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Author Topic:   Does silver corrode?
fidda

Posts: 45
Registered: Nov 2004

posted 01-24-2005 11:26 AM     Click Here to See the Profile for fidda     Edit/Delete Message   Reply w/Quote
Can silver actually corrode like iron? I am not quite sure about this yet. I know some certain rubbers and plastics can cause corrosion but can silver corrode by itself in contact with air (like iron)? I think tarnish is its way of corrosion but then again i am not sure since tarnish does not actually break down the surface like rust does to iron... or does it? Do the very stubborn black spots on silver that don't go away (unless heavy polishing is applied) corrode the surface of silver or does it simply stain it without effecting the metals strength?

Please help me out here since I have been pondering on this for the last few months and can't seem to find an answer anywhere.

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nihontochicken

Posts: 289
Registered: May 2003

posted 01-24-2005 01:48 PM     Click Here to See the Profile for nihontochicken     Edit/Delete Message   Reply w/Quote
The following is from my experience only, and others' experiences may differ. Silver is almost a noble metal like gold and platinum, but with an Achilles heel, which is sulfur. When chemically attacked by sulfur, it can both form a very thin, even brown-black patina that may be polished away with slight metal loss, and it can form black fissures and pits. Strong acid and salt conditions appear to increase the pitting attack, such as seen in salt dishes and mustard pots, hence the gilding often applied. Note that rubber bands contain sulfur, and so tend to leave black stripes on banded flatware. The black sulfur corrosion product is a chemical conversion of the base silver metal and not an extraneous surface stain, and its removal by means of polishing removes what used to be base silver metal. Conversion products like Tarn-X appear to reduce the silver sulfide back to elemental silver, but they leave a whitish texture because the reduced silver is microscopically spongy instead of solid. Polishing will restore the shine, but mostly by removing the spongy reduced silver layer, and so the result may not be much more conservative than skipping the chemical reducing agent and simply polishing in the first place. Finally, each piece seems to have its own personality. Some pieces seem to tarnish more easily than others, and on some pieces, the tarnish seems to be much more difficult to remove than on others. For heavily tarnished pieces that are very resistant to polish, I recommend using Tarn-X or the like (more than one application may be necessary, wash thoroughly afterwards), and polishing away the resultant whitish patina later. An exception is three-dimensional pieces, such as repousse, where the chemical reducing agent should NOT be introduced into the crevices. FWIW.

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Kimo

Posts: 1232
Registered: Mar 2003

posted 01-24-2005 02:06 PM     Click Here to See the Profile for Kimo     Edit/Delete Message   Reply w/Quote
Corrode? Yes.
Like Iron? No.

Iron corrosion is mainly the result of its chemical reaction with oxygen in the air or water resulting in the creation of iron oxide which is commonly known as rust.

Silver is a fairly stable metal and under normal circumstances does not corrode quickly. Silver corrosion is mainly the result of its chemical reaction to sulfur in the air or water resulting in the creation of silver sulfide which is commonly called tarnish. Silver can also react with chlorine or bromine in a similar way. Silver does not react at the same speed to these as iron reacts to oxygen and so tarnish typically does not wind up converting large percentages of the metal into the corrosion products unless subjected to high concentrations of sulfur, chlorine or bromine over a long time. Sulfur can be sorbed down below the surface forming a "stain" that can only be removed by polishing the metal away until you get down to "unstained" metal. The best way to avoid all of this is to keep silver away from any possible sources of sulfur, chlorine or bromine. One of the biggest sources of sulfur in a typical house is from cooking or heating gas. As such, the worst place to store silver would be in or near a kitchen with a gas stove or in or near a basement with a gas heater.

Silver objects are rarely made of pure silver. They are typically alloys of silver plus copper plus other metals, or they are plated over these other metals. These other metals tend to be more aggresively reactive to elements found in air or water, and the higher percentage of these other metals, the more likely the item is to have these additional reactions.

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fidda

Posts: 45
Registered: Nov 2004

posted 01-24-2005 02:41 PM     Click Here to See the Profile for fidda     Edit/Delete Message   Reply w/Quote
hmm ok ok I see thanks a lot.
2 more questions left -
  1. will tarnish spread? What I mean is if there is a spot of tarnish that is not polished out but the silver is maintained properly by washing etc. will the spot of tarnish still grow?

    and

  2. Will a stubborn spot of tarnish ''eat away'' the silver underneath by time and as a result pit the silver? in which case better removed and if harmless not worth the bother of heavy polishing to remove a few small spots at the back of a piece.
Thanks for all the help.

