When I think of Numismatics as an academic discipline, I envision it as resting on three pillars or foundations of knowledge. The first two, art and history are the ones we typically think a lot about and then there is the one we don't spend too much time on. That would be metallurgy.
If you have taken the ANA course on the modern minting process, you have gotten some exposure to the basic information about metallurgy. My own involvement in this course work has provoked a great deal of thought and questions about metallurgy as it applies not only to U.S. coins, but to coins from all nations worldwide.
What exactly is metallurgy anyway? Metallurgy is the domain of materials science and engineering that studies the physical and chemical behavior of metals. It is divided into two categories, chemical metallurgy and the one that we focus on in Numismatics, physical metallurgy.
So lets take that deep plunge into the field of physical metallurgy. Let us start by brushing off your old Chemistry knowledge and taking a look at the periodic table to get an idea about the chemical properties of the elements we most often use in coining. ( I knew that 11 hours of chemistry I took in college would come in handy someday!).
Elements are organized by similar properties called groups. The group most commonly used for coining is group 11 followed by group 10. We also use one from group 12. A select few metals fit that category. From Group 11 we have Copper, silver and gold. From group 10 we have nickel, palladium, and platinum along with Zinc from group 12. Of course there are some countries that use Aluminum from group 13 as well.
All are highly electroconductive. Why those particular groups, you may ask? It's because they all have the properties of being malleable and ductile. That means they can be made into shape without breaking and they can be stretched. They are favored for their relatively low chemical reactivity. They also have a suitable level of hardness, making them resistant to wear.
The development of alloys was crucial in making coins from the beginning of coinage in world history. In American coinage, all half cents and all large cents were struck in copper. The first U.S. coin to be made of an alloy was the Flying Eagle cent. The reason small cents came into existence was the cost of copper content became prohibitive to continue make large cents. The composition of the Flying Eagle Cent was .880 copper and .120 nickel.
Various other alloys and their content specific specifications for each coin beginning with the Flying Eagle Cent can be found in the Redbook. All of our circulating coins are made of alloys. Many of us think that when you buy a non circulating bullion coin, you are buying a coin that is 100% gold, silver or platinum content, but this is not true. Silver Eagles are "three nine fine", meaning they are composed of .9993 silver and .007 copper. Gold Eagles are composed of .9167 gold, .03 silver and .0533 copper. Gold Buffalos and Gold Spouse coins are "four nine fine", meaning it is as pure as can be coined at .9999 gold content. Platinum Eagles are composed of .9995 platinum. The Redbook does not list the other content, nor could I readily find it on the internet.
Beginning in 1965 the Mint began issuing clad coins, Cladding involves the bonding of two or more different metals to each other. U. S. dime, quarter , half dollars and SBA dollar clad coins are composed of two separate layers of a copper nickel alloy that contain .750 copper and .250 nickel clad to a pure copper core. Golden Sacajawea, Presidential and Native American dollars have a pure copper core with brass overlays consisting of .770 copper, .120 zinc, .070 manganese and .040 nickel.
How are these metals bonded together? There are two methods that can be employed. There is the high impact explosive method which employs the use of explosives on the outer layer of the metals to create a uniform weld . The second method is called the cold rolling process in which sheets of metal are rolled under high pressure to create a weld. The mint experimented with both processes before settling on the cold roll method.
For our coinage, two sheets of copper nickel alloy are rolled with a layer of copper sheet through a mill. To the best degree that I could determine, the Mint has settled on outsourcing to obtain their clad sheeting for minting dimes, quarters and half dollars rather than produce it in house. This is supplied by PMX Industries in Cedar Rapids, Iowa. Personally, I think that having only one source is a poor practice for several reasons. It raises anti trust issues and also raises the price of readily available material resources in the absence of free market competition.
Modern U.S. cents are made by electroplating a 20 micron layer of copper to a zinc core. 1943 cents were also electroplated. They consisted of a steel core with zinc electroplate.
I hope that this article will demonstrate and provide a working knowledge on how metallurgy is important to the modern numismatist in understanding how our coins are made and the processes that underlie their creation, Hopefully it won't be too dry and will also be interesting.