Why you don’t want a copper and silver mokume gane ring.

AgCuDay10_1
Corroded copper and silver ring with the copper almost completely gone

No, I will not make you a copper & silver mokume ring.

It is not because I don’t like the color contrast. I love the color contrast that copper alloys have with silver in mokume gane. The original Japanese  work in mokume gane was almost all done in copper alloys and copper alloys with silver. Those strong color contrasts are one of the things that originally attracted me to mokume gane.

It is not because it is cheaper than the precious metals that are in most of my rings. The rings I make are labor intensive. The mokume process is very time consuming and exacting. We hand make every ring for a customer; we do not mass-produce or machine-produce these rings. I and my studio assistant make every piece of mokume gane. We cut, clean, stack fire and forge the mokume billet from the individual sheets of metal. Most of the metals I alloy, cast, forge and roll into sheet myself to get the color and working properties I want for my mokume gane billets. Almost all the rings we make are intended to be wedding or engagement rings. They hold great significance for my customers as the visible symbol of their love and commitment for one another. Because of this I strive to make the best mokume gane rings we can possibly produce for each and every person who has entrusted us with the job of making his or her ring. So even if we were to make a copper and silver mokume gane ring it would not be inexpensive due to the time and care we put into each and every one of the mokume rings we make.

The reason I will not make you a silver and copper mokume gane ring is that they self-destruct.  Copper is a base metal as opposed to being a noble metal. In chemistry, noble metals are those that are resistant to corrosion and oxidation in moist air. The noble metals are gold, platinum, palladium, silver, iridium, osmium, rhodium, and ruthenium.  In and of itself copper being a base metal is not the problem. By itself when worn on the skin copper will corrode and turn your skin green; this is a nuisance but many people are ok with these phenomena and wear copper bracelets or rings. The problem comes from a physical property of metals: galvanic corrosion.

A galvanic cell is what is created when you connect two different metals in the presence of an electrolyte. It makes an electrochemical cell otherwise known as a battery and electrical current will be produced. So what has this got to do with a copper and silver ring?  There is a battery formed by the copper and silver when the ring gets wet; salts on the skin, lotions and soaps or other substances in the water create an electrolyte. This current will flow from the more negative metal to the more positive one. When this happens,  galvanic corrosion causes the more positive metal to dissolve or corrode into the electrolyte and the more negative metal is inhibited from corroding. Copper is the more positive metal in the copper silver pair and it begins to dissolve every time it gets wet. The speed with which this happens is controlled by many variables and it is impossible to predict how fast the process will be for any individual. However it will happen! Any ring made from a base metal (such as copper) in contact with a noble metal (such as silver) will corrode. Rings made from noble metal pairs (such as gold and silver) will still form galvanic cells but their resistance to corrosion (nobility) keeps them from being dissolved into the electrolyte.

When I first started making mokume gane rings I did not understand this. I made rings with gold and shakudo and with silver and shakudo. Shakudo is a traditional Japanese decorative copper alloy that is about 95% copper with the balance being gold. It takes on a dark black color that is very striking when laminated to high karat gold metals.  Another artist told me about galvanic corrosion and I began to research it. After learning more about it I decided self-destructing rings were probably not a good idea so I quit making them. But over the years I have had the occasion to see rings that I had made where this corrosion was very obvious.

Top photo after about 18 months of wear. Bottom photo the same ring when it was brand new.
Top photo after about 18 months of wear. Bottom photo the same ring when it was brand new.

The experimental ring.

To illustrate the problem of galvanic corrosion I decided to make a copper sterling silver ring and perform an experiment with it.   I published my experiment on another blog on a jewelry-making site. Since we often get requests for copper silver rings I thought I would share that post here as well.

Since this was first posted several people have somehow assumed this is a problem with all mokume gane.  It is not! The corrosion will occur only if one or more of the metals in the ring are a base metal and two metals are connected in a wet environment. This corrosion will not happen with rings made entirely of precious metals. Rings worn daily are the jewelry items that will typically be affected due to the fairly constant wetting of ones hands.

This test is an accelerated aging test so you will not see this level of effect with normal wear in this short a time, but it will occur. How quickly will vary widely with the individual and their environment. I have seen this level of corrosion over the period of a couple of years on some individual’s rings that had copper or shakudo elements in contact with gold.  Two metals joined together in the presence of an electrolyte create an electrolytic cell that is in essence a battery. In a ring the electrolyte is provided by the water you constantly expose your hands to through washing, sweat, swimming etc. One of the metals will be more electrically positive called anodic and one will be more electrically negative called cathodic. The difference between these poles is measured in volts. When exposed to the electrolyte the anode will dissolve and supply ions to the electrolyte. The higher the voltage the greater the activity and the faster the anode will dissolve.  The higher a metal is on a galvanic series chart the more noble it is and the more cathodic or negative it is. The precious metals are at the top of the chart which why you will occasionally hear them referred to as noble metals.

So what all does this mean? If you combine silver (noble metal) and copper (base metal) as in the ring above you will have an electrolytic cell where the silver is the cathode and the copper is the anode; the copper/anode will corrode.

