LBMA Assaying & Refining Conference 2015
8-10 March 2015
Analyses of the chemical and structural integrity of
gold bullions by eddy current probes
Daniel Sutter, Helmut Fischer AG Switzerland
Quentin Bochud, Metalor SA Switzerland
LBMA Assaying and Refining Conference, London, March 2015
LBMA 2015, Bochud-Jodry-Sutter
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Content
 Introduction: Basic idea of using eddy-current probes to study the properties
of precious metal bullions
 Part I: method and instrumentation
 Part II: applications on bullions
 Conclusions
___
 Abbreviation: electrical conductivity, s (Sigma)
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8-10 March 2015
Introduction
Fake gold bars:
 Gold coated tungsten bars (>)
 Gold bars with tungsten rod inclusion (>)
Bars with defects
 Pores, bubbles, inclusions
 Hammerings (>)
 Bars with poor quality in other aspects
 Instrumentation available to identify
both fakes or low quality bars?
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Part I: Methods and instrumentation
 Element composition: Fire-assay, WD-XRF, ED-XRF, ICP-OES,
Spark-OES, etc.*
 Structural integrity: Ultrasonic methods, transmission X-ray
methods, eddy-current flaw detection technologies, radio wave
transmission, density analysis, metallography, etc.**
Ultrasonic**
Inspection
of bullions
Analytical integrity,
composition
Eddy-current
LBMA 2015, Bochud-Jodry-Sutter
ED-XRF*
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Part I: Metals and electrical conductivity
 Electrical conductivity, s , units MS/m
 Gold: 45, Silver: 63, Copper: 60 MS/m
Materials properties affecting s
 Microstructure: perfect crystal lattice → maximal s
Any defect reducing s (impurities, alloying, inclusions, precipitations, pores,
stress&strain, grain boundaries, surfaces, etc.)
 Temperature: T s
s
s
Ref: ESA Edu site
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Part I: Metals and electrical conductivity
 Electrical conductivity, s , units MS/m
 Gold: 45, Silver: 63, Copper: 60 MS/m
Materials properties affecting s
 Microstructure: perfect crystal lattice → maximal s
Any defect reducing s (impurities, alloying, inclusions, precipitations, pores,
stress&strain, grain boundaries, surfaces, etc.)
 Temperature: T s
LBMA 2015, Bochud-Jodry-Sutter
Parameter
Implication
Alloying elements
of interest
Inclusions, precipitations, pores
of interest
Grain boundaries
avoid influence
Stress&strain
avoid influence
Temperature
avoid influence
Surfaces
avoid influence
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8-10 March 2015
Part I: Phase-sensitive eddy-current method
 Following ISO 2360 for coating thickness measurements and DIN
EN 2004-1 / ASTM E1004 for conductivity measurements
 Simple and industry approved instrumentation
Sigmascope®
by Helmut Fischer
LBMA 2015, Bochud-Jodry-Sutter
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Part I: Phase-sensitive eddy-current method
 Coil system generating an alternating magnetic field which is
inducing eddy-currents in el. conducting materials. Eddy-currents
reduce the total magnetic flux, change of flux is detected by the
probe
 Evaluation:
amplitude > distance from material+thickness
phase > conductivity of material+thickness
 Calibration:
phase vs. s of certified reference
materials
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Part I: Phase-sensitive eddy-current method
 Coil system generating an alternating magnetic field which is
inducing eddy-currents in el. conducting materials. Eddy-currents
reduce the total magnetic flux, change of flux is detected by the
probe
 Evaluation:
amplitude > distance from material+thickness
phase > conductivity of material+thickness
 Calibration:
phase vs. s of certified reference
materials
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Part I: Phase-sensitive eddy-current method
 Performance (Sigmascope Gold B, FS14 probe at 1 kHz)
Repeatability: 0.03 MS/m
Accuracy: 0.3 MS/m
Measuring time: 2 sec
 Robust against
Contact quality: can be operated in non-contact mode
Surface quality: neither grease nor roughness a problem
 Sensitive against
Thickness: frequency dependent probing depth
Temperature: to be monitored and corrected (sensors available)
Stamping: has minor influence
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 Performance
Repeatability:
Accuracy:
Measuring time:
relative deviation [%]
Part I: Phase-sensitive eddy-current method
5
4
Metals can be analyzed
through plastic foils, no
coupling medium needed
3
2
1
0
0
500
1000
1500
polymer foil thickness [um]
 Robust against
Contact quality: can be operated in non-contact mode
Surface quality: neither grease nor roughness a problem
 Sensitive against
Thickness: frequency dependent probing depth
Temperature: to be monitored and corrected (sensors available)
Stamping: has minor influence
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Part I: Phase-sensitive eddy-current method
 Performance
Repeatability:
Accuracy:
Measuring time:
Probe frequency for different bar types
(it’s the only parameter!)
depth 
1
  f  0  s
 Robust against
Contact quality: can be operated in non-contact mode
Surface quality: neither grease nor roughness
a problem
In doubt stack
two bars…
 Sensitive against
Thickness: frequency dependent probing depth
Temperature: to be monitored and corrected (sensors available)
Stamping: has minor influence
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Part I: Phase-sensitive eddy-current method
 Performance
Repeatability:
Accuracy:
Measuring time:
s changes ~ 5% per 10°C! Software
applies corrections to measured s with
input of material and temperature.
