provided a
suitable metallic ion was present that was capable of acting as the
anodic half of the process. Plating out of gold would be negligible
if it occurs at all.

    2) Do I understand correctly that your explanation does not
require the introduction of iron but rather 'gets the (plating) job
done' as a result of the effects of the peroxide? 

No, the plating is not as a direct result of the peroxide. The
peroxide is necessary as a first step to provide silver ions. The
metal ions that could act in the place of iron include copper, tin,
lead, nickel, cobalt, cadmium, chromium and manganese. Copper and
nickel would be the most likely to be present as the result of usual
procedures. Chromium and manganese could be present if sodium (or
potassium) dichromate (Na2Cr2O7) or potassium permanganate (KMnO4)
were used as the oxidant. The others would be present thru error by
contact with pewter, soft solder, brass, etc. The key is that all
these metals exist as ions in two or more oxidation states as does
iron. For the process I am suggesting to take place the metal in
ionic form must be present in the lower oxidation state. It is thus
available to be oxidized thereby providing the anode half of the
reaction. This is what has to be happening in the case of iron
causing copper to plate out as described with equations in my first
post. This process is not spelled out very clearly in any of the
material relating to metalsmithing, jewelry, etc. that I have read.

     3) I think I would take issue with your statement  "The
metals of greatest interest in Janet's report are copper, silver
and gold." 

A good point. In my attempt to reduce the size of the post I
inadvertently eliminated a sentence that said I was considering only
copper in the post because the chemistry of nickel was not clear cut
in the system being discussed. There is confusion in what I have
read as to exactly which oxide of nickel is involved in firescale of
nickel white gold. Nickel is, in fact, a good candidate as the key
species in your observations. While I don’t feel comfortable
writing equations for the nickel system, I am confident that it
would operate exactly as in the case of the copper and iron systems.
The main consideration in any of these systems is the formation of
the lowest oxidation state of the metal ion involved. This is the
case in the iron system because ferrous ion (Fe 2+) is the species
formed on reaction with acid. That is not the usual case with
copper because it requires an oxidizing acid and thus forms the
cupric ion (Cu 2+). The situation with nickel is not clear to me at
the present. There are at least three oxides of nickel (NiO, Ni2O3,
and NiO2. There are two schools of thought on Ni2O3. One school
claims it is a solution mixture of NiO and NiO2. Some literature
research is obviously needed. Maybe someone on the list is
knowledgeable on nickel oxide chemistry.

  Once you say this (and from the jist of the rest of the post),
your discussion would then be about the usual gold-silver-copper
items in the usual peroxide-acid system. But this
silver(?)plating-effect has not usually occurred in my experience
and I don't believe other Orchidians who often use peroxide-acid
have either reported a similar experience or confirmed my
experience. 

That is why experimental work is needed. It happened! Therefore
there is a reason. The experimental work will require careful
control of the possible factors involved. In my career I have
investigated many cases of problems including explosions. The
cause in most cases was not determined in spite of exhaustive
efforts with experimental setups and a large expenditure of
resources. That could well be the ultimate outcome here. I hope
not.

    4) Is this an error: "This would make the sterling part of the
jewelry piece immersed more noble than the silver" ? General
comment: Perhaps I might mention that in principle I am actually
not an advocate of doing a lot of experiments to solve these sorts
of questions---there are always way too many variables in a
metalsmith's studio! 

It is my understanding that silver was plated out beautifully on the
silver part of your piece as well as on the gold. This is
incidental to the primary problem of why did the plating occur in
the first place. What I was trying to say, and not very well at
that, is that the electrical nature of the intimate combination (if
soldered) of the metals would be between the electrical nature of
silver and gold. The electrical nature referred to is frequently
spoken of as the “osmotic pressure” of the metal in a solution of
metallic salt(s).

I conclude you must have thought that I was suggesting that you
should do the experimental work. Not at all. I would guess that an
art jeweler like yourself would not have the experience and training
that would be needed. I would like to do the work myself if and
when I could acquire the necessary reagents and equipment. There
are other folks on this forum that have chemical expertise judging
from what I read in their posts. Maybe they would be in a position
to check it out. I think it is really imperative that the issue be
examined. My suggestions are based on theoretical considerations.
We all know that what really occurs does not always gybe with
theory. The data I used is from reduction potentials at standard
state. That is, one molar concentrations at 25 deg. C, etc. Your
system was far in excess of standard state in terms of acid and very
low in terms of metal ion concentrations. Therefore, the
suggestions gleaned from standard tables could well be considerably
off the mark.

I tend to favor a theoretical answer (usually arrived at with the
help of a 'real' a scientist) and then 

If by real scientist you mean someone currently active in the type
of chemistry we are dealing with, then you are probably correct that
a good answer might be arrived at very quickly with very little
laboratory work needed to confirm the process. If, on the other
hand, you mean someone with extensive training and experience in a
scientific field, then I would probably qualify (Ph.D., organic
chemistry, Leland Stanford Junior University 1957. Eighteen years
as a research chemist at a chemical division of Eastman Kodak, the
last ten years as head of the organic section of R & D,
author/co-author of 55 U.S. Patents). I must admit that I am not as
up to date as I should be, altho I try. When I retired from Eastman
in 1975 I returned to my other love, the sea. I held a Merchant
Marine Masters license for 25 years. Hence the title “Captain”.
Chemistry is always present aboard ship, especially concerns
involving metals in a conductive solution which, of course, is what
we have been discussing.

I know this post leaves many questions. Further comments and
questions or suggestions could best be discussed via e-mail with a
brief post to the forum as needed. I seem to have trouble being
"brief". Too many years writing research reports and ship’s logs
that are always possible evidence in a court of law I guess. Janet,
I thank you for your comments. It has been fun communicating with
someone in Israel. The wonders of the modern age! I would very
much like to see some of your work, especially the ancient
techniques you reconstructed. Maybe you have a web site you could
steer me to?

“Marlinespike Seamanship in Precious Metals”
@Alden_Glenda_Blood