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Since copper is quite soft, it should produce a muted, round sound. A test way back in 1867 described copper's sound as "dead," while brass emitted a "clear ringing note." This might be tough to judge without experience, but learning it could prove useful for an antiquing or scrap collection hobby. This works best for thick, solid metal objects. Brass objects made for industrial purposes often have a code stamped on them to identify the exact alloy. In both the North American and European systems, the codes for brass begin with a C and are followed by several numbers. Copper is often left unlabeled, but if you want to be sure, double check the code with this quick guide:  The UNS system in North America uses brass labels beginning with C2, C3, or C4, or fall between C83300 and C89999. Copper, if labeled, can use codes from C10100 to C15999, and C80000–C81399. The last two digits are often dropped. In the current European system, both copper and brass begin with C. Brass ends with the letter L, M, N, P, or R, while copper ends with A, B, C, or D.  Older brass may not conform to these systems. Some older European standards (which were used quite recently) list the element symbols followed by a percentage. Anything that contains "Cu" and "Zn" is considered brass. This test is usually not too helpful, since brass is only slightly harder than copper. Some types of treated copper are particularly soft, so you may be able to scratch them with a US penny (which is never true of brass). However, in many cases there is no convenient object that will scratch one object but not the other. Copper is easier to bend than brass as well, but it's difficult to draw exact conclusions from that test (especially without damaging the object).
Strike the metal and listen to the sound. Look for stamped codes. Check how hard the metal is.