 Robust against
Contact quality: can be operated in non-contact mode
Surface quality: neither grease nor roughness
problem
> more onathis
in Part II
 Sensitive against
Thickness: frequency dependent probing depth
Temperature: to be monitored and corrected (sensors available)
Stamping: has minor influence
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Part I: Phase-sensitive eddy-current method
 Performance
Repeatability:
Accuracy:
Measuring time:
 Robust against
Surface features with significant
Contact quality: can be operated
in non-contact
mode
conductivity
have an impact
on how
Surface quality: neither grease
nor roughness
a problem
fields propagate.
Deep stampings
affect the measurements (especially
 Sensitive against
at high frequencies).
Thickness: frequency dependent probing depth
Temperature: to be monitored and corrected (sensors available)
Stamping: has minor influence
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Part II: Applications on bullions
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Part II: Analyses of bars 999.9 ‰ and defects
Bullion 1
Bullion 2
Bullion 3
Bullion 4
Bullion 5
Bullion 6
Mean
44.56
44.40
44.32
44.50
44.43
33.52
Std.dev.
0.08
0.10
0.13
0.13
0.10
0.68
What’s the problem?
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Part II: Analyses of bars 999.9 ‰ and defects
Bullion 1
Bullion 2
Bullion 3
Bullion 4
Bullion 5
Bullion 6
Mean
44.56
44.40
44.32
44.50
44.43
33.52
Std.dev.
0.08
0.10
0.13
0.13
0.10
0.68
What’s the problem?
 Eddy Currents allow the detection of bullions with defects:
 Bubbles
 Inclusions
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Part II: Integrity of gold bullions Analysis
Pure gold
Gold with tungsten
rods
45.15
31.46
45.22
33.50
45.20
30.93
45.26
34.41
45.25
33.56
Mean
45.22
32.77
Std.dev.
0.04
1.50
 Difference of response between pure and fake bullions
 Worse repeatability on fake bar (not homogeneous)
 Response is similar to that of bar with defects
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Pure gold
Gold with tungsten
rods
45.15
31.46
45.22
33.50
45.20
30.93
45.26
34.41
45.25
33.56
Mean
45.22
32.77
Std.dev.
0.04
1.50
 Difference of response between pure and fake bullions
 Worse repeatability on fake bar (not homogeneous)
 Response is similar to that of bar with defects
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Part II: Gold between 995 and 999.9‰
 A Significant correlation R2 > 0.6
 But an important variation without a specific rang
 >100 bars with titles in
995 - 999.9 ‰ were tested
LBMA 2015, Bochud-Jodry-Sutter
Conductivity [MS/m]
 Influence of temperature
 Influence of minor elements
 Influence of surface
46
45
44
43
42
41
40
39
38
37
36
y = 1.1778x - 1133.7
R² = 0.6253
995
996
997
998
Au title [‰]
999
1000
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conductivity [MS/m]
Part II: Gold between 995 and 999.9‰
 Influence of temperature
 Without control (>)
45.5
45.0
44.5
44.0
43.5
43.0
15
20
25
30
temperature [°C]
 under control (with a stabilization of sample and equipment)
at fixed temperature: 20°C
at variable temperature
(corrected by Sigmascope)
Au5N
Au 997.5
Au 995
Au5N
Au 997.5
Au 995
Average
46.69
44.38
41.33
Average
46.64
44.36
41.28
SD
0.11
0.18
0.09
SD
0.15
0.17
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0.12
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Part II: Binary alloys analysis
 Possibility to do a calibration (in range of 995-999.9 ‰)
48.0
y = 1.1007x - 1053.9
R² = 0.9951
conductivity [MS/m]
47.0
46.0
45.0
44.0
43.0
42.0
41.0
40.0
994
995
996
997
998
999
1000
1001
Gold title [‰]
 But it works only with Binary alloys
 Or to perform QC on home-made alloys
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Part II: Binary alloys analysis
 Why only with binary alloys ?
Au 995 ‰
Ag 5 ‰
Au 995 ‰
Cu 5 ‰
41.45
36.41
41.42
36.51
41.41
36.53
41.39
36.40
41.35
36.49
Mean
41.40
36.47
Std.dev.
0.04
0.06
 Conductivities are different with
different minor elements!
 Exactly the same purity of gold (995 ‰)
 But the conductivity of gold with copper is lower than that of gold with silver
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Conclusions
Quality control information
Title
Pores
Inclusions
Other defects…




Monitoring of production
Refiner
Tolerance limits display
Banks /
Industry
Recycler
Gold
Trade
LBMA 2015, Bochud-Jodry-Sutter
Assay
offices
Check integrity of bar
Judgments based on simple criteria!
 s > 44 MS/m: 999.5 grade bar
 44 > s > 35 MS/m: LBMA grade
bar
 s < 35 MS/m: suspicious bar →
further investigation
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Conclusions
 Questions….
 Thank you for your attention.
___
 Additional Information
www.goldanalysis.com
www.helmutfischer.com
 Contacts
Daniel Sutter, daniel.sutter@helmutfischer.com
Quentin Bochud, quentin.bochud@metalor.com
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