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nihontochicken

Posts: 289
Registered: May 2003

posted 01-24-2005 05:00 PM     Click Here to See the Profile for nihontochicken     Edit/Delete Message   Reply w/Quote
Again, my opinion only.
  1. Yes and no. If the item is kept completely clean, dry, and unexposed to sulfur (and, as mentioned by another, chlorine/chloride and bromine/bromide), then the spot won't grow. If the storage is less than ideal, then the tarnished spot may tend to grow before clean metal tarnishes, because the spot provides a favored place for condensation and the collection of aggravating impurities such as salts and salt contaminated skin oils.

  2. Yes, the tarnish spot may result in a deep pit if not removed, as indicated above. Any spot of tarnish is already a "pit" in that sulfide (most typically) attack has eaten into the metal at least a little bit. Often the "pit" is rather shallow and not noticeable when the sulfide layer is polished away. It is probably best to try to polish tarnish spots out if they are not deep. OTOH, in some cases, as often happens with salt dishes, the pitting is so deep that too much metal would have to be removed to get the pits out. You have to eyeball it when you polish and determine how deep the pit really is.

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Kimo

Posts: 1232
Registered: Mar 2003

posted 01-25-2005 09:43 AM     Click Here to See the Profile for Kimo     Edit/Delete Message   Reply w/Quote
Once silver has been converted to silver sulfide (or silver chloride or silver bromide) it can not be converted back other than by re-smelting the metal which of course entirely defeats the whole purpose of preserving of the object. All that you can do once tarnish has formed is to remove the stuff which on most items is a simple light hand polish. Yes, this does remove the tarnish which by definition is removing a tiniest bit of what used to be the surface layer of silver of the object and the rubbing will also often remove a tiniest bit of unconverted metal as well. Occassional polishing by hand for this purpose does not remove much and a silver object can last hundreds of years in this way with only minimal loss of surface metal and detail. Frequent or vigorous polishing/scrubbing, or using a dreaded mechanical buffing machine, will hasten this loss process dramatically. Most silver collectors and conservators cringe at the thought of this kind of drastic approach - especially buffing machines - in that it not only removes too much metal but it also removes an objects patina which is a large part of old silver's beauty. However, in certain circumstances such as on low value items of little historical importance it might be justified. If an object has high value and/or historical importance my advice is to simply clean it as best as you can with a soft cloth, good quality silver polish and a light touch - then store it appropriately to reduce the tarnish process. Leave the spots as part of its history and patina.

Another viable approach to long term conservation for objects that are not used or displayed can be to simply leave the tarnish and keep the object in a cool, dry place with no ready source of sulfur, chlorine or bromine. If you think about ancient silver coins, you can find ones today that are 2,000 years old that when cleaned up look almost as fresh as the day they were buried. These indicate that these were not near sources of sulfur, chlorine or bromine. The initial tarnish layer acted as a protectant layer and prevented additional conversion of the silver. On the other hand if you think about some silver coins found in 300 year old shipwrecks that were buried in high sulfur anoxic sea bottom mud - some of these have been converted completely or almost completely to silver sulfide and are just black lumps now.

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nihontochicken

Posts: 289
Registered: May 2003

posted 01-25-2005 11:04 AM     Click Here to See the Profile for nihontochicken     Edit/Delete Message   Reply w/Quote
I agree with most of your response, Kimo, excepting the following, "Once silver has been converted to silver sulfide (or silver chloride or silver bromide) it can not be converted back other than by re-smelting the metal which of course entirely defeats the whole purpose of preserving of the object. All that you can do once tarnish has formed is to remove the stuff which on most items is a simple light hand polish."

I believe TarX and similar "dip" type products utilize a thiourea in an acid solution that chemically reduces the silver sulfide back to metallic silver. The same effect may be obtained by immersing the tarnished silver in hot water to which has been added sodium bicarbonate (baking soda), with the silver object in electrical contact with aluminum metal immersed in the same solution (aluminum foil is often used). In this case, some aluminum is oxidized, reducing the silver ion in the silver sulfide to elemental silver. I believe this method has been mentioned elsewhere on this forum. Again, it should not be used for repousse work, because it will reduce the tarnish in crevices which should appear dark.

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Arg(um)entum

Posts: 304
Registered: Apr 2002

posted 01-25-2005 01:03 PM     Click Here to See the Profile for Arg(um)entum     Edit/Delete Message   Reply w/Quote
"... reducing the silver ion in the silver sulfide to elemental silver. "

I quite agree with you that this happens. But from the limited experimentation I did with it some time back, I concluded that:

  1. The regained elemental silver sits on the surface so loosely that it is lost by the still necessary slight polishing following the procedure. If there is any benefit to using the method then it is that this polishing can be less vigorous than it would need be without the prior electrolytical treatment. Thereby there may be a slight reduction in the loss of metal.

  2. I found that recessed areas such as in shaped (cast) handles do not get recuced as effectively as broad flat areas or high points. So I suspect that repousse work shouldn't suffer if watched carefully while under treatment.

