07/29/2010 (4:39 am)
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I used a search strategy for "metal identification +bronze +brass +copper" and immediately came across interesting information. At "Eureka, I Found It", (http://www.eureka-i-found-it.com/antique/metals.html) an antique site that has a page on identifying metals in cast antiques, you will find the following:
1. If a magnet sticks, its iron
2. If a worn spot or a spot on the bottom that does not show plating is yellow, then it is brass or bronze. If it is grey, then it is zinc.
3.When tapping the piece lightly with a pencil and listening closely, you'll find that a thud is zinc, a faint dull ringing is brass and a clear ringing tone is bronze. They suggest tapping an arm or extended piece of a sculpture or, in the case of a plaque, holding one corner while tapping the opposite corner.
As with any technique that requires the senses, an experienced person will probably be better able to determine whether the ringing indicates the dullness of brass or the clarity of ringing of bronze.
Brass, being an alloy of copper and tin, has some give and will bend some before a breaking failure. It weighs 4.94 ounces per cubic inch.
Bronze is composed of an alloy of copper and zinc and is much more brittle and will general break with bending. It weighs 4.71 ounces per cubic inch.
So, using the above information you can also dismiss some common falacies that an object's heft will tell you if it is brass or bronze as they are so close in specific gravity. You can also not determine by color as the object may be plated and some bronze is the color of brass.
Most cast art objects, by the way are not made of copper due to its maleability. This page (http://www.jewelrysupplier.com/2_copper/Copper_properties.htm) notes these characteristics of copper:
Color
Copper’s color is a unique softly reflective brown red to deep brick red. Exposure to Oxygen causes Copper to tarnish and turn a Teal Green as is the case with one of America’s most famous monuments the Statue of Liberty.
The Henry Ford Museum pages at http://www.hfmgv.org/research/cis/brass.html#1 have a discussion on caring for brass and bronze antiques and discuss identifying the composition of an object. They offer good descriptions of the properties and uses of these alloys, but fall short in methods of identification. You might find it useful reading none the less. Their section on care and storage seems very well done.
Metal identification can also be done chemically. A paper on this appears at http://web.fccj.org/~smilczan/Two5/tony/steel.html and offers a chart that shows that a small sample of brass, removed eltrically, will react with Nitric Acid by quickly turning green, while bronze will quickly turn blue-green in the same test. Again, it may take a more practiced eye to determine the subjective difference between green and blue-green. I would have suspected that the intensity of the green is directly related to the copper content of the sample, however it turns out to be just the opposite as the brass has less copper content than the bronze.
I have also found this page: http://www.geocities.com/metalworking2000/casting.html#identifying
Which offers some common sense tests and some you may not wish to try including sparking and drilling. He claims that different metals sound different when being drilled and also come off the drill bit differently (spiraling or powdering, for example) depending on their composition. Caveat emptor, but I offer it for additional information and amusing reading.
And finally, positive identification can be made with spectometry and highly specialized laboratory equipment. This Russian page (also in English) lists a variety of manufacturers of this type of equipment. http://www.spectrometer.ru/manufacturers-suppliers/metal-analyzers.htm
I hope this helps you to identify the composition of your art figure. Feel free to ask for any clarification I might be able to provide.
-=clouseau-ga=-
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