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

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.

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.

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

Day 1: The crystal structure of the copper is clearly visible where the sweat solution has begun to etch it.

Day 3: So I decided to check back in another 2 days. At this point the etching was quite pronounced.

Day 7: The ring was severely etched.

Day 7 close up: In fact in some places the copper had been totally eaten away.

Day 10: The end of the test.

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