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08/24/2011 / thriftomancer

Restoring tarnished silver, with SCIENCE!

Quick! Look at your clock! What’s the time?

Your clock is wrong.

“Then what time is it?!” You ask? I’ll tell you what time it is. It’s time, for science!

Today’s subject is, as you might’ve guessed from the post title, how to remove tarnish from your silver stuff (jewelry, flatware, tchotchkies, ect.) and restore it to its former, shiny glory using naught but science. It’s a simple, safe procedure/experiment that you can do in your own kitchen without using any carcinogenic polishes or dips. Even better, this process doesn’t remove any silver from your piece like traditional cleaning methods; it just strips away the sulfur that bonded to the elemental silver to create the tarnish. Practical chemistry, saving the day.

In addition to giving you all the instruction you’ll need to start working, I’m also going to explain the chemistry behind why it works because I’m all about the why. (Best question you can ask, that.) It’s much more fun to do this if you understand what’s happening. As such, this post will be split into two sections: Part I where I’ll cover the chemistry details and Part II where I’ll show you how it’s done. (If you’re lame and hate learning you must, feel free to scroll on down to Part II. The steps will work even if you don’t know why. I’ll just be very disappointed.)

Part I: Learning is fun

Let’s start at the beginning with how tarnish forms. Tarnish is the result of an oxidation reaction between a metal and some chemical in its environment. Not always oxygen, mind you, though O2 is an exceptional oxidizer. (Note: I can’t use subscripts here on WordPress, so if there’s a number following any symbols in my chemical equations, assume it’s a subscript.) Unlike rust, which continues to eat away at the underlying metal even after it’s covered the metal’s surface, tarnish stops at the surface. It forms a patina, a layer of already-oxidized metal compound, that protects the remaining metal below it by excluding the oxidizing agent; preventing it from making further contact with the fresh metal.

In the case of silver, the oxidizing chemical isn’t oxygen or water and it’s not the copper that’s oxidizing either. Some sources will tell you that sterling silver tarnishes because it’s an alloy of 92.5% silver and 7.5% copper and the copper is reacting with skin oils or oxygen or some other such rot.

That’s just plain wrong.

The gray-black compound that forms on silver as tarnish is silver sulfide (Ag2S). It forms when elemental silver (Ag) is exposed to hydrogen sulfide (H2S), the gas responsible for the familiar “rotten egg smell” of sulfur (S), or other sulfur compounds. (Silver can also be tarnished by exposure to chlorine (Cl), but sulfur is the more common perp.)

When the two chemicals meet, they react. The sulfur breaks away from its two hydrogens to form two new covalent bonds with silver atoms. This makes silver sulfide and releases hydrogen gas (H2). Written properly:

2 Ag + H2S —–> Ag2S + H2

In order to restore the silver, we need to reverse this reaction (which is hard) or find a way to move the sulfur elsewhere.

(Okay, there is a third option of using polishes, but those solve the problem by just scraping off valuable silver instead of restoring it. It takes an undue amount of effort and isn’t at all science-y, so it’s not really an option after all.)

What we’ll do is forget about trying to reverse the tarnishing process entirely and run another reaction instead. The point of this one is to shuffle the sulfur atoms clinging to the silver off on to some other substrate, a metal that’s more reactive than silver and so more attractive to the sulfur. That metal is aluminum (Al), an exceptionally reactive and obliging metal. It’s so incredibly reactive that it doesn’t occur in a pure elemental form at all in nature.

Aluminum’s elemental form has a much higher affinity for sulfur than silver does and will bond with it readily, even if it has to yank the sulfur away from silver to do so. Lucky for us, there’s a ready source of pure, elemental aluminum present in nearly every modern kitchen. That’s right, we turn to tin foil! (It’s actually made of aluminum these days.)

This reaction is really simple. We take yon silver sulfide and put it in a reaction chamber (a bowl, jar, non-aluminum pot, whatever) containing water and aluminum. The aluminum and silver sulfide need to be in contact with each other for the reaction to occur. At this point we only need one more thing, a chemical that will dissolve into charge-conducting ions in water. Most sources recommend baking soda (Na2CO3), but salt (NaCl) or other ionic compounds will work as well. Substituting a strong acid like lemon juice or vinegar for the water will also work because the hydronium ions (H30+) that make it acidic will assist in carrying charges as well.

