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Oxidation or patination?

More on the polishing saga…

The other day the entire batch of stuff that I had polished
patinated while in a bucket of soapy water (I have to scrub polishing
compound off with a toothbrush after polishing). Every single piece
patinated. They came out looking bronzed, with irridescent red and
blue highlights. Kind of a rainbow effect, with a brassy gold and
the irridescent red and blue instead of a copper color. Some of this
then turned black after sitting and drying for about three hours.

At first these folks told me I had not polished properly (I had)
then they said that copper OXIDIZES IN WATER. They insisted that
water is the cause of oxidation, and not exposure to oxygen. In any
case, this coating looked nothing like any oxidized copper I have
ever seen.

Possibly I just haven’t seen all the different ways copper can be
oxidized, so I’m asking you folks.

I believe this stuff patinated because of acid or other chemical
residue. Some of these pieces smelled particularly strongly of the
acid bath. I have used copper tongs extensively to fish stuff out
of soapy water in the ultrasound and I’ve never seen this effect
before.

These folks have been doing this for years (copper plating and
selling these statues) and they insist that they never leave the
pieces in water after polishing for more than a few minutes because
the copper will oxidize under water if its in there more than about
10 or 15 minutes. They repeatedly told me that metals oxidize under
water, which is contrary to what I thought I knew about oxidation
(eg, rust is oxidation of iron, but if you have carbon steel shot
you’re SUPPOSED to keep it under water to prevent rusting, eg, to
stave off OXIDATION).

Enlighten me…

Sojourner

Hi Sojourner, Cynthia Eid has a great section in her recent article
about Argentium silver that discusses the correct meanings of the
words oxide and oxidize. Basically, jewelers tend to say "oxidize"
even though this is inaccurate. When we use an antiquing agent to
darken silver, a sulfide forms - not an oxide. Likewise tarnish is
not an oxide - it is a silver and copper sulfide.

Best Regards,
Kevin Whitmore
Rio Grande

Hello Kevin; I sent this a couple of days ago, but somehow it got
sent to a chemical supplier rather than Orchid. How that happened
is beyond me. I apologize for being late.

It seems to me that you are saying in your post that the term
"oxidize" is not correct when talking about the formation of black
silver sulfide on silver metal because an oxide is not formed.
Maybe that is not what you intended to say? Chemically speaking the
term “oxidize” is correct under the conditions given.

Oxidation is defined as the loss of electrons. Oxygen or an oxide
is not necessarily involved. For silver metal to form the sulfide
it is necessary that the silver give up electrons. That can be
written as;

Ag(zero charge) = Ag(1 plus) + e(1 minus)

When something is oxidized something else has to be reduced.
Reduction is defined as the gaining of electrons. The species being
reduced in the case of sulfide patination of silver is a little more
complicated than, for example, the reaction of zinc with
hydrochloric acid. Looking at that reaction for a minute we have;

Zn(zero charge) + 2 H(1 plus) + 2 Cl(1 minus) = Zn(2 plus) + 2
Cl(1-minus) + H2

Here the zinc gave up two electrons to become the zinc cation. The
electrons were accepted by the hydrogen ions to give hydrogen gas.
The zinc was oxidized and the H(1 plus) was reduced. The oxidizing
agent (that which caused the oxidation of the zinc) was the hydrogen
ions and the reducing agent (that which caused the reduction of the
hydrogen ions) was the zinc. Getting back to the case of sulfide
patination, the oxidizing agent is the hydrosulfide anion no matter
what sulfur compound is used. This is because there is essentially
no sulfide anion in aqueous solutions of sulfide salts (Na2S for
example). The sulfide anion is an extremely strong base which
abstracts a hydrogen ion from water to give the hydrosulfide anion
as follows;

(1) S(2 minus) + H2O = HS(1 minus) + OH(1 minus)
The H(1 plus) part of the HS(1 minus) accepts the electron given up by
the silver and is thereby reduced to H(zero). The reactions are:

(2) Ag(zero) = Ag(1 plus) + e(1 minus) Oxidation
(loss
of electrons)

(3) HS(1 minus) + e(1 minus) = H + S(2 minus) Reduction (gain
of
electrons)

(4) 2 Ag(1 plus) + S(2 minus) = Ag2S (on the Ag
surface)

(5) H(zero) + H(zero) = H2 (very
fast)

Any sulfur anion from reaction (3) that is not close enough to the
silver ions on the silver surface to form Ag2S is instantly
converted to HS(1 minus) via reaction (1).

To summarize, any time a metal reacts to produce a metal compound
the metal has been “oxidized” even if oxygen was not involved. Be
aware that it is not necessary that the oxidized entity have a zero
charge as is the case with metals. For example, the half reaction;

2 Cl(1 minus) = Cl2 + 2 e(1 minus) is an oxidation. It is an
oxidation because the chloride anions lost an electron to become
zero charge chlorine atoms.

