THE SPECIFIC GRAVITY TEST FOR COPPER, BRONZE, AND BRASS

I'm writing to you as I came across your very interesting pair of articles on copper alloys, specifically:

"What's Copper? What's Bronze? What's Brass? What's the Difference?"
-The E-Sylum: Volume 13, Number 10, March 7, 2010, Article 11

"More on the Difference Between Copper, Bronze, and Brass"
- The E-Sylum: Volume 13, Number 11, March 14, 2010, Article 9

I'm coming to this from a non-currency angle, as I have a burgeoning interest in bells and chimes, where the differing alloys of copper can apparently produce different quality sounds. My first thought, which I had before I read the first article, was to use a specific gravity test to tell the difference. Like the author of the first article, I found that it would be ambiguous at best. Even if one could perform the test to better than a percentage point, as the second author points out, the difference between brass and bronze is the alloying metal (zinc or tin, respectively). But because of their differing densities, you could not uniquely distinguish the two alloys.

In order to help me think through the specific gravity test, I made the above chart, which may be of interest to others. There are two solid lines on the chart that show pure bronze (CuSn) and brass (CuZn) alloys specific gravities (vertical axis) as a function of copper percentage (horizontal axis). As you can see, depending on the copper content, you could have the same specific gravity for two different copper percentage brass and bronze alloys. I also included the specific gravity of some other copper alloys shown as a circle for bell metal bronze and diamonds and triangles for various other alloys as defined by Wikipedia. See: http://en.wikipedia.org/wiki/List_of_copper_alloys .

The other thing that struck me was this comment :

I immediately thought of x-ray fluorescence, which I am somewhat familiar with from my day job. It is non-destructive, and more than capable of giving metal compositions for the elements of interest for most metal alloys. I also knew that the components for these systems were produced for commercial purposes. A quick internet search turned up this product which might fit the bill : http://www.olympus-ims.com/en/xrf-xrd/delta-handheld/delta-alloy/ .

One limitation of this method is penetration depth of the x-rays. I haven't done a calculation yet, but it would only return an analysis based on the first centimeter at a guess. It also only illuminates a small area of the sample. Perhaps this measurement combined with a specific gravity test would give confidence on the composition of an entire object. I am pursuing the x-ray angle for my own entertainment.

To read the earlier E-Sylum articles, see:
WHAT'S COPPER? WHAT'S BRONZE? WHAT'S BRASS? WHAT'S THE DIFFERENCE? (www.coinbooks.org/esylum_v13n10a11.html)
MORE ON THE DIFFERENCE BETWEEN COPPER, BRONZE AND BRASS (www.coinbooks.org/esylum_v13n11a09.html)
NUMISMATIC VOCABULARY: BRONZE VS BRASS (www.coinbooks.org/esylum_v15n11a10.html)
ON BRASS, BRONZE, AND GOLDENE (www.coinbooks.org/esylum_v15n12a18.html)
QUERY: WHAT IS THE DEFINITION OF BRONZE (www.coinbooks.org/esylum_v17n45a10.html)
ANSWERS: WHAT IS THE DEFINITION OF BRONZE? (www.coinbooks.org/esylum_v17n46a08.html)

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