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Posted 07/05/2008 07:09 am
Yes, gold and silver won't stick to a magnet. But then, neither will most metals. Iron, cobalt and nickel are the only metals normally attracted to a magnet, and even many alloys containing these elements aren't magnetic. Cupronickel, the alloy used to make US "nickels", is 25% nickel; it looks "silvery" and doesn't fly to a magnet. A magnet doesn't really tell you much about whether or not a "gold" or "silver" coin is really made of precious metal. There are some other tests that can be done, depending on how much trouble you want to go through.
Test Kits
You can buy "test kits" which use a chemical reaction with a small scraping of metal from the coin to determine whether it's pure gold or silver. This test is basically a variation of the ancient method of using a touchstone. Please note that word "scraping"; this technique will do a small amount of physical damage to the coin, and is not recommended for collectable coins.
Weight
If you have an identified coin and know what the coin is supposed to weigh (reference to a coin catalogue will normally tell you this) all you need to do is weigh it and see if the weight matches. A matching weight isn't necessarily proof of genuineness, but a wrong weight (way too high or way too low) is convincing proof that it's fake. All you need for this is access to a balance accurate in grams to at least 2 decimal places.
Density (Specific Gravity)
Gold is a very dense metal - meaning it's very heavy, compared to other metals. A 1 centimetre cube of gold will weigh 19.28 grams; compare this with lead (11.34 g/cm³), silver (10.50 g/cm³), copper (8.96 g/cm³), iron (7.87 g/cm³) and tin (7.29 g/cm³).
The only metals which are as dense or denser than gold are either rare metals which are even more valuable than gold is (such as platinum) or artificial, radioactive elements (like plutonium). This means that a "fake" gold coin would most likely be made of something considerably less dense than gold.
To measure density, you need a specific gravity rig on a balance that can weigh to an accuracy of at least 1/100th of a gram.
If a "gold" coin passes the SG test, you can be confident that it really is gold. Silver is harder to be confident about by SG, since it's possible to take cheap, denser metals and alloy them with cheap, less dense metals. For example, an alloy of lead and tin is apparently favoured by the Chinese fakemasters for their lower-quality fakes.
Bring out the Heavy Gear
X-Ray Fluorescence Spectroscopy (normally abbreviated XRF) is a quick, non-destructive test that can accurately determine the precise composition of an object. Unfortunately, the XRF Spectrometer you need to do this test isn't exactly something you can buy off the shelf. They're normally big, bulky things like the one in the Wikipedia article, though I see that they've got some nifty little handheld XRF guns nowadays, which are as close as you'll get today to owning a Star Trek tricorder.
Either way, they're expensive to buy, but your local university or analytical laboratory might have one they'd make available to the public... for a fee. The university where I work has only got one XRF, in the archaeology department... but they don't like letting outsiders use it.
Test Kits
You can buy "test kits" which use a chemical reaction with a small scraping of metal from the coin to determine whether it's pure gold or silver. This test is basically a variation of the ancient method of using a touchstone. Please note that word "scraping"; this technique will do a small amount of physical damage to the coin, and is not recommended for collectable coins.
Weight
If you have an identified coin and know what the coin is supposed to weigh (reference to a coin catalogue will normally tell you this) all you need to do is weigh it and see if the weight matches. A matching weight isn't necessarily proof of genuineness, but a wrong weight (way too high or way too low) is convincing proof that it's fake. All you need for this is access to a balance accurate in grams to at least 2 decimal places.
Density (Specific Gravity)
Gold is a very dense metal - meaning it's very heavy, compared to other metals. A 1 centimetre cube of gold will weigh 19.28 grams; compare this with lead (11.34 g/cm³), silver (10.50 g/cm³), copper (8.96 g/cm³), iron (7.87 g/cm³) and tin (7.29 g/cm³).
The only metals which are as dense or denser than gold are either rare metals which are even more valuable than gold is (such as platinum) or artificial, radioactive elements (like plutonium). This means that a "fake" gold coin would most likely be made of something considerably less dense than gold.
To measure density, you need a specific gravity rig on a balance that can weigh to an accuracy of at least 1/100th of a gram.
If a "gold" coin passes the SG test, you can be confident that it really is gold. Silver is harder to be confident about by SG, since it's possible to take cheap, denser metals and alloy them with cheap, less dense metals. For example, an alloy of lead and tin is apparently favoured by the Chinese fakemasters for their lower-quality fakes.
Bring out the Heavy Gear
X-Ray Fluorescence Spectroscopy (normally abbreviated XRF) is a quick, non-destructive test that can accurately determine the precise composition of an object. Unfortunately, the XRF Spectrometer you need to do this test isn't exactly something you can buy off the shelf. They're normally big, bulky things like the one in the Wikipedia article, though I see that they've got some nifty little handheld XRF guns nowadays, which are as close as you'll get today to owning a Star Trek tricorder.
Either way, they're expensive to buy, but your local university or analytical laboratory might have one they'd make available to the public... for a fee. The university where I work has only got one XRF, in the archaeology department... but they don't like letting outsiders use it.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis
