Author Topic: Making errors with bimetallic coins  (Read 8736 times)

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andyg

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Making errors with bimetallic coins
« on: January 09, 2010, 02:17:42 AM »
Here's a picture of something which cropped up on another forum, this little beauty is for sale on an auction site.  A novel little fake you might think, well there were two listed (the other has finished (possibly withdrawn)) - the problem is that the last time I looked the other coin (not this one) was selling for over £100  :o
« Last Edit: January 16, 2010, 06:49:52 PM by Figleaf »

translateltd

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Re: £2 coin error, or not....
« Reply #1 on: January 09, 2010, 06:45:34 AM »
I notice that the list of diagnostics for fake pound coins includes rotated dies, yet die rotation isn't all that uncommon in business strikes - many examples are known in NZ coinage (produced by a variety of mints) that are all perfectly genuine.


Offline Bimat

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£2 coin error, or not....
« Reply #2 on: January 09, 2010, 06:47:40 AM »
I have a similar error coin (check it here).Though mine is only off-center strike,yours has die axis rotation too :o

Aditya
It is our choices...that show what we truly are, far more than our abilities. -J. K. Rowling.

andyg

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Re: £2 coin error, or not....
« Reply #3 on: January 09, 2010, 12:27:54 PM »
I have a similar error coin (check it here).Though mine is only off-center strike,yours has die axis rotation too :o

Aditya

This £2 is not an error, if you look closely it's the centre of one coin in the ring of another.
That is why the portrait is on the wrong side.....
« Last Edit: January 09, 2010, 06:21:57 PM by AJG »

RHM22

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Re: £2 coin error, or not....
« Reply #4 on: January 09, 2010, 06:14:15 PM »
I recall reading somewhere that you can put these in the freezer, remove them and then simply pop out the center ring. Apparently the cold caused the center to shrink, making it slip out easier.

Offline Figleaf

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Re: £2 coin error, or not....
« Reply #5 on: January 09, 2010, 06:22:30 PM »
I don't know how British coins are struck. If this were a euro coin I could say with certainty that it was homemade. In the case of the euro coins, pill and ring are struck at the same time, so something like this would be impossible to produce officially. To make something like this yourself, remove the ring of one coin (a little metal saw will do the trick), drill the centre of another coin. Join the two pieces. Cooling the pill and heating the ring would probably help.

Peter
« Last Edit: January 09, 2010, 08:18:58 PM by Figleaf »
An unidentified coin is a piece of metal. An identified coin is a piece of history.

andyg

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Re: £2 coin error, or not....
« Reply #6 on: January 09, 2010, 06:28:59 PM »
I don't know how British coins are struck. If this were a euro coin I could say with certainty that it was homemade. In the case of the euro coins, pill and ring are struck at the same time, so something like this would be impossible to produce officially. To make something like this yourself, remove the ring of one coin (a little metal saw will do the trick), drill the centre of another coin. Join the two pieces. Cooling the pill and heating the ring woud probably help.

Peter

I guarantee that is how the UK coins are made too, as sometimes one finds the ring design ever so slightly struck over the copper-nickel centre.

Galapagos

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Re: £2 coin error, or not....
« Reply #7 on: January 09, 2010, 06:41:20 PM »
I recall reading somewhere that you can put these in the freezer, remove them and then simply pop out the center ring.

I have a blank of a trial bimetallic two pound somewhere. There are spikes inside the inner ring, and when the blank is struck, these spikes slam into the pill, holding the two fast together, so I doubt that your information is true.

RHM22

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Re: £2 coin error, or not....
« Reply #8 on: January 09, 2010, 07:13:41 PM »
I know this could be done with a certain bi-metallic coin, but I don't remember which. I'll check around to find out for sure.

RHM22

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Re: £2 coin error, or not....
« Reply #9 on: January 09, 2010, 07:20:51 PM »
It's a Canadian toonie ($2 coin). I don't know if the newer coins will separate, but the older ones apparently do.

Offline chrisild

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Re: £2 coin error, or not....
« Reply #10 on: January 16, 2010, 12:55:28 PM »
I have a blank of a trial bimetallic two pound somewhere. There are spikes inside the inner ring, and when the blank is struck, these spikes slam into the pill, holding the two fast together

Pretty much the same here. I have a €1 coin that I got quite a while ago, badly battered and, well, my curiosity won. :) The pill has a "groove" line, much like what the 2 cent coins have, and the ring has a matching "tongue" line. These two do not simply fall apart ...