    But again, I think that the benefit of the method is generally minimal though I wouldn't discourage anyone with truly heavily tarnished items from giving it a try.

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fidda

Posts: 45
Registered: Nov 2004

posted 01-25-2005 01:14 PM     Click Here to See the Profile for fidda     Edit/Delete Message   Reply w/Quote
Thanks a lot for your help.

I managed to polish out the spots, a bit of hand polishing and they were out!

Do you recommend removing the tarnish in the chased decoration? If so how? By the use of dips or by carefully and painfully long #(to get into the hundreds of lines) hand polishing?

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nihontochicken

Posts: 289
Registered: May 2003

posted 01-25-2005 02:38 PM     Click Here to See the Profile for nihontochicken     Edit/Delete Message   Reply w/Quote
Agree with your comments, Arg (per my original remarks re the "whitish texture" left by Tarn-X and the like). However, while it may not be significantly more conservative, I have found it easier to use the "dip" on heavily tarnished pieces and afterwards remove the whitish cast by polishing than it is to remove the tarnish by polishing alone.

Re your question on "chasing", fidda, do you mean repousse work (which "chasing" more properly implies), or engaving (your reference to numerous "lines")? Arg covered the former well. Re the latter (engraving), I personally don't think it is worthwhile or advisory to try to remove tarnish inside engraved lines, other than that which is coincidentally removed by normally polishing the adjacent flat areas. I suppose you could try using a thin bristle (soft) toothbrush and polishing agent, but this is not conservative practice, IMO, as it will eventually round off the edges of the incised lines and "soften" the engraving. Plain polishing does this fast enough!

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fidda

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Registered: Nov 2004

posted 01-25-2005 04:32 PM     Click Here to See the Profile for fidda     Edit/Delete Message   Reply w/Quote
Well it is sort of engraving you see, it is a flat chased salver i think you get the idea it is not embossed it is done with a hammer and punch u believe.

Well now I get rather confused because if tarnish ''harms'' the silver metal itself and will pit it etc why do some consider tarnish in for example reposse as good in order to shadow the features etc.

Will this not damage the silver surface in time and so better removed? I do not really get it when we establish that tarnish corrodes the metal and then we say we might as well leave it in engraved lines, reposse work and in chasing.... is this tarnish not damaging?

Thanks for all the replies.

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nihontochicken

Posts: 289
Registered: May 2003

posted 01-25-2005 05:50 PM     Click Here to See the Profile for nihontochicken     Edit/Delete Message   Reply w/Quote
Okay, was kinda hoping to avoid this, because corrosion is such a complicated topic, as much art as science. There are a number of kinds of corrosion, the major types being general corrosion, pitting and crevice corrosion, and stress cracking corrosion. The first two are of importance for silverware. In general corrosion, the metal surface is chemically oxidized more or less uniformly. The resultant oxide (or in the case of silver, sulfide) layer may or may not be protective of the metal underneath. The typical porous, flaky red rust of iron and steel (hematite, Fe2O3) in common atmospheric conditions is fairly non-protective, and corrosion continues at a fairly constant rate. But it can be converted by a reducing agent (like Corroseal TM) into magnetite (Fe3O4), a hard, tight and protective layer. Some metals are "self-protective" under mild conditions. Zinc and magnesium are fairly reactive metals, but under mild atmospheric conditions tend to form a solid, self-protective oxide layer, slowing the general corrosion rate. The sulfide layer formed on silver tends appears to be hard and tight, and somewhat protective in mild conditions (not wet and not aggravated by chlorides).

However, pitting corrosion is a different animal. Here, as the name implies, the corrosion is not uniform, but rather it is concentrated in certain small spots. It is often initiated in a microscopic defect in the metal surface, a small nick or microcrack. Or it can start where a bit of salty dust settles and provides a condensation spot when the humidity gets high. Usually pitting corrosion requires at least intermittent wetting, either water immersion or at least surface condensation. It is very much promoted by salt contamination, particularly by chlorides as in common table salt, dust, and sweat. In pitting corrosion, the metal oxide (or, for silver, sulfide) "rust" cap, instead of protecting the metal, actually assists in an auto-catalytic reaction utilizing charge transfer through chloride ion transport that results in quite highly acidic conditions in the base of the pit. Once started, the pit just gets worse and worse, as long as it is wet. And condensation can occur in the pit long before it is apparent on the bare metal surface, and at less than 100% humidity. It is therefore best to polish out pits when they are small.

So a uniform sulfide layer on a silver piece, while not aesthetically pleasing in places, is not so bad a thing by itself. But any underlying pits are bad, of no redeeming social value. And yes, engraved lines can be considered a metal surface "defect" conducive to the initiation of pitting. But what can be done? To eliminate the "defect" is to erase the engraving! And attempting to get all the sulfides out of an engraved line via toothbrush and polishing cream is indeed erasure in action. Better to just keep the engraved portion clean and dry.