Once all that is together, you wait.

What happens is the ionic solution conducts electrons between the silver and aluminum, which helps to break the covalent bonds between silver and sulfur atoms. The sulfur atoms then migrate to the aluminum pieces where they’ll form new bonds oxidizing aluminum to create aluminum sulfide (Al2S3). For this to work, however, the silver and aluminum need to be able to conduct electrons between themselves and so must be touching.

The actual reaction occurring is:

3 Ag2S + 2 Al —–> 6 Ag + Al2S3

When all’s done, the sulfur has been stripped way and re-deposited elsewhere, turning the silver sulfide tarnish back into shiny elemental silver.

Yup, science is pretty awesome.

Part II: How to restore tarnished silver

Now that you fully appreciate the workings of this reaction, let’s do it! (And those of you who skipped down from the top, I’m disappointed and shaking an admonishing finger at you.)

I’m going to assume you’ve got a good idea of what silver of yours needs cleaning and that you’ve already fetched it and have it ready to go. I had a whole bunch that needed work, as you can see.

Tarnished silver stuff!

The first thing to do when setting up is make sure your workspace is well-ventilated. Windows open, fans on. You don’t need a respirator or fume hood or anything major, but a decent amount of airflow is good. This reaction, in addition to forming aluminum sulfide, sometimes releases hydrogen sulfide. You’ll smell it if it does, little whiffs of rotten eggs coming from your reaction vessel.

That gas is toxic in high concentrations and will give you headaches at lower concentrations, like what you might get in an unventilated room. Don’t worry overly much, but if you start to smell some avoid breathing it and turn the fans up to whisk it away.

Once you’ve got your space picked and prepped, you’ll need a reaction vessel. Make sure it’s large enough that you can fully submerge the piece of silver you want to clean and that the vessel isn’t made out of aluminum. (Don’t want to oxidize the inside of a perfectly good pot, now, do you?) I chose a Pyrex baking pan , 9″ x 9″, for the greater surface area on the bottom. It’ll be helpful since I’m going to be cleaning a lot of chains and want them to lie flat.

Pyrex pan or reaction chamber?

Next, grab some shiny new aluminum foil and tear off an adequately-sized piece. If you’re not planning to clean much, maybe a ring or two, a piece about the size of your palm should suffice, but the more aluminum you use, the more tarnish you can potentially remove.

Since I’m trying to remove a whole lot of tarnish, I lined the pan with foil as well as tearing up a few pieces to float around and bump into stuff. I feel like I didn’t need the floating pieces though.

Adding Al

With the foil secured in your vessel, add in your ionic compound. I used baking soda with a bit of salt every now and then as the reaction ran. Throw in your silver pieces, then add enough water to cover them. It should look something like this.

Reaction time

Let the chemistry do it’s thing and chill out for a bit. Make a cup of tea, do some jumping jacks, compose a sonnet, commune with the squirrels at your birdfeeder, whatever you like.

Give the solution two or three minutes, then check on the silver. Lightly tarnished pieces should be clean or nearly so by now. More heavily tarnished pieces will take longer, five or seven minutes. If your silver isn’t squeaky-clean, put it back in and wait a bit more then re-check. If things still haven’t changed appreciably by the next time you check you can try adding more salt/baking soda or foil to the mix, but be patient. Sometimes you’ve just got to wait.

When a piece is done remove it from the solution, wash it off with good old H2O, and dry it thoroughly. You might want to use distilled water just to be sure no sulfur will get on the silver (I have well water, so that’s very possible for me), but tap water should be fine. Just be sure to only use cold water to rise. Hot water can cause silver to develop a yellowish cast.

Also, if you’re cleaning a lot of silver at once you might want to hold off on drying the pieces as you bring them out. As I mentioned before, this reaction can produce hydrogen sulfide, the gas that tarnishes silver in the first place; and the more tarnish you remove, the more chances you have to produce some. If any of your freshly cleaned silver comes into contact with the gas as if wafts from the reaction vessel, it’ll start tarnishing again.

