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 microscopy2) Sample preparation3) 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?

AboveRoman coin (from London):Constantine I

BelowAustralian coin

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

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Characteristic chlorine peak

Characteristic carbon peak

Energy of packetsin thousands of electron volts

Amount of packets Characteristic oxygen peak

EDS output from X-rays

Lets start by looking at relatively modern coins

1958 sixpenceEquivalent to 5 cents

5 cent coin: Australian

1 cent coin: Australian

What is a sixpence made of?

Above:secondary electron image of coin shows surface detail

Left: backscattered electron image shows average atomic number [by contrast]

020020

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The spectrum (left) shows the X-ray analysis of a sixpence (on blue cross)

Note the silver peaks

What else is present?

Why is a sixpence silver to look at?

Standardless Quantitative Analysis [phi-rho-z method]

Element keV (energy) Mass % Error % Atomic %

C 0.277 6.02 0.11 35.63

Cl 2.621 0.36 0.12 0.73

Cu 8.040 4.14 0.82 4.64

Ag 2.983 89.48 0.33 59.00

Total 100.00 100.00

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 imageWhat are the dark spots?

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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.

Standardless Quantitative Analysis [phi-rho-z method]

Element keV (energy) Mass %

Error % Atomic %

C 0.277 8.60 0.11 31.98

O 0.525 1.13 0.09 3.17

Ni 7.471 23.96 0.25 18.24

Cu 8.040 66.30 0.33 46.62

Total 100.00 100.00

If the whole composition is the same as the surface then there is about 66 mass % Cu and about 24 mass % Ni

On the area we tested we can calculate composition

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?

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Note the copper peaks on the right

Is anything else present?

Standardless Quantitative Analysis [phi-rho-z method]

Element keV (energy) Mass % Error % Atomic %

C 0.277 6.44 0.24 24.65

O 0.525 3.42 0.20 9.83

Cl 2.621 0.55 0.13 0.72

Cu 8.040 89.59 0.69 64.81

Total 100.00 100.00

The 1 cent coin has about 90 mass % Cu so it is brown because of the copper

On the area we tested we can calculate composition

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?

Note: never clean an old coin – it removes value! This one has a patina of green corrosion

Roman coinFrom AlexandriaConstantine I

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?

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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

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The white blobs are about 50 mass % silverWhat 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)

View of both sides of Roman coin from London