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The E-Sylum: Volume 13, Number 11, March 14, 2010, Article 9

MORE ON THE DIFFERENCE BETWEEN COPPER, BRONZE AND BRASS

Bob Neale submitted these thoughts in response to Dick Johnson's item on copper, bronze and brass medals. -Editor

The difference to me, as a chemist in my former life, is that bronze is mainly copper plus tin (not zinc) in various proportions, and brass is mainly a copper-zinc alloy. For example, cartridge case (brass) cents contained zinc, and thereafter both tin and zinc (like the early bronze small cents).

How to tell the difference just by looking? Bronze might appear a bit grainy and dull, without the brighter and shinier appearance of most things brass. Think of yellow brass hardware. Bronze is less stable to oxidation in general and darkens faster than brass. Think statues.

Of course, all comparisons depend on the percentage composition of the components. There is really no clear-cut delineation. Once you go to mainly tin, with copper and other elements, you get pewter. And once you go to centuries-long aged medals, there's likely no visible difference between most bronze and brass. So what I might disagree with is that:

"I checked my entry on Bronze in my Encyclopedia of Coin and Medal Technology and found 26 names of bronze alloys that have been used as the composition for coins and medals. It is all a matter of the amount of zinc (generally) alloyed with copper."

It may well be that any bimetallic alloy of copper with either tin or zinc might is called bronze in some circles. But I'm pretty sure that the term 'brass' should connote zinc, rather than tin, as the major component with copper (but usually along with some tin).

Next, Bruce W. Smith adds his thoughts -Editor

I suspect you will hear from several people about the comment in the last The E-Sylum about Copper vs Bronze vs Brass, in which Dick Johnson wrote that bronze is copper mixed with zinc. Though these metals do cause numismatists a lot of trouble, he is dead wrong about bronze and zinc. Copper is an element, a basic metal which does exist in nature as a metal. In its natural metallic state or as a pure refined metal, it is -- well, copper colored ---- a lighter or darker brown depending on the amount of surface oxidation.

Bronze is an ancient alloy of copper and another metallic element, tin --- not zinc. The Bronze Age was named for the widespread use of this alloy in ancient times, both in Europe and Asia. Coins and objects made bronze, when new, are reddish brown or chocolate brown in color. In time they patinate with a layer of usually green oxidation (though the patination can be blue or red or other colors depending on burial conditions).

Brass is an alloy of copper and the element zinc, and is usually yellow in color, though it will darken to brown in time. Brass is a relatively modern "invention." Coins and other objects made of brass did not exist before the 1400's AD. There are some who claim that some ancient brass objects are genuine, but this is not true. So lets be clear -- ancient coins in brass are not genuine. The same applies to other objects which may look old and may have a green patina, but they can not be genuine if they contain zinc.

The reason for this is the peculiar nature of zinc. It does not exist in metallic form in nature, and generally can not even be an accidental alloy in an ancient object. Zinc has a very low melting point, and when heated, it passes directly to a gas and evaporates. So even if zinc ore was thrown into an ancient melting pot, the metal would simply evaporate and disappear.

The only way to produce metallic zinc (which can then be mixed with copper) is to heat it in a covered crucible. The zinc gas flows out a pipe in the top, is lead away and cooled, and solidifies as pure metallic zinc. This process was discovered in India in the early 1400's and spread from there to China. The metal was introduced to Europe from China -- Europeans had no idea what it was or how it was made. In the 1400's the Chinese changed their coinage from bronze to brass because zinc is much cheaper and more readily available than tin.

Trace amounts of zinc do turn up in ancient objects, but only in very small amounts (less than 1 percent) due to some fluke in production. So the presence of zinc (more than 1 percent) in a purported ancient object, is evidence that it is not genuine. The object must have been made after 1400 by mixing some brass with copper or bronze. The same is true with aluminum, which though extremely common in the earth's crust, does not exist in nature as a metal. The first metallic aluminum was not produced until the early 1800's and as late as 1850 there was not a kilogram of aluminum on the entire planet.

Some supposed ancient coins have turned up in recent years with traces of aluminum -- say, 1 or 2 percent. These too, despite their ancient look and wonderful fake patina, can not be genuine because of the presence of aluminum. Whoever made these fakes, must have thrown some aluminum-bronze metal into the melting pot when mixing the metal for the coins. Unfortunately there is no way to spot the presence of aluminum from appearance when in such small amounts.