To prevent that, you may want to have a bowl of distilled water, full enough to completely cover your pieces, sitting ready to act as an exclusion chamber. Like so:

Cleaned pieces

When a piece is done bring it out of the solution and rinse it as normal, but instead of immediately drying it off, throw it in the bowl. The distilled water will exclude any gas from getting to it and protect it until you’ve had time to clean all your pieces and air out the room.

And there you go! How does it feel to bend the raw forces of the universe to do your bidding? Pretty cool, isn’t it? Think about it, you just manipulated atoms using naught but water, baking soda, and tin foil. I call that a good day.

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12 Comments

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  1. Molly / Aug 25 2011 6:52 am

    Pretty cool! I usually just ask my Dad to polish my few silver things but this looks like much more fun. My kids still love to do science experiments around the house so I think I’ll give this a go this weekend. Thanks for sharing :)

  2. Charlie A / Oct 14 2014 1:25 pm

    I had been scouring the internet on methods for cleaning silver. I was already aware of the foil & baking soda method, but your page (very nicely presented by the way) was the first that took the trouble of explain the role of baking soda in the reaction. So many thanks for the information, and keep up the good work. Charlie

  3. Daniel Starling / Apr 25 2015 5:33 pm

    I also have been scouring the internet for help with understanding silver sulfide. Last year I went to sulfur hot springs and my sterling silver bracelet tarnished (obviously) but to my surprise after about a week it was back to normal, and I hadn’t done a thing to make it that way except wear it. This year I went in a hot tub and it tarnished again and again it’s going back to normal, even the parts which aren’t touching my skin (so I don’t think it’s skin oil).

    So I thought that it was oxygen:

    Ag2S + O2 –> 2Ag + SO2

    But when I consider the oxidation states

    2Ag+, S2-, 2O0 –> 2Ag, S4+, O2-

    Then I see that BOTH Silver AND oxygen are being reduced while sulfur is oxidized. Of course, this throws the method of half reactions out the window. So my questions to you are:

    Is oxygen even the right culprit?
    If so, how would you balance it?

    If you answer this, I promise I’ll be happy.

    Sincerely,
    Daniel Starling

    • thriftomancer / Apr 27 2015 10:26 pm

      I like it when people are happy!

      As far as I know, it’s not oxygen that’s reacting with the sulfur to remove it from the silver. Even as electronegative and reactive as oxygen is, Ag2S is a stable compound. The only reason that aluminum can strip the sulfur away is because its pure metallic form is so insanely reactive, and even then it needs an ion bridge to react in a useful way. I doubt atmospherically available oxygen even comes close to the necessary level of reactivity.

      I haven’t got a concrete answer for you (since I haven’t found anything decisive in the wide web), but I do have three theories based on what I already know that may help:

      The first is that the tarnish is being removed by a combination of skin oil and physical polishing. The surface of your skin is slightly acidic and, combined with the stress of daily wear like rubbing against your skin, the inside of your sleeve (which also has your skin oil on it once you’ve touched it), ect, it may be enough to physically polish away the tarnish that accumulated.

      My second idea is that your bracelet may be made of a different alloy than normal sterling silver (92.5% silver, 7.5% copper). There may some a small percentage of a different metal in there that is disrupting the reaction and inhibiting the tarnish from sticking as solidly as it would otherwise. (Not so sure about this one.)

      The third, and I think most likely, is that the tarnish you’re seeing might be a different compound than Ag2S. The tarnish from the hot springs likely was sulfur-based, but there’s another type of patina that forms on silver called firescale.

      http://en.wikipedia.org/wiki/Firescale

      It’s a reaction with the copper, not the silver, and does involve oxygen. It occurs when the metal is exposed to heat. Like in a hot tub. It also tends to dissipate without you actually having to do anything. (I get it on my rings after washing pots/pans/dishes all the time. It fades in a few hours to a day.)

      If the tarnish is more of a dark bronze/gold color than straight black, it may be what’s on your piece and explain why it’s vanishing on its own.

      • Daniel Starling / May 6 2015 3:13 pm

        I am indeed VERY happy! Thank you for sharing this with me, I would have never found this information by myself.