No matter how the two metals are joined (bonded as in mokume, soldered, riveted), it will always create an electrolytic cell. When copper is placed in contact with an electrolyte the copper will give off ions to the electrolyte and dissolve. How quickly is the next question, which is what I wanted to know. When copper is placed in contact with an electrolyte the copper will give off ions to the electrolyte and dissolve. I wanted to know how quickly this happens so I set up a test to find out.

Believe it or not there are defined standards for test solutions to simulate human sweat for testing properties such as colorfastness of fabric dyes; the EU uses another one to test for nickel release in jewelry items. I looked up several of them and picked one that seemed to be both easy to make and not too concentrated. I chose 7.5g/l NaCl, 1.2g/l KCl, 1g/l urea, 1ml/l lactic acid with a pH of 4.57. I placed this mixture into a beaker at room temperature and suspended the ring in it with nylon fishing line. I thought I would check on it once a week or so but I took a peek at day one to see if anything had happened. Much to my surprise etching had become visible in only 24 hours.

Silver Copper Day0
Day 0: The experimental ring, highly polished, non-etched in sterling silver and copper before beginning the test. 
The copper showed definite signs of etching in just 24 hours
The copper showed definite signs of etching in just 24 hours
AgCuDay1Close1
Day 1: The crystal structure of the copper is clearly visible where the sweat solution has begun to etch it.
AgCuDay3
Day 3: So I decided to check back in another 2 days. At this point the etching was quite pronounced.
AgCuDay7
Day 7: The ring was severely etched.
Day 7 close up: In fact in some places the copper had been totally eaten away.
Day 7 close up: In fact in some places the copper had been totally eaten away.
AgCuDay10_1
Day 10: The end of the test.
At this point the copper rich areas of the sterling were beginning to be affected by the solution and in many places the copper was totally gone. In another few days the ring would have fallen apart.
Day 10 close up: At this point the copper rich areas of the sterling were beginning to be affected by the solution and in many places the copper was totally gone. In another few days the ring would have fallen apart. Quite beautiful in a very distressed way.

What should you take away from this?  Copper-silver rings will corrode/etch over time….it may take months or years but it will happen.  It often starts subtly so it may be a long time before you notice.  However if you want a ring that will last a lifetime buy a ring made from a combination of noble metals (platinum, gold, palladium and/or silver).

Thanks for reading,

Jim


28 thoughts on “Why you don’t want a copper and silver mokume gane ring.

  1. I am curious if the ring were to be sealed with a clear polymer coating. That should keep it dry. Is there any reason not to do (something like) this?

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    1. Unfortunately coatings are not really suitable for the wear that rings receive. No coatings can stand up to the rigors of day to day wear, for other jewelry items coatings can work but may not be absolutely necessary.

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      1. What about a clear enamel coating? Enamel is basically just glass, and if it wears, cracks, or falls off/out, it can be replaced with a new enamel coating. For example, if you “pre-etch” a ring to the extent of day 7 in your test, the copper is recessed at a depth below the silver, so you could enamel that recessed depth or use some other durable sealant to protect it.If it’s enamel, it might not even require maintenance, unless the ring is dropped hard, which would damage any ring.

        I can think of a few other possible ways to create, protect, or simulate the contrast provided by copper, without the durability issues from galvanic corrosion. The reason I’m thinking about this topic is because I have proposed the use of mokume gane or similar techniques for the production of coinage, which you can view here:

        https://forum.coincompendium.com/index.php?topic=5206.msg23176#msg23176

        It might not be done due to the expense, but if a mokume gane artisan like yourself were contracted to produce planchets, it could be a helpful line of business because coin collectors are much like jewelry buyers in their desire for uniqueness, rarity, and beauty – they’re both willing to pay well for the things they like.

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      2. Enamel will not work in this case first due to the melting point of the enamel. Copper/Silver mokume will begin to melt at 1435°F, enamel needs to fire between 1400°-1500°F with better results at the higher end of the range. The second problem is enamel will not survive intact on a ring for any length of time. That said the problem is only really an issue for rings as they are exposed to water on a regular basis which is required for the galvanic reaction. Other jewelry or eve coins would be less affected as long as kept dry.

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  2. Thank you so much for the information! Would there be a good alternative to copper that contrast well with silver? Something that is not as precious as gold.

    Alternatively would shibuishi be better than regular copper in terms of longevity?

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    1. There is not really any substitute that is not a gold alloy. There are only two colored metals copper and gold, all the others are some shade of grey. Silver is the closest to a pure white but it is also slightly grey. So alloys that have a color other than grey are made using either gold or copper or both. Any of the traditional Japanese copper alloys like shibuichi or shakudo will have the same issue as copper in that they will readily dissolve when used in a ring. Other types of jewelry are generally not a problem but rings are so exposed to water that electrolysis is a real issue.