Sorry for the bandwidth, hope no one banged his/her forehead on the monitor as consciousness faded!

[This message has been edited by nihontochicken (edited 01-25-2005).]

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adelapt

Posts: 406
Registered: May 2003

posted 01-25-2005 08:43 PM     Click Here to See the Profile for adelapt     Edit/Delete Message   Reply w/Quote
It is interesting to note that Arthur Grimwade (of sainted memory) in his book "Silver For Sale" mentions visiting one of the Rothschild mansions, and seeing there a display of silver "au naturel", left for ages with its layer of uniform tarnish. Not only did this save the butler a heap of work, it also saved the surfaces of the rare objects involved the stress and wear of regular cleaning. Evidently this approach was not uncommon among older European collections.

There is a view that the range of colours on a tarnished object has its own attractiveness. There can be some wonderful deep purples in there!

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fidda

Posts: 45
Registered: Nov 2004

posted 01-26-2005 05:41 AM     Click Here to See the Profile for fidda     Edit/Delete Message   Reply w/Quote
OK thanks a lot guys that was very helpful.

One last thing.... I use silvo liquid polish anyone knows how abrasive this is? and also I noticed the same company produces silvo dip that comes in a jar and looks like water, can one spread this on a tray fro example since dipping is impossible (you are never going to get a tray in that jar!) and if so do you recommend I do this just once to get rid of most tarnish in the cast work and then just keep the piece as clean as possible?

Thanks a lot !

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nihontochicken

Posts: 289
Registered: May 2003

posted 01-26-2005 08:26 PM     Click Here to See the Profile for nihontochicken     Edit/Delete Message   Reply w/Quote
Sorry, I don't have much experience with the different "dip reducing" type polishes, will have to punt on this one.

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fidda

Posts: 45
Registered: Nov 2004

posted 01-27-2005 12:38 PM     Click Here to See the Profile for fidda     Edit/Delete Message   Reply w/Quote
Its OK thanks for all the help so far!
Hmm OK I did it, I used the dip and.......... I'm amazed at the result! it really is effective. I would say 95% of tarnish went leaving a few stubborn spots that can easily be polished out by hand. now I know many are not going to agree with me on this but its just personal taste and I prefer my silver to be ''tarnishless'' so to speak! yep no tarnish in the cast work or in the engraving, but its the way I like it.

Just the last few questions ....

  1. what is the best way to remove the whitish residue left by the tarnish?
  2. can this just be left on, it really doesn't show, or will it turn back in to tarnish or is it harmful?

    and

  3. It says not to leave the silver in for more than 5 minutes I guess it will start attacking the surface after that, now if i use the dip again will I have another 5 minutes or will it continue to act upon the previous time? i.e., can you use it for 5 minutes lets say for 4 separate times or 5 minutes in all for a single piece?

    and....the last! ....

  4. what happens to the surface if left for long in dip (Goddard's silver dip), I heard the surface turns brownish is this true and if so can it be fixed?

Thanks so much I really am full of questions but I think these are my last!

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Scott Martin
Forum Master

Posts: 8747
Registered: Apr 93

posted 02-01-2005 12:04 PM     Click Here to See the Profile for Scott Martin     Edit/Delete Message   Reply w/Quote
Fidda - I deleted your other post and have included it here since it really seems like it is a continuation of this thread:
quote:
Regarding silver dips that remove tarnish (such as Goddard's silver dip) do these really discolor (turn brownish) the silver if left for too long?

Also if that is the case can it be fixed?

Also if true will repeated use of silver dip cleaner result in this or will it only discolor the silver if it is left for too long in one particular wash?

AND....... I know I am full of questions!.....

Is it OK to leave the white deposit remaining where there was tarnish or will it turn back into tarnish by time? and if better removed will polishing do the job?

Thanks in advance!


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fidda

Posts: 45
Registered: Nov 2004

posted 02-01-2005 01:45 PM     Click Here to See the Profile for fidda     Edit/Delete Message   Reply w/Quote
Well OK you are partly right it is a continuation although it doesn't fall under the title of corrosion of this tread but Thanks anyway.

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nihontochicken

Posts: 289
Registered: May 2003

posted 02-01-2005 09:02 PM     Click Here to See the Profile for nihontochicken     Edit/Delete Message   Reply w/Quote
  1. Simple polishing will remove the whitish haze left by the dip type cleaners.
  2. The whitish residue can be left on, it is just spongy silver, but it will likely be a little bit more susceptible to tarnish due to the micro crevices and high surface to volume ratio as compared to solid silver.
  3. & 4. After a time, the thiourea oxidizes and deactivates, but the acid does not. Apply in separate doses, and wash off in between. Don't try to leave it on as a preservative, it isn't.

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