        I’m pretty sure you’re right with the firescale, as the color you described is closer than that of tarnished silver, and both times this has happened, it has been after exposure to heat. I could not have hoped for a better answer.

        Sated and pleased,
        Daniel

  4. Anonymous / Oct 13 2015 10:37 am

    Great information! What do you do when there are stones like a London topaz in the jewelry too?

    • thriftomancer / Oct 24 2015 8:25 pm

      If I’m sure that the stone won’t be damaged by the acidic environment of the bath I just submerge the whole thing. You may notice there are a few pieces with amethysts and citrines among the items I cleaned.

      In my experience quartz varieties and other tough gems tend to make it, so any topaz should be fine as long as the setting itself can withstand the treatment (i.e. it’s something like a prong or bezel setting that has a physical piece of metal restraining the stone and not glue that may melt) and there are no decorative coatings (like “peacock topaz” which is treated with a fine layer of titanium) that could be marred.

      When a stone might be damaged by high levels of acidity (like jet, coral, or pearls) I use substitute ketchup for the salt bath. The ketchup’s acidity works just as well, but because it’s thicker it can be used to selectively polish the silver without submersion and you can keep it from coming into contact with the stone. (Though I did successfully clean coral by submersion. The heavy cuff bracelet in my pictures is set with three coral cabochons.)

  5. Lizzy / Apr 28 2016 1:47 am

    This worked PERFECTLY! Go science!

    I have a sterling silver chain anklet that suddenly turned coppery after I waded in the water at the beach today. It struck me that the water smelled sulfur-y, so i googled how to un-sulfur jewelry and found your website. I used a piece of aluminum foil in a pyrex cup with some salt and warm water and it was restored within seconds. The reaction happened so fast I was able to watch it. And I got a slight whiff of the sulfur gas coming off.

    So neat, thank you!! Until I read this, I thought I accidentally bought some cheap jewelry and I was considering just tossing it out. Yay!

  6. Ted / Jul 10 2016 10:49 pm

    Thanks for the science. Is there a method for removing the tarnish caused by chlorine exposure?

    • thriftomancer / Jul 22 2016 8:55 am

      From what I’ve been able to find (not much, unfortunately) it doesn’t seem like there’s any specifically tailored reaction you can run to restore silver that’s been exposed to chlorine. (I’m not even sure exactly what compound is forming in that case, since silver chloride isn’t formed by a reaction with pure silver like silver sulfide is.)

      What I did find suggested trying one of the three basic silver cleaning methods until one takes:

      1) Wash with soap and water. If the substance being formed by chlorine exposure is actually silver chloride (AgCl), then it’s insoluble in water and I don’t know how much this will actually work. Never hurts to try though.

      2) Use baking soda/ketchup and aluminum as with silver sulfide tarnishes. Since the chlorine is an even more strongly reactive ion than sulfur is, the aluminum may work to separate it from the silver just like it does sulfur.

      3) Polish. Definitely a last choice because of the issues involved with polishing (removing material, possibly scratching the piece, takes more time/effort, etc.), but try a mild polish like toothpaste on a rag or paper towel. (Toothpaste is the most inexpensive source for pharmaceutical grade calcium carbonate you can hope for, and it’s already in a gel suspension so you don’t have to bother with mixing it. Plus, your jewelry will smell minty fresh.)

      I’ll be doing more research to try and find exactly what the reaction with chlorine is (mayhap there’ll be a new post in the future), but for now try one of those three. Good luck!

      (And let me know if any of the treatments work so I can recommend it to others with more confidence than ‘Might work, give it a shot.’)

      • Ted / Jul 25 2016 9:34 am

        Thank you. I did try the foil method. It’s reactive but spotty. I ended up polishing and polishing. It’s not like it was heirloom quality material, just a few spoons. Eventually got it off before I hit base metal. Thanks again.

  7. Shirlee Crandall / Feb 11 2017 10:36 am

    Very interesting! You’ll be happy to know I read parts 1 & 2. : ) I love your writting style, just thought you’d like to know; now I’m off to make a cuppa & commune with some squirrels!

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