Conventional
techniques for precious metal quality assessment
Historically, inspection of gold
quality has mostly involved chemical analysis to determine purity, using
techniques such as fire assay[1]. But as this is a
wholly destructive technique, there are obvious disadvantages. More recently
minimally or non-destructive techniques such as mass spectrometry [1] or
X-ray fluorescence [2,3], and even low temperature
photoluminescence [4] have been used. These approaches may also be applied to
other precious metals such as silver and platinum.
These can provide relatively
rapid inspection with negligible damage to the material, because they probe
only the outer surface and rely on the assumption that the material is
homogeneous. The problem here is that large regions of impurity – even
a completely different metal – will be undetected by these techniques
providing the outer layer is of pure metal.
The only conventional technique
that is both non-destructive, and able to detect impurities deep within
precious metal objects, is the accurate determination of net density (via
weight and displacement volume). This is a simple but highly effective
approach. However, it cannot detect substituted materials that have the same
density as the precious metal that they displace. While no two metal elements
have exactly the same density, some are quite close – for example
tungsten has a density of 19,254 kilograms per cubic metre,
compared to 19,281 kilograms per cubic metre for gold[5]. This is sufficient to fool density testing in
many cases.
Ultrasonic techniques
An ultrasonic wave can propagate
through solid materials such as metals without doing damage. Where the wave
encounters a region of material with different physical properties –
particularly the density and elastic constants – to the rest of the
metal, the beam is affected in a number of ways:
 Scattering (attenuation) can occur as
a portion of the wave is reflected or reradiated from small regions of
different material, or from an irregular interface between two larger regions
of different material. This manifests itself as a reduction in the amplitude
of the wave
A change of arrival time of the wave
at the detecting transducer can occur if the wave passes through a
significant region of material where the ultrasonic velocity is different
Refraction/reflection of the wave
– or where there is a relatively (compared with the wavelengths of
ultrasound used) smooth interface between two regions of different material,
a portion of the wave will be reflected from this interface. Impurities will
result in the wave being refracted at a different angle
All of these effects can be
detected and used to determine the existence of, and in some cases quantify,
impurities (see figure 1). A material that has similar density
to a precious metal, and thus may be used as a dopant that can fool a density
measurement as discussed previously, may have very different ultrasonic
properties. For example, although tungsten has an almost identical density to
gold, it has 40% and 60% higher shear and longitudinal wave velocities,
making it relatively easy to detect using ultrasound.
Practical application
Ultrasound can be generated and
detected using a wide variety of transducers, including piezoelectrics,
electromagnetic acoustic transducers (EMATs), lasers and capacitive
transducers. By sending an ultrasonic pulse through the bulk of the precious
metal and monitoring the signal that returns, impurities and structural
defects in the metal can be determined by observing the arrival time and
amplitude of the returning wave. Other effects such as additional returning
waves or changes to the waveform shape can also provide information about any
impurities and defects.
Additional transducers can also
be used to detect the ultrasound transmitted through the object, or to look
for ultrasonic waves scattered, reflected or refracted from, or by, impurity
in the metal.
The technique can be further
improved by using a phased array ultrasonic system. In its simplest form this
allows the angle of the ultrasonic wave to be controlled by moving the
elements of the array slightly out of phase with each other (see figure 2). Additionally, more advanced phased
array techniques such as full matrix capture and synthetic aperture focussing can be used to generate a detailed
three-dimensional map of the metal – although these techniques are more
complex and can lead to longer inspection times. Technological advances in
this field are advancing at a rapid pace.
Summary
Ultrasonic inspection is an important
tool in the arsenal of stockists and traders in
precious metals and precious metal artifacts
Its advantage over most other
techniques is that it can detect “buried” regions of impurity
within the metal, even if the impurity has the same density as the
surrounding metal
Understanding the physics behind the
ultrasonic interactions is important for meaningful interpretation of the
results
Ultrasonic scanning is both
non-destructive and rapid, allowing routine measurements to be made on all
samples
References
[1] Origin
and Effects of Impurities in High Purity Gold, David J Kinneberg and Stephen R Williams, Gold Bulletin Vol.31
(2), 1998, pp.58 – 67
[2] Non Destructive Analysis of
Gold Alloys Using Energy Dispersive X-Ray Fluorescence Analysis,
Volker Rößiger and Bernhard Nensel, Gold Bulletin Vol.36 (4), 2003, pp.125 –
137
[3] Application of Energy-dispersive
X-ray Fluorescence to Jewellery Samples determining
Gold and Silver, A. Jurado-López,
M.D. Luque de Castro, R. Pérez-Morales, Gold
Bulletin Vol.39 (1), 2006, pp.125 – 137
[4] Photoluminescence of gold,
copper and niobium as a function of temperature, Helen Armstrong,
D.P.Halliday, Damian P.Hampshire,
Journal of Luminescence Vol. 129 (2009) pp. 1610–1614
[5] Tables of Physical and Chemical
Constants, G. W. C. Kaye and T. H. laby,
(c) 1973, 1986 Longman Group Ltd. 15th Ed
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