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G series Ep. "Greek" possible factual error (appar

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wikinerd
679066.  Sat Mar 06, 2010 12:49 am Reply with quote

Correct me if I'm wrong, but I remember on the 14th episode, "Greek", Stephen Fry mentioned that a one of the practices of testing gold is "biting", in which he said that biting gold would not leave a toothmark.

Gold, being a "soft metal", led me to [a rather quick] search onlineóto which the results below put gold quite low on the Moh (hardness) scale (~2.5-3Moh):

http://chemistry.about.com/od/geochemistry/a/mohsscale.htm
http://www.slate.com/id/2136438/

And the second article specifically discusses the fact that biting gold would leave a toothmark because gold ranks lower than teeth enamel (5Moh).

Archive that the Slate article points to for the Moh rating of enamel:
http://web.archive.org/web/20080519072922/http://www.lib.umich.edu/dentlib/Dental_tables/Mohshard.html


PS. I do know that these references may not be the best, if I'm wrong please do update me on the facts

PPS. If I post this in the wrong section/forum etc, do point me to the right one (I'm new here). (Thanks)


Last edited by wikinerd on Sat Mar 06, 2010 6:35 am; edited 1 time in total

 
wikinerd
679071.  Sat Mar 06, 2010 2:00 am Reply with quote

PPS. The part about how biting lead (1.5Moh) will leave a dent is true; however,teeth marks on a medal would not distinguish genuine gold from gold-coated lead, unless the force of biting is kept consistent and the depth of the marks are measured (possible?). On the other hand, the practice of biting gold is to distinguish gold from other less malleable metals.

 
mckeonj
679146.  Sat Mar 06, 2010 4:58 am Reply with quote

'Biting' gold is a matter of taste.
Some experts will taste or smell a 'gold' item to estimate its purity; this was seen recently on the BBC 'Antiques Road Show'.
Pure gold (24 carat) is chemically inert and does not oxidise, nor does it have a taste or smell. Ignoble metals do have characteristic tastes and smells, notably iron, copper, and tin; and their alloys.
Biting on a piece of metal which is also in contact with the tongue can have a very interesting effect, especially if any teeth have amalgam fillings, which sets up a couple. Try biting a piece of aluminium kitchen foil for a very odd sensation, which is the flow of electrons across the taste buds.
Some metals are highly dangerous; I would not recommend tasting sodium or arsenic.
To recap; pure gold has no smell or taste, and no reaction to biting other than indentation.

 
Flash
679180.  Sat Mar 06, 2010 5:42 am Reply with quote

wikinerd - the point being made was that gold coins were not pure gold, but an alloy. Consequently they were harder than gold, so biting them did not leave a mark. Coins which tried to counterfeit gold, on the other hand, were made of coated lead (so that they would feel as heavy as gold). If you bite a lead coin you do leave an indentation - so indents indicate a forgery.

Here's the transcript of what Stephen said:

Quote:
If it leaves a tooth mark, then itís false. Because gold coins, when they were made, were always a mixture of other metals too, and those other metals hardened it, but fake ones used lead, and they did leave an impress, so you were testing to see that it wasnít lead. So itís if it left a mark, it wasnít gold.

 
PDR
679198.  Sat Mar 06, 2010 5:55 am Reply with quote

The Moh hardness scale is related more to scratch resistance than resistance to indentation - it was invented to allow geologists to identify minerals by seeing whjich scratched the other.

To look at resistance you indentation you need to use one of the "engineering" hardness scales (eg Vickers, shore, Brinnel or Rockwell).

It's true that "pure" gold is rather soft, but it's almost never used in this form. 22ct gold has a hardness of around 70 Vickers, but the various gold alloys (different carat ratings) can vary from 40 to about 230ish Vickers, which means it overlaps lead, aluminium, copper (in some states) etc.

In summary, the bite test isn't really that reliable for establishing the purity of gold!

PDR

 
RLDavies
679754.  Sun Mar 07, 2010 6:52 am Reply with quote

mckeonj wrote:
Some experts will taste or smell a 'gold' item to estimate its purity; this was seen recently on the BBC 'Antiques Road Show'.
Pure gold (24 carat) is chemically inert and does not oxidise, nor does it have a taste or smell. Ignoble metals do have characteristic tastes and smells, notably iron, copper, and tin; and their alloys.

Experts often feel an object with the lips or teeth, as well, since these are more sensitive than the fingers. You can sense cracks or repairs in china this way. High-carat gold feels almost buttery or oily.

 
bobwilson
680025.  Sun Mar 07, 2010 11:15 pm Reply with quote

So - to fool an expert you take a fake a gold coin and make it out of diamonds which the buyer won't be able to bite into - that'll fool them.

Actually, I thought it was a naff question - the most interesting part of the whole thing was Jupitus's interjection predicting Chocolate as one of the Klaxon answers.

 
RLDavies
680232.  Mon Mar 08, 2010 2:56 pm Reply with quote

Here's something else you can test by biting: trilobites!
Quote:
Biting the trilobites with your front teeth will sometimes serve as a test of trilobite authenticity. Fake trilobites might feel soft to the bite as a plastic would feel. This simple and safe test utilizes the sensitive nerves in your teeth, and would certainly never damage an authentic trilobite that is markedly harder than fakes made of resin. Only slight pressure is needed; don’t try and bite into it for obvious reasons!

 

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