And yes, nickel silver and German silver are different names for copper-nickel. There is no silver in the mix, normally, and the amount of copper is normally higher than the amount of nickel (which is more valuable). Usually it's around 75 percent copper and 25 percent nickel, but the ratio varies depending on the alloy desired. Other metals might also be present.

This brings us to the other metal problem facing numismatists -- "white metal." In the 19th century many tokens and medals were made in imitation of silver, using various combinations of cheaper metals, especially lead, zinc, nickel and tin. After 1890 aluminum was also used for this purpose. These various alloys are often difficult to identify, particularly if the medals or tokens can not be handled (in cases, for example, or slabbed), so in the 19th century numismatists began using the term "white metal" for any white colored alloy other than silver.

One medal or pattern coin might be described in one place as copper-nickel, in another as tin, in another as pewter, in another as aluminum, and in another as white metal. Do all of these varieties exist or have some been confused with others? The problem continues because new descriptions in catalogs or auctions are often just as wrong as the old descriptions.

What we need is a small, inexpensive device which can identify several different metals using a non-destructive method. The jewelry industry has two or three different kinds of hand-held devices for determining whether a diamond is real. We need something like that.

Hear, hear! That's a great idea. But is such a device possible? Could it by manufactured and sold cheaply enough to make it ubiquitous in the hobby? If not, we may be vexed by this problem for a long time to come. Bruce continues: -Editor

Among stamp collectors there are several different color charts available, though those made in England and those made in the USA sometimes have different names for the same color shade. But I have never seen such a chart for metal. Companies which supply non-ferrous sheet metal might have a website with images, but I'm not sure we could rely on their color control on the website. Moreover, color alone probably wouldn't be enough to identify metals.

I collect aluminum type coins, pre-1890 aluminum medals, and medals relating to the aluminum industry. I can tell you that it is often necessary to check the weight of a medal to know whether it is aluminum or something else. Aluminum can sometimes look very much like silver. Tin-zinc compounds also tend to be light weight, but the color is different than aluminum. Aluminum coins or medals with a proof or proof-like surface are easy to distinguish from tin or pewter or other white metal compounds with a proof-like surface. But once they start toning, they can be very confusing.

As I said, what we need is some sort of affordable device which performs a test (chemical or using radiation) to identify metals present in a coin. Such a device could also be useful to quickly spot counterfeits. If the supposed ancient coin contains zinc or aluminum, then we know it isn't genuine. Testing for trace elements could also provide a clue to where the metal came from.

For example, a copper mine in one area might produce metal with small amounts of iridium, while a mine in another place might produce metal with trace amounts of tungsten (but not iridium). Copper coins produced in both areas with the same design could be separated into those containing iridium and those containing tungsten, and those containing neither. Knowing where the metal came from might allow us to attribute the coins to particular mints or particular political entities.

Celtic coins, for instance, were made in an area stretching from Ireland to the Black Sea. Identifying the metal source might narrow down where such coins were made. The same is true in China for Pan Liang and Wu Chu coins, which were made across most of the country. Identifying the source of the metal might allow us to determine where specific varieties were made and perhaps even provide a rough date in some cases. Wu Chu coins, for example, were made for 700 years (114 BC to about 600 AD). If we find that the metal in one variety comes from a place which wasn't settled until around 400 AD, then we can infer that those coins were not made until 400 AD or later.

Of course a device which could date copper or bronze objects would be invaluable not only in the numismatic field but also in the antiquities field. There was a fellow back in the 1980's who was working on a C14 dating method using the tiny amounts of air trapped inside cast bronze. The object would have to be destroyed, of course, to get at the air. As far as I know, he never was successful with the process. Perhaps we should focus on the tin. Does tin have radioactive forms which break down and can be measured? Are there any physicists out there reading this?

John W. Adams adds:

I would recommend that the curious set themselves up to do specific gravity testing. The available equipment has improved over the years, so those who may have dismissed it in the past should give it a second look.

There is such a big difference between the specific gravity of copper (8.96) and zinc (7.13) and tin (7.31) that mixed compositions are relatively easy to quantify. For a whole bunch of technical reasons, which I will discuss in my new book on the Vernon medals, color may reveal the metal SOME of the time but is not a reliable guide overall.

To read the earlier E-Sylum article, see: WHAT'S COPPER? WHAT'S BRONZE? WHAT'S BRASS? WHAT'S THE DIFFERENCE? (www.coinbooks.org/esylum_v13n10a11.html)

THE BOOK BAZARRE

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Wayne Homren, Editor

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