Download
1 / 27
270 likes | 474 Views
Faces and Fakes: Ancient and Modern coins - Summary of a CyberSTEM presentation. We will be looking at a range of modern and ancient coins We will talk about: 1) Scanning electron microscopy 2) Sample preparation 3) Chemical analysis of metals and oxidation
E N D
Faces and Fakes: Ancient and Modern coins - Summary of a CyberSTEM presentation We will be looking at a range of modern and ancient coins We will talk about: 1) Scanning electron microscopy 2) Sample preparation 3) Chemical analysis of metals and oxidation 4) Modern, ancient and fake coins, their value and identification Text and images by the Centre for Microscopy & Microanalysis, University of Queensland, Australia, August 2007
Coins are made out of metal. What metals are used and why? Below Australian coin Above Roman coin (from London): Constantine I
Coins were originally worth their weight in the metal from which they were struck or cast But not all coins are pure metal Bronze is a copper alloy (copper plus other metals such as tin and other elements) Some coins are precious metal mixed with metal of lesser value and others are plated, that is one metal over another of lesser value. Useful references: http://en.wikipedia.org/wiki/Bronze http://en.wikipedia.org/wiki/Coins
What are modern coins made of? How can we analyse the elements present in a coin? Is modern money worth the value of the metal that makes it? What makes an ancient coin precious? What can they tell us?
Sample preparation Coins ready to go into a scanning electron microscope
Scanning electron microscopy (SEM) can be used to look at the surface features of coins This can tell us about history – in the case of ancient coins since details were changed regularly The X-rays produced when electrons interact with the sample can tell us about the surface composition of coins [for details on science and background of technique see the PowerPoint presentation called Background theory and terminology] We use energy dispersive X-ray analysis (EDS or EDX) to find out what elements are present at the surface
EDS output from X-rays Amount of packets Characteristic oxygen peak Energy of packets in thousands of electron volts Characteristic carbon peak Characteristic chlorine peak
Lets start by looking at relatively modern coins 1 cent coin: Australian 1958 sixpence Equivalent to 5 cents 5 cent coin: Australian
What is a sixpence made of? Left: backscattered electron image shows average atomic number [by contrast] Above: secondary electron image of coin shows surface detail
Why is a sixpence silver to look at? The spectrum (left) shows the X-ray analysis of a sixpence (on blue cross) Note the silver peaks What else is present?
Standardless Quantitative Analysis [phi-rho-z method] If the mass composition is the same as the surface then there is about 89 mass % Ag
A sixpence looks silver because it contains a lot of silver The coin weighs 2.8 grams What would it be worth in today’s metals market? At $480 AUS/ kg for silver the coin is worth about $1.20 AUD
An Australian 5 cent coin also looks silver but does it contain silver? Above: backscattered electron image What are the dark spots?
The spectrum (left) shows the X-ray analysis of a 5 cent coin Note the copper and nickel peaks The dark spots on the previous image will be element with a low atomic number such as carbon – they may be an area of oxide or dirt.
On the area we tested we can calculate composition Standardless Quantitative Analysis [phi-rho-z method] If the whole composition is the same as the surface then there is about 66 mass % Cu and about 24 mass % Ni
Why does the coin look silver? Nickel mixed with copper produces a silver look to the coin This example 5 cent coin weighs 2.9 grams By looking up the current value of copper and nickel the value of the metal can be calculated
A copper 1 cent coin looks brown What is it made of?
Note the copper peaks on the right Is anything else present?
On the area we tested we can calculate composition Standardless Quantitative Analysis [phi-rho-z method] The 1 cent coin has about 90 mass % Cu so it is brown because of the copper
Our copper coin weighs 2.7 grams With copper selling at about $8.94 AUD/ kg What is the metal in the coin worth? About 2 cents!
What can Roman coins tell us? Roman coin From Alexandria Constantine I Note: never clean an old coin – it removes value! This one has a patina of green corrosion
Roman coin from Alexandria (ear region of Constantine I) seen using backscattered electrons Ear region
What are the white blobs in the coin on the previous image? What is the surrounding material?
The Roman coin contains copper, lead, tin, and may have some dirt (clay: Al, Si and Fe) on the surface. There is oxide present too. We can say the basic coin is bronze
The white blobs are about 50 mass % silver What does this mean? Was the ore the coin was made from contaminated with silver? Or was the crucible in which the copper was melted the source of the silver?
Bronze coins are thought to have remained near where they were made because they were not particularly valuable and therefore not carried far So, their metal composition reflects the mines and local manufacturers techniques Silver is not found in the bronze coin found in London [analysis not shown here], only in the one from Egypt (Alexandria)
