Ultrasonic inspection of precious metals

2012-JUL-16

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