Each week, CoinWeek, in collaboration with the Numismatic Bibliomania Society, brings you a highlighted feature from the current volume of the E-Sylum eNewsletter.
In last week’s E-Sylum, reader Dave Baldwin submitted the following:
Bronze pieces are usually described as chocolate brown with copper pieces being more red. But two Buchanan Residence medals came up with the redder piece being 99.46% copper, .32% platinum, .17% zinc and the brown piece 99.68% copper, .22% platinum. And bronze is usually described as an alloy of copper and tin yet none of the ones tested showed any trace of tin. Small amounts of zinc were found and almost always some platinum.
Online searches for definitions of bronze are pretty inconclusive and it seems the term copper alloy is preferred. So as numismatists what do we mean by bronze and how is that determined? I have also seen the term ‘bronzed copper’, is that bronze or a copper piece with bronze plating? Any help would be much appreciated.
This week, we look at some of the responses Wayne received to this surprisingly complicated question.
WHAT IS THE DEFINITION OF BRONZE?
Regarding Dave Baldwin’s question on the definition of bronze, Jack Howes writes:
The terms brass and bronze have become so muddied that they don’t mean much if anything specific anymore. I would call all those medals copper with trace amounts of alloy with the one exception that was 2% zinc which is a true copper-alloy.
Gosia Fort writes:
The same question was nagging me when I was researching medals from my collection. In one catalog a medal can be described as copper, in the other – as bronze. What proof does the author have to choose one name over the other if he did not test the alloy? (I do not think that either Freeman or Storer examined the chemical content of each medal when they were compiling their great works). So I came to a very simple conclusion – that numismatists use the metal name in a common sense; they describe more the look of it than the content. For a chemist it may matter that the brass is an alloy of copper and zinc, while bronze is a mix of copper and tin, but for a numismatist (especially a newbie like myself) it may be OK to go with a common sense and a duck test for those few medals I could not find authoritative reference. I will call a copper anything that looks like a copper, a bronze – any shades of brown, a brass – anything with hues of yellow. After all, I am looking at the medals and describing them as objects of art.
Paul Schultz submitted a response on this topic back on March 18, 2012. Here it is: -Wayne
I read the brass & bronze definitions in last week’s E-Sylum, and thought I might clarify it from a metallurgist’s point of view.
The problem comes about in the origin of the names, from tradition dating back hundreds of years for some specific alloys, vs. modern definitions for general groups. The modern metallurgical definitions I was taught in my courses for a Master’s degree in metallurgy reveal a confusing difference between alloy groups and specific alloys.
For the general categories, the definition of a brass is copper alloyed with less than 40% zinc, and no other elements. Copper with over 40% zinc is Muntz metal. Copper alloyed with anything other than zinc, (especially tin, but also aluminum, nickel, etc, and it may also include zinc along with these other elements) is considered a bronze alloy. An aluminum bronze is copper with aluminum plus possibly other elements, nickel bronze is copper with nickel plus possibly other elements, etc.
The confusing problem is the overlapping traditional naming for the very specific alloys, which go back hundreds of years, and which are not fully consistent with the above modern system. “Commercial Bronze”, composed of 90 copper and 10 zinc, is really a brass alloy by modern definition. “Aluminum brass”, at 76 copper, 22 zinc, and 2 aluminum, is really in the bronze series.
The situation creates more confusion than it alleviates. Specifically for coinage, I believe that most of the copper zinc alloys that are used have a more golden color, and the copper tin alloys have a lighter color. For coinage, it may be more practical to use the modern category definitions which are more consistent with the colors we see in coinage, and ignore the specific traditional names like “Commercial Bronze”. Reference for most of the above is “Physical Metallurgy for Engineers”, Donald S Clark and Wilbur R Varney, Van Nostrand 1962, pp 392-393.
Paul was responding to a note by Dick Johnson. As it turns out, Dick was very much in agreement with Gosia’s assessment.. -Wayne
So what is the difference between bronze and brass? If the zinc is less than 10% it is bronze. But numismatists are not satisfied with the term at this precise formulation.
For numismatists — who can best describe an item by inspection — the term is determined by color. If the piece is brown it is bronze. If it is golden yellow it is brass. But the color of the metal alloy doesn’t change until the zinc content is above 15%, or as expressed in the metalworking trades, copper 850 zinc 150. This adds to 1000, but sometimes expressed .850 and .150 to add to 1 — the total amount.
Technically that area between 10% zinc and 15% zinc alloyed with 90% to 85% copper is called red brass. At zinc 160 copper 840, one percent more zinc you find a solid yellow-brass color. This continues as the amount of zinc increases. However red brass is not permanent. Like a freshly struck U.S. cent it is copper-red color that after about six month’s time harsh exposure or much handling in circulation has turned brown. It is the copper content that causes the color change.
We could not have all the world’s coins if we did not have copper. The alloys of copper for numismatic items have been called many things. But it’s okay to call brown coins or medals “bronze” and golden yellow coins and medals “brass.”
Dick Johnson submitted these additional thoughts for today’s issue. Thanks! -Wayne
WHAT IS BRONZE? WHAT IS BRASS?
When I went to work at Medallic Art Company I was charged with writing about the company and the products. I quickly realized I needed to learn the lingo, the language of the medallic field. I started writing down the words, the terms everyone was using that I didn’t know for sure.
The first two were cartouche and cliché. I would ask what each word meant. The workmen, foreman, and management were all kind, they took the time to answer all my questions.
Bronze was one of those words that was widely used — didn’t everyone already know? — but late to be defined. Years later I studied the use of the term “bronze” in all of its uses not only in numismatics, but also in metalworking. It is so closely associated with brass I had to study that as well.
What ended up was a chart of 24 kinds of bronze-brass terms. Coins were made in a select few of these, medals were made in just about every one. But they all fit in that one chart.
For a three-year period I wrote a Monday Report for the new management of the company. I wrote one of those Reports on bronze-brass and included that chart. Management liked it so well they placed in on the firm’s web site available for all.
In numismatics we most often describe an item by its color. Bronze will tone brown, brass is a golden color. Both are a copper alloy. What I learned that the alloy changes color because of a second metal copper is alloyed with.
When copper is alloyed with zinc if the zinc content is more than 16% the metal is a golden color — brass. When it is alloyed with 15% or less zinc it will become a brown color — what everyone calls bronze. The alloy changes color between 15 and 16 percent zinc.
Of course, when copper or bronze is freshly struck it is red. It tones brown in short time; the brown color is permanent. Brass will tone but still retains its golden color. This may not be the last word on the subject, but you can read that report online. [Link below – Wayne]
To answer Dave’s question about “bronzed copper” or “chocolate bronze”, these are terms of its patina. These are not terms of the composition. However, patinas are a subject for another article.
To read Dick Johnson’s Medal Blog article, see:
Dave Baldwin adds:
Anyone who wishes to continue the conversation can contact me at [email protected] . And John Kraljevich said bronze was “any non-red copper alloy”. That’s pretty simple!
To read the earlier E-Sylum articles, see:
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