I hope the above is germane to the previous posts on the topic.
Confusion about “redox” reactions seems to be very common. If I have
not read the previous posts correctly, perhaps the here
will still be worthwhile. I just hope I have not created more
confusion than may already exist.

Captain Blood
"Marlinespike Seamanship in Precious Metals"
@Alden_Glenda_Blood
Cap

    The other day the entire batch of stuff that I had polished
patinated while in a bucket of soapy water (I have to scrub
polishing compound off with a toothbrush after polishing).  Every
single piece patinated. 

Hi Sojourner, I was thinking it could be the soap if you’re using a
dishwahing liquid. The common ones contain ammonia and ammonia
patinates copper blue.If that’s the problem, not letting them sit in
the soapy water and rinsing the soap off immediately should work for
you.

HTH,
Laurie
www.laurieevinger.com

It seems to me that you are saying in your post that the term
"oxidize" is not correct when talking about the formation of black
silver sulfide on silver metal because an oxide is not formed.
Maybe that is not what you intended to say? Chemically speaking the
term "oxidize" is correct under the conditions given. 

Wow! That was impressive.

But what I was actually asking in the original question, which I may
have worded badly because I do natter on at times, is this:

Is it more likely that the gold, blue, and red irridescence that I
saw on the COPPER metal I had painstaking polished and then dropped
in a bucket prior to cleaning the polishing compound off with a
toothbrush and dish soap… Is it more likely that this was caused
by acid and other chemical residues (which there are a LOT of, most
involving sulfur) on and inside the copper pieces that diffused
through the water resulting in patination of the copper surface…

OR is it more likely that this sort of irridescence would be caused
by “oxidation”, which when used by these folks means exposure to
plain water or humidity (but which when used by ME means exposure to
oxygen or normal atmosphere that results in a usually gradual change
to the surface of the metal, typically greenish for copper and ugly
black/grey for silver)?

Thanks.
Sojourner

    I was thinking it could be the soap if you're using a
dishwahing liquid. The common ones contain ammonia and ammonia
patinates copper blue.If that's the problem, not letting them sit
in the soapy water and rinsing the soap off immediately should work
for you. 

I don’t know, there are so many other chemicals involved I think it’s
probably a mix of things. Today while I was polishing, some of the
liquid from inside one of the statues ran out and down my leg. It
burned. It was diluted acid, but it was still acid. I’m glad I wear
leather gloves and had a water bottle handy.

Doctor’s appointment Friday so I can get permission (I hope) to quit
this crazy job, LOL!

Sojourner

        Is it more likely that this was caused by acid and other
chemical residues (which there are a LOT of, most involving sulfur)
on and inside the copper pieces that diffused through the water
resulting in patination of the copper surface.... 

Most likely sulphur

        OR is it more likely that this sort of irridescence would
be caused by "oxidation", which when used by these folks means
exposure to plain water or humidity (but which when used by ME
means exposure to oxygen or normal atmosphere that results in a
usually  gradual change to the surface of the metal, typically
greenish for copper and ugly black/grey for silver)? 

This is also caused by sulphur in the air. If you are anywhere near
a coal or oil burning heater then you will get some air borne
sulphur.

Bill Bedford

Morning Captain,

I believe Kevin was trying to explain the diffence in corrosion
product as opposed to the actual reaction mechanics. You are
correct in that the metal has undergone oxidation as it looses
electrons. In metals, this reaction is called corrosion regardless
of the actual corrosive element involved (oxides, carbonates,
sulfides, chlorides…). What Kevin was explaining is that the
reaction of sulfur on silver causes sulfur corrosion products, called
tarnish or sulfides, and not oxygen corrosion products, oxides. He
was pointing out that jewellers are not refering to redox reactions
when they use the terms oxide and oxidise but to the corrosion
process/product in which oxides are formed. The confusion is simply
that jewellers have grouped all corrosion - sulfide, chloride, oxide,
chloride - as oxides rather than naming them for what they are. Clear
as mud? :slight_smile:

Eileen

Thanks Eileen,

It is confusing. Sorry I helped further the confusion. Captain was
correct, I misspoke - it is accurate to say something has been
oxidized, even when no oxygen is present. It is also accurate to say
that it is not oxide on an antiqued or tarnished piece of sterling.
The darkening is almost always a sulfide. The initial post referred
to keeping steel shot under water to prevent rusting, and compared
that to oxidation. In the case of rust, the metal is reacting with
oxygen. In the case of inadvertent patination while under water, the
metal is likely reacting with sulfur.

Kevin Whitmore
Rio Grande
7500 Bluewater Road NW
Albuquerque, NM 87121
USA
(505) 839-3114 (voice)
(505) 839-3135 (fax)
@Kevin_Whitmore

    It is confusing. Sorry I helped further the confusion. Captain
was correct, I misspoke 

Thanks. That’s what I thought. It’s not the water that does it -
its the chemical residue that diffuses into the water. When they
complain of “oxidizing” when exposed to “humidity”, its probably due
to the chemical FUMES from the unvented copper plating process
causing the problem, and not the humidity per se.

Sojourner