Christian

RHM22

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Re: £2 coin error, or not....
« Reply #11 on: January 16, 2010, 06:22:34 PM »
Pretty much the same here. I have a €1 coin that I got quite a while ago, badly battered and, well, my curiosity won. :) The pill has a "groove" line, much like what the 2 cent coins have, and the ring has a matching "tongue" line. These two do not simply fall apart ...

Christian

Hey Christian, if you ever get curious about alchemy and Lincoln cents, take a look at this:

http://chemistry.about.com/od/chemistryhowtoguide/a/goldsilverpenny.htm

Offline chrisild

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Re: Making errors with bimetallic coins
« Reply #12 on: January 17, 2010, 02:50:31 PM »
And in school I have always thought that chemistry was a somewhat boring subject. ;) Here is a page from sammler.com which shows similar effects, also involving zinc. (These are links to two different parts of the same page.)

http://www.sammler.com/mz/euro_abarten_faelschungen.htm#golden_gemacht
http://www.sammler.com/mz/euro_abarten_faelschungen.htm#Herdplatte

Christian

Offline Bimat

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Making errors with bimetallic coins
« Reply #13 on: January 17, 2010, 03:07:11 PM »
As a chemical engineer,I can always try to explain :)

These errors are just because of the difference in the values of coefficient of thermal expansion.I had explained these things a couple of weeks ago to Bill ;) This is what I wrote:

Most substances expand when they are heated.Thermal expansion is a consequence of the change in average separation between constituent atoms of the object.(Assume atoms of an object to be connected to each other by stiff rings,for simplicity)In case of solids,this average separation is quite small (about 10-10 to 10-12 meters)As temperature increases,the atoms oscillate with greater amplitude and the separation between atoms increases,consequently the object expands.Reverse process occurs in case of cooling.

Linear Expansion: Suppose you have a rod of length 'L' at a temperature T Celsius,and its temperature changes from T to T1,then the change in length of the rod is given by L*Alpha(greek letter)*(T1-T),the Greek letter alpha is called coefficient of linear expansion.The new length of the rod now will be L+ L*Alpha(greek letter)*(T1-T)  Obviously,higher the temperature difference,higher is the expansion/contraction.
Now,because the linear dimensions of the rod change,it follows that its surface area and volume changes as well. The change in volume is given by V*Gamma(Greek letter)*Change in temperature. (V is the original volume). Gamma is called coefficient of volume expansion.Alpha and Gamma are related as : Gamma=3*Alpha

Average values of alpha for some metals are- (per degree Celsius) (Multiply alpha by 3 to get value of gamma)
Steel-1.2*10-5
Copper-1.7*10-5
Brass-2*10-5
Aluminum-2.4*10-5

These values are extremely small,and the thermal expansion process might not be significant for small temperature changes.However,if you keep a bimetallic in oven at about 200-250C,you will find a change in its structure.

(These values and above expressions involve a lot of maths and experimentation)

Now in the case of coin Andrew has posted..The outer ring is nickel brass and inner ring is cupro nickel.From above values,the value of alpha for copper is less than that for brass.That means,for the same change in temperature,the outer ring will expand/contract more than the inner ring.Therefore,an allowance must be provided for expansion in case there's high temperature difference.(I think) the spikes are provided for the same reason.Note that it may not be necessary to provide such an allowance in a country like India where we don't have large temperature fluctuations like you have..

Any more doubts? :D

Aditya
« Last Edit: January 17, 2010, 03:18:09 PM by numismatica »
It is our choices...that show what we truly are, far more than our abilities. -J. K. Rowling.

Offline Figleaf

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Re: Making errors with bimetallic coins
« Reply #14 on: January 17, 2010, 03:19:18 PM »
Well done, numismatica. I could understand your message by reading the post once while I know practically nothing of the subject. This was all the more striking since I am in the process of translating a presentation on pension systems - a subject I do know a lot about - and I am having great trouble understanding what the author, a university type, is trying to say.

Peter
An unidentified coin is a piece of metal. An identified coin is a piece of history.