The Chemistry of Silver Tarnish

Ted BeyerEvery Dark Spoon has a Silver Lining – Using Chemistry on Silver Tarnish

by: Ted Beyer

The holidays have just passed, and for most people, part of the celebration involves getting out the “good” china and flatware to set a festive table. Like most people, I have a few pieces of silver and silver plate that don’t get a lot of use, but are nice to use on special occasions. Thing is, over time, silver tarnishes – it turns dark, and if allowed to continue long enough, it turns black. So out comes the polish and you get to spend all kinds of time polishing it. And with silver plated items, if you do that often enough, eventually, you will polish the silver completely off!

chemistry silver tarnish

Tarnished Silver Platter-2

But wait – science will come to the rescue!

The tarnish is actually the result of a chemical reaction between the silver and sulfur-containing substances in the air. The silver is actually combining with sulfur and forming silver sulfide. Silver sulfide is black. When a thin coating of silver sulfide forms on the surface of silver, it darkens the silver. That’s what we call ‘tarnish.’ There are two ways to remove the offending silver sulfide. One way is to strip it from the surface. The other is to reverse the chemical reaction and turn silver sulfide back into silver. Polishes that contain an abrasive, shine the silver by rubbing off the silver sulfide and some of the silver along with it. Another kind of tarnish remover dissolves the silver sulfide in a liquid. These polishes are used by dipping the silver into the liquid, or by rubbing the liquid on with a cloth and washing it off. These polishes also remove some of the silver. Obviously, the better way to do away with the tarnish is to reverse the reaction that created it in the first place, and save your silver! This is something that is easy to do at home with commonly found items. You will need:

  • Your tarnished piece of silver
  • A container large enough to completely immerse the silver in (think roasting pan…)
  • Aluminum foil to cover the bottom and sides of the pan
  • Tap water (enough to fill your pan so you can completely immerse your silver)
  • A pot to heat the water
  • Pot holders – you will need these to handle the pot of  hot water
  • Baking soda (about 1 cup per gallon of water)

Line the bottom of the pan with aluminum foil. Place the silver object on top of the aluminum foil. Make sure the silver touches the aluminum.  Place the pan in an empty sink.

imgp1149

Heat the water to a full rolling boil. Remove it from the heat – the next step will make the water froth a bit and it may spill over, so be careful! Add about one cup of baking soda for each gallon of water. (If you need only half a gallon of water, use half a cup of baking soda, etc.). As I mentioned, this will cause the water to froth quite a bit and it may boil over the top of the pot. That’s ok. Immediately pour the hot baking soda/water mixture into the pan, and so that it completely covers the silver. Almost immediately, the tarnish will begin to disappear. If the silver is only lightly tarnished, all of the tarnish will disappear within several minutes – or even seconds! If the silver is badly tarnished, you may need to reheat the baking soda and water mixture, and give the silver several treatments to remove all of the tarnish.

tarnished silver sugar bowl

You can see that only half of this sugar bowl was placed in the solution of baking soda and water.

imgp1154

You will notice that often the water will look slightly yellowish and dirty and smell faintly of rotten eggs. That is the sulfur that has been detached from the silver. I found what when using this process, trying to re-use the solution more than twice really slowed the reaction down. Since Baking soda is cheap, I just make up a new batch of water and baking soda every two or three treatments. You will also notice that the silver will dry with a bit of a white residue on it. That is just residual baking soda – a quick rinse in clean water and a dry with a towel will make that disappear, leaving you with nice bight silver.

And now for the real science:

As previously mentioned, this tarnish-removal method uses a chemical reaction to convert the silver sulfide back into silver. Many metals in addition to silver form compounds with sulfur. Some of them have a greater affinity for sulfur than silver does. Aluminum is one of them – and luckily, most people already have lots of it in a convenient to use form in their kitchens. In this case, the silver sulfide reacts with aluminum. In the reaction, sulfur atoms are transferred from the silver to the aluminum, freeing the silver metal and forming aluminum sulfide. Chemists represent this reaction with a chemical equation.

3 Ag2S

  +

2 Al

————>

6 Ag

  +

Al2S3

silver sulfide

aluminum

silver

aluminum sulfide

The reaction between silver sulfide and aluminum takes place when the two are in contact while they are immersed in a baking soda solution. As with almost all chemical reactions, the reaction is faster when the solution is warm. The solution carries the sulfur from the silver to the aluminum. The aluminum sulfide may adhere to the aluminum foil, it may be found floating in the water having detached from the foil or it may form tiny, pale yellow flakes in the bottom of the pan. The silver and aluminum must be in contact with each other, because a small electric current flows between them during the reaction. This type of reaction, which involves an electric current, is called an electrochemical reaction. Reactions of this type are used in batteries to produce electricity. Hopefully, this little bit of science will make your future holiday preparations faster and easier – not to mention saving your silver to be enjoyed for many years to come.

