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Kelsey Serwa, Julia Murray Julia Murray & Kelsey Serwa


In movies you commonly see treasure hunters, pirates, thieves, ...

bite down on a gold coin to verify it's real gold.

(Gold Medal winners often pose with the medal between their teeth, and I'm guessing it derives from that trope)


My question:

  • Is the "Bite Test" a working method to verify the authenticity of a gold coin?
  • 1
    In other words, were any coins ever forged in gold so pure that biting would have an effect? – Sklivvz Apr 7 '12 at 19:36
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    Wikipedia gives two contradictory explanations: (a) 24 karat pure gold is softer than fakes and biting will show this softness; (b) 22 karat gold (alloyed with copper or silver) as used for coins is harder than fake gold-plated lead and biting will show this hardness. – Henry Apr 7 '12 at 23:05
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    Wikipedia says that Olympic gold medals are 99% silver, with the gold on the outside. TV Tropes says that silver isn't soft. So I doubt it would help the two people photographed. – Andrew Grimm Apr 8 '12 at 14:37
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    I want to note something related here. If you ever come across a really old (e.g. ~2000 years old) coin, which appears to be made of gold - whatever you do - do not bite it. Biting is likely to dent such coin, and in the process will both devalue the coin (since the value is historic rather than the metal's value) and damage an irreplaceable piece of history. Biting found gold coins is a real problem in areas with a long history of gold coin usage. – Ofir Oct 20 '13 at 8:08
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The Wikipedia article on Crown Gold has a nice summary for this topic. Essentially,

  • there are pure gold coins, e.g. the Canadian Maple Leaf, as well as
  • alloyed gold coins, e.g. the Krugerrand of which e.g. the 1 oz coin contains 1 oz of gold plus another 2.8 g of copper.
  • Historically, this alloy (22 karat crown gold) has been introduced (in 1526) because it is more durable than pure gold. At that time, gold coins were used for payments, and (in addition to shaving) wear due to normal use as a means of payment was a concern.

  • According to the Wikipedia article, until the mid 20th century, all gold coins were intended for circulation and used these durable alloys

  • Nowadays, also pure gold coins for collection purposes are produced.

To sum up:

  • Unless "historically" means > 500 years old, the biting is likely not to work, because the gold coins were alloys that were deliberately chosen to be not too soft.

  • For recent (< 60 a) collection coins, biting may dent them.


Update: An unknown editor of this post found the following closely related manuscript Why do pirates bite gold coins they are given? which

  • concludes that the coin biting is most probably a cliche in literature and movies.
    The manuscript points out that there are many references to coin biting form early 20th century but not from older (contemporary to the setting) sources e.g.

    Brecht based his play Mother Courage on Grimmel- hausen’s Courage [6]. The book, published in 1670, makes many references to coins, money, ducats and pistoles but at no time is a coin bitten to determine whether or not it is genuine. One must conclude that Brecht and Cendrars merely invented the scene of biting the coin

    They put a possible origin to the cliche to 19th century gold prospectors distinguishing pyrite from gold nuggets by biting.

  • that 19th century false coins of lead with gold plating would have been softer than the real coins (possible bite detection) but that the weight difference would have been quite distinct and easier to judge and (consequently? anyways?) most fake coins were better falsifications where hardness was not so distinct.

  • And show experimentally that typical mint gold alloys in the shape of typical historical (e.g. 15th century) coins could be dented by biting strongly but the dents are unfortunately difficult to judge as the biting strength [and tooth shape] would play a crucial role. Note that those coins are much thinner than modern coins.

-4

This places the stress of bite at about 0.6MPa

http://rspb.royalsocietypublishing.org/content/early/2010/06/14/rspb.2010.0509.full

This places gold's hardness at 216 MPa using diamond tip. http://www.webelements.com/gold/physics.html

I don't think any human could really dent gold- we are probably either smearing given it high ductility or deforming it due to high malleability. Biting maybe a gold test of gold's other properties which may be reflective of purity.

  • Because tooth enamel is harder than gold, shouldn't a bite at least be able to scratch gold (although, admittedly, not to "bend" it enough bite though it)? Also I'm note sure about the 0.6MPa figure: is that the pressure measured over the surface area of a whole molar, or of the whole jaw, instead of over the (much smaller) area of an incisor? – ChrisW Sep 17 '13 at 12:49
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    (-1) Bulk modulus is not of any interest here: the bulk modulus is about a pressure you need to apply (from all sides) to a material in order to decrease the volume. On contrast, when denting a solid material (no foam), you displace the volume, but do not reduce it. – cbeleites supports Monica Oct 17 '13 at 18:05
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    The "bite stress" number you provide is the mean stress of the entire cranial-mandibular system when biting down on something with the second molar -- in short, it's a number entirely unrelated to how high a bite pressure a human can produce. – Mark Apr 22 '17 at 1:00
  • Gold is ductile. It bends easily. This depends on the thickness of the coin. Think gold foil, which you can find on some fancy chocolates. It can be completely chewed up by most any set of teeth. – phoog Apr 22 '17 at 14:49

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