Ultrasonic Testing of Gold Bars
Nondestructive verification of the physical integrity of gold bars.
The rise in fraudulently adulterated gold bullion bars by inserting slugs of inexpensive base metal of similar density causes much concern in the gold markets. These insertions are difficult or impossible to detect through weighing or radiography technologies. X-ray fluorescence is a very reliable way to confirm the purity, but XRF is a surface technique. Some precious metals processors have even resorted to drilling or cutting bars to verify integrity. However, a simple ultrasonic test can quickly and reliably locate inserts with no need to drill, cut, or otherwise alter the bar. Combined with XRF surface testing, the processor can be confident of the purity and integrity of their gold.
Any Olympus flaw detector or phased array instrument can be used for this test. These include the EPOCH 600, EPOCH 650, EPOCH 1000, OmniScan SX, and OmniScan MX2. The recommended transducer frequency will typically be 2.25 MHz.
Adulteration of a gold bar by means of inserts will cause predictable changes in the way ultrasonic waves pass through the metal. Inserts of material other than gold in a bar will change the pattern of wave reflections, a response that can also be caused by internal voids. Large inserts that fill most of the volume of the bar can also be detected through changes in sound velocity.
1. Pulse/echo reflection method
Ultrasonic waves traveling through any medium will continue propagating in the same direction until they strike a boundary with a different material, which will cause them to reflect back to their source. Ultrasonic flaw detectors and phased array instruments generate pulses of high frequency sound waves with small, hand-held transducers. The sound energy is coupled into the test piece and the instrument monitors and displays the pattern of reflected echoes. Reflections coming from inside a gold bar, rather than the opposite surface, change the pattern and indicate either an insert of another metal or a internal void.
This test is set up by coupling the transducer to a known good bar and identifying the echo from the bottom surface. A flaw gate may be used to monitor the interval ahead of this back wall. Any echoes appearing within the region marked by the gate indicate that the sound beam is reflecting off a discontinuity, and the bar should be further inspected by other means. Typical screen displays are seen below.
Conventional flaw detector images from solid metal (left) and metal with discontinuity (right).
Note: Echo appearing in region marked by red gate.
Phased array images from solid metal (left) and metal with discontinuity (right). The discontinuity appears as an indication in what should be a white region of the image.
2. Velocity method
The sound velocity in pure gold is 3,240 m/S or .1275 in/uS. The harder gold alloys used in jewelry will typically be somewhat faster, but will similarly have a specific velocity associated with a given alloy. If the sound velocity in a bar deviates from the expected value, then the metal content has been altered.
This test is set up by coupling the transducer to a known good bar and identifying the backwall echo. That backwall echo may be marked by a flaw gate. If the position of that echo changes with no accompanying change in bar thickness, then the sound velocity in the metal has changed and the bar should be further inspected by other means. The most common adulterants increase sound velocity and thus move the echo to the left, as seen above.
Note: With small modifications these test can be applied to other precious metals such as silver and platinum as well. Contact VM ViSion for further information.
Products used for this application :
- Epoch 1000 series
- Epoch 650
- Goldexpert Mobile XRF Analyzer
- OmniScan MX2
- Omniscan SX