23 Responses to The Chemistry of Silver Tarnish

  1. Doug Koch says:

    I have been told that using ‘washing soda’ (Na2CO3) is more efficient/less time consuming and perhaps easier on the silver piece than Sodium Bicarb. Are these statements correct?

    • Tami O'Connor says:

      Hi Doug,

      Thanks for asking. We sent this question out to Ron Perkins, the founder of EI and retired Chemistry of over 30 years and this is his answer.

      Most of the methods published use aluminum and hot water containing baking soda in order to make a slightly basic solution. A few recipes suggest substituting baking soda with washing soda which would give a stronger basic solution and cause the reaction to occur faster, but this substitution is definitely not recommended for valuable silver pieces.

      Although this electrochemical process will indeed remove the tarnish and make the silver appear shiny again, the process should not be used on valuable silver pieces such as coins, antique silver, or sterling. The problem is that the process returns the silver from the tarnish to the surface of the metal object but probably, not where the silver atoms were originally. Under a microscope, the originally smooth silver surface can be seen becoming more pitted and rougher each time the tarnish removal process is used.

      I hope this helps. Please let us know if you have any other questions.
      Tami

  2. beadspitter says:

    Loved this article. Thank you so much for posting it – I love having the explanation as well as the technique.

    Will this work on copper/brass? From what I’m reading, the darkening/verdigris on copper is more likely to be copper sulfide than copper oxide, which made me wonder whether the sulfur could be reclaimed from that, too. Do you know?

    • Tami O'Connor says:

      In order to find the answer to your question, we reached out to Ron Perkins, the founder of Educational Innovations and Chemistry teacher for over 30 years. This is what he said:

      If the tarnish is a sulfide of copper, the same method may work to obtain pure copper. I’ve never thought of trying it nor have I read anything about it. Sounds like a good experiment for some student to try.

  3. Eva Wild says:

    Sooooo, having a little trouble with an unclear antecedent. When you suggest that we not use “this” treatment on antique silver, you mean the entire concept of the electrochemical reversal of the tarnishing? or just the washing soda version? I have used washing soda for tarnish removal decades ago, but was not happy with the result, probably for the reason you give. The texture was never as silky and lovely ever again. I now deal in fine antique silver and just yesterday could feel the temptation to start boiling when a particularly intricate teapot was giving me fits. I suspect you meant either medium would be damaging.

    • Tami O'Connor says:

      Hi Eva,

      This from our founder (and chemistry expert), Ron Perkins:

      The problem is that although the chemical reaction with Al, and baking soda removes the sulfide from the silver sulfide, the remaining silver doesn’t end up in its original place on the object, making the object no longer shiny and smooth.

      You can find some of the best silver cleaning advice is in this article:
      http://www.925-1000.com/aPolish.html

      One rule: never clean antique coins !!!! Sell them “as is” and let the buyer decide what to do.

      Hope that helps!
      ~Tami
      Educational Innovations

  4. Mary says:

    Im wondering if silver gets tarnidhed and is left in that state, will the tarnish “eat away” at the silver over time, thoroughly corroding it, such as rust does to iron?

  5. Miss Razz says:

    Thank you so much for your cool post. We homeschool and I love thrifting, so for an outing we specifically searched for silver items at the Goodwill in our neighborhood. We found some amazing candle sticks and your pots totally cleaned them, restored brilliantly!

  6. Dr Dirt says:

    Would the reaction be expedited by insulating the silver target from the aluminum and applying a voltage between the two? (positive to aluminum, negative to silver)

    • Tami O'Connor says:

      In electrochemical reactions, one always needs a complete circuit, otherwise nothing happens. If the Ag is insulated from the Al it is not a complete circuit.

      This would be like attaching an Al wire to the positive terminal and a silver wire to the negative terminal of a battery and not letting the wires touch. — one would not expect anything to happen. There can be no flow of electrons.

      Electrochemistry is fun to think about and relatively safe to see what happens if you use only a single flashlight battery, a D or C cell.

      Have fun experimenting.

      Ron Perkins

      • Lawrence says:

        I tried the experitment – DC powere supply, + to aluminum, – to silver, sodium carbonate (baking soda) solution. It works!. Rapid evolution of H2S where previously was getting nowhere with just hot solution. Current starts to flow at about 2.7 Volts.

