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