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  3. I read with interest your article. Are you saying then that 9k gold as we use in New Zealand would also corrode the same. Have you tested this as I thought it would be fine as much jewellery here has silver and 9k gold even before the advent of mokume gane. I was hoping say shakudo (4%gold) and say 3k (12.5%gold) would be alright. Your comments appreciated. Regards Brian Dodds

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    1. Yes 9k would corrode if you were to join it to 14k or 18k. It would be much slower than the copper corrodes but yes it will. Basically any alloy of less than 50% weight gold or lower than sterling silver will corrode if paired with a more noble metal. So 9K against shakudo the shakudo would corrode, 18k against 9k the 9k would corrode.

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      1. Thanks James I now see you already answered my question before. Just learning how this blog system works

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  4. This is a very interesting article, and it stands to reason why one wouldn’t want to make any items with this combination.
    I’m wondering, if silver is often alloyed with copper to make sterling why this doesn’t happen then. Maybe I missed it in the article, and I’m sure it has something to do with science and the way it is alloyed. In that case, can something be done differently to the copper, so this doesn’t happen in the Mokume gane process?
    Also, can mokume gane be achieved base metal to base metal
    Thanks so much for the information…invaluable!

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    1. It actually does happen in sterling but it gets more complicated to describe. If you look closely at the 10 days picture you will see pitting in the surface of the sterling. Sterling is what is called a two phase alloy. At room temperature there are two different crystal structures in sterling one that is mostly silver with a tiny bit of copper in it this is the alpha phase, the beta phase is mostly copper with a tiny bit of silver in it. Because there is way more silver than copper in the mix sterling is mostly alpha phase with small bits of beta phase in it. The beta phase will corrode just like the copper in the experimental ring. It is just so small an area you don’t normally see it. Unfortunately there is no magic trick that can be used to fix the galvanic corrosion issue, it is a property of the basic atomic structure of the metal elements. This is why steel is coated with zinc to “galvanize” it. The zinc is the more anodic (+) metal and it will preferentially corrode instead of the steel.
      Base metal mokume is fairly common but not suitable for rings because of the corrosion issues.

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  5. I am a jeweller from New Zealand. We mainly use 9 ct gold and combine it often with silver with many rings long before Mokume gane became popular. I never heard of the silver or the 9 ct gold dissolving with those rings. Do you think there would be a problem with Mokume gane if using those combinations? Have you ever done a test whether on this combination. I was hoping may be 3 ct gold (12.5% gold) would be all right. Even hoping Shakudo (4% gold) would be all right. Look forward to your comments. Regards Brian Dodds

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    1. I can definitely tell you shakudo will not work, I am very doubtful 12.5% gold will fare much better. We don’t use 9K here but I do know it suffers from some corrosion issues as does 10K. But it is real easy to test. Solder two pieces of sheet back to back with one being the alloy under test and one being the highest karat gold in the desired laminate and suspend the resulting plate on a piece of nylon fishing line in a solution of 1 liter of water with 7.5g NaCl, 1.2g KCl, 1g urea, 1ml lactic acid. This is an artificial sweat solution. Pull the plate every day and see what happens. The degree of corrosion will be your guide to how much damage you are likely to see in use.

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  6. could always just add a liner inside the ring as many jewelers do prohibiting contact of mokume and skin, thus leaving mokume outside and visible for looks and liner inside the ring for practical function.

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    1. Unfortunately a liner will not inhibit the corrosion and in some cases can make it worse. For example the use of a gold liner will make the corrosion happen even faster. The corrosion happens because the different metals are in direct contact with each other and exposed to water. This cannot be avoided except by never allowing the piece to get wet. Since we are talking about rings that are worn daily that is not really possible, even the moisture on your skin will allow the process to occur.

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  7. Woah, I learned something new today! Came here from Pinterest and this was a real fascinating read.
    “Quite beautiful in a very distressed way.” – I was thinking the same. Is it be possible to make a piece of jewelry so that the copper would corrode but the noble metal would still be wearable, or does everything fall apart once the copper goes?

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  8. I am no jeweller and stumbled upon this article for completely unrelated reasons, but I must say: the final result (with copper almost completely corroded out) is strikingly beautiful. If there is a way to keep the ring structurally sound on that stage, it can make a great new technique capable of producing very exciting designs. I’d pay top money for something like this.

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  9. The process of making mokume is extremely fascinating to me. I have a question. What if you used nickel-silver with copper or say brass, would there be erosion with these combinations?

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  10. finally I have found what is probably he solution to why I have so much trouble with plated jewellery obviously a scratch lets in the water/sweat and galvanic corrosion does the rest- I have even de-plated the earwires from a pair of gold plated earrings within 24 hours- however I have recently acquired a pair of rings which, although with the plate gone are copper inside no longer leave the green deposit around my fingers most copper and bronze tends to – I’m really curious to find out why this might be

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  11. Thank you! I really appreciate your time spent sharing this intel. I made a copper/Argentium Mokume Gane ring for my husband about a year ago. I took pictures of it the other day when I went to make it a half size smaller for him. It shows the corrosion from your 1-3 day test pictures. He doesn’t wear it at night or hanging out at home. When this one falls apart I’ll do one for him in gold/Argentium.

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