WHAT'S COPPER? WHAT'S BRONZE? WHAT'S BRASS? WHAT'S THE DIFFERENCE? Público Deposited
- Dick Johnson submitted these thoughts on an interesting question. Just how DO you tell the difference between copper, bronze and brass without being a metallurgist or chemist? I'm just a dumb numismatist and like most of us, I often take others at their word when they catalog a piece as one or the other, assuming that they know. Guess what? They don't always get it right. -Editor
Medal researcher Ron Abler wrote me this week about a problem he is facing identifying the composition of medals in the American centennial period he is cataloging.
"Speaking of alloys," he writes, "one of the problems that I'm grappling with is proper identification and designation of bronze, copper and brass medals. It seems that 19th century catalogers were not very precise in their use of these terms. In fact, if one were to cross Holland's list with Frossard's list and contemporary auction descriptions (as I have done), one might draw the conclusion that copper medals should be more prevalent than they appear to be today."
Ron is attempting to be precise in his catalog descriptions, for which he should be greatly admired. He noted that 130 years of toning of his 1876 medals further amplifies the problem. Correct. Copper and all copper-based alloys tone in time. Nature of the beast.
Copper is a popular coinage metal because of its (somewhat) low cost and high coinability. It has been used for coins since 450 BC. And as every Lincoln cent collector knows, untarnished copper red quickly turns brown in circulation.
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. While copper is the major component it is alloyed with zinc and/or tin with other metals as trace or impurities.
Even nickel-silver is on this list. It is a copper alloy (as much as 72 percent) despite the fact it bears a compound name of two other coinage metals. It has no silver, but does have nickel (up to 18 percent).
As Ron Abler observed it is difficult to determine an EXACT composition because these alloys have a wide range of copper-alloy formulations. There is one clue: color. A copper-zinc alloy remains red in just-struck or as-cast state until the zinc reaches 15 percent (with 85 percent copper). From 85 to 72 percent copper the alloy is brass and has the typical brass yellow color. Below 72 percent copper the color becomes gray like copper-nickel (or silver-nickel).
By inspection alone any other determination is impossible. Ron considered doing specific gravity tests but learned these test results are so close to be inconclusive. Yes, other laboratory tests could be conducted, but is it that important to learn a few percentage points of zinc one way or the other?
Most mints or medal makers order their strip stock in whatever is required or most convenient. Thus the amount of zinc is not always that precise.
He mentioned that three coin dealers suggested calling the composition "bronze" and let it go at that. I concur. I would like to know more about the artist, the design, the reason for the item's issue and other historical factors rather than the exact amount of zinc in the item's composition.
With that in mind I must relate one incident about "bronze." In 1913 coin dealer Thomas Elder wanted to issue a series of plaques of famous people. He contracted with sculptor Jeno Jusko to model 13-inch plaques, which he would issue that size (and two smaller size reductions). When he began writing his sales pitch he felt the word "bronze" did not have enough éclat. Promoter that he was he stated that all his plaques would be cast in "Corinthian Bronze."
Corinthian Bronze? I refuse to make that the 27th term in my list of bronze names!
Ah, shades of Ricardo Montelban. Remember the ads for car interiors with "rich, Corinthian leather"? Maybe he stole that line from Elder. -Editor - 2010-03-07
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