    • Lawrence says:

      I tried the experiment – DC power supply, + to aluminum, – to silver, sodium carbonate (baking soda) solution. It works!. Rapid evolution of H2S where previously was getting nowhere with just hot solution. Current starts to flow at about 2.7 Volts. Silver cleans up nicely but needed a minor final once over with silver cleaner.

      • RW says:

        hi lawrence,

        could i bother you to give me more information on this experiment you did?

        id really appreciate it, thanks!

  7. Priscila says:

    Thanks for sharing your experiments!
    Reading through it I saw that the reaction is partially incorrect. The product of the reaction is aluminium oxide, Al2O3 and not aluminium sulfide, Al2S3. The latter is actually unstable in aqueous medium, decomposing to Al2O3. See overall reaction below:
    OXIDATION: 2 Al(s) + 6 OH– (aq) –––> Al2O3(s) + 3 H2O (l) + 6 e–
    REDUCTION: Ag2S(s) + 2 H2O (l) + 2 e– –––> 2 Ag(s) + H2S (aq) + 2 OH– (aq)
    Overall reaction: 3 Ag2S(s) + 2 Al(s) + 3 H2O (l) –––> 6 Ag(s) + 3 H2S (aq) + Al2O3(s)

    • Tami O'Connor says:

      Thanks so much, Priscilla! We always appreciate input! For those reading, Priscilla correctly noted that Al2S3 in water decomposes spontaneously into H2S and Al2O3. For those taking advanced or college chemistry, Priscilla even provided the two Half-Reactions:
      OXIDATION: 2 Al(s) + 6 OH– (aq) –––> Al2O3(s) + 3 H2O (l) + 6 e–
      REDUCTION: Ag2S(s) + 2 H2O (l) + 2 e– –––> 2 Ag(s) + H2S (aq) + 2 OH– (aq)
      Overall reaction: 3 Ag2S(s) + 2 Al(s) + 3 H2O (l) –––> 6 Ag(s) + 3 H2S (aq) + Al2O3(s)

      Ron Perkins
      Educational Innovations

  8. Jahanvi says:

    Please can you tell what is the role of aluminium foil in this activity.

  9. Jennifer says:

    Hello,
    I have a ring that I would like to do this for. The ring has a sapphire stone in it. Will the sapphire react with the baking soda, aluminum, or hot water?

    Thank you 🙂

    • Tami O'Connor says:

      Hi Jennifer,

      To be perfectly honest with you, that’s a much better question for a jeweler than for us. I wouldn’t think so, but I wouldn’t take a chance without asking a professional jeweler.

      Tami
      Educational Innovations

    • Katie Donovan says:

      Color gem stones often get their color from the presence of “contaminants” which are usually AL-based. This is usually called, “corundum.”
      Sapphires are no exception.
      So I would expect any such gemstone to become “clouded” on exposure to base solutions.
      Such clouding would probably stay on the “surface” of the stone, but the remedy for it will probably be more expensive and time-consuming than simply rubbing the silver piece with a polishing compound, or dipping it in a jewelry cleaner.

  10. Jazmine says:

    Hello thank you for your the information but why did the experiment use hot water? Do you think tarnish removal would be faster or slower in the water at room temperature? Also, what are your recommendations for improving the speed of tarnish removal in this process?

    • From the author, Ted Beyer:

      The rate of reaction is vastly increased by the use of hot water (many reactions occur more rapidly at higher temps). The same reaction will occur at lower temperatures, but will take far far longer. I know — I tried using the water solution more than once without re-heating it…

      To my knowledge, there is no way to improve the speed of tarnish removal. Experimentation on my part caused the tarnish to disappear virtually instantly when using boiling water and the method indicated — and instantly is about as fast as any reaction can go… If proper contact between the silver and the aluminum is not maintained, the reaction, regardless of water temperature, will not occur, and for some irregularly shaped objects that can be a bit of a challenge.

  11. Sarah says:

    Hi Can you tell me when the chemical process “silver black” was created? I’m finding a lot of Native American jewelry is being chemically processed to make it look vintage when it’s really new. Just wondered how long ago this process came into being. It will help me date pieces.

  12. Sherry says:

    25 years ago, I bought silver oxidation preventer papers for our Moms for Christmas. They were long, maybe 1.5 inches wide, light blue gray. They put them inside their selves drawers, inside their displays of silver tea services, punch bowls and other things that were out all the time. Now I can’t find anybody who ever heard of them! Anyone tell me where I can find some of these.? I think they came in ziplock type bags of 10 or twenty with writing on each slip of paper.

Leave a Reply

Your email address will not be published. Required fields are marked *