As the global bullion market continues to modernise, the tools used to assure trust in physical gold have evolved alongside it. Among these, non‑destructive testing (NDT) techniques have taken on increasing importance. Vault operators, refiners and assayers are expected to verify the integrity of Good Delivery Bars without compromising their physical condition, chain of custody or marketability. Ultrasonic testing – long established in industrial applications – has therefore attracted growing interest within the precious metals sector.
For the past couple of years, Phased Array Ultrasonic Testing (PAUT) has been introduced as a potential enhancement to conventional ultrasonic inspection of gold bars. PAUT offers higher data density, improved coverage and recordable scan data. However, experience from both operational use and targeted investigations shows that while PAUT can provide valuable insight into the internal structure of bullion, it also raises new challenges around interpretation, sensitivity and standardisation.
This article explores how PAUT works, why it matters to the gold market, and what recent investigative testwork has revealed about its strengths and limitations when applied to cast gold bars.
From Conventional Ultrasound to Phased Array
Traditional ultrasonic testing uses a single transducer to transmit and receive sound waves. Reflections from internal interfaces are interpreted to determine material thickness or identify discontinuities. While effective, this approach relies heavily on probe movement and operator technique, offers limited spatial coverage per scan, and can be sensitive to defect orientation.
PAUT builds on this foundation by using a probe containing multiple piezoelectric elements. These elements are pulsed independently with controlled time delays to create a focused beam of sound waves. This technique allows for the inspection of materials and welds with high precision and accuracy to ensure a comprehensive defect detection and analysis in materials and welds.
PAUT is widely employed across numerous industries due to its flexibility and essential function in maintaining safety and quality standards. In the oil and gas industry, it serves to inspect pipelines and pressure vessels. The aerospace sector uses PAUT for examining aircraft components, while manufacturers rely on it for monitoring weld quality. It also plays a vital role in power generation by inspecting turbines and in the automotive field for checking engine parts. Across these sectors, this type of testing is primarily performed to prevent catastrophic failures.
Why PAUT Appeals to the Bullion Market
Gold bars are dense, visually uniform and chemically homogeneous. Once cast and stamped, their integrity is typically verified through a combination of visual inspection, mass and dimensional checks, and assay. However, these methods provide little insight into internal features.
PAUT offers the ability to ‘see inside’ a bar without cutting or drilling. It can identify large internal voids, density changes or foreign material inclusions that disrupt the propagation of sound. For vaults and customers, this strengthens assurance without introducing radiation hazards or damaging the physical bar. For refiners and assayers, it provides an additional data layer that can be retained and referenced to ensure the integrity of the bars.
At the same time, PAUT introduces a level of sensitivity far beyond that of conventional ultrasound. This sensitivity is both a strength and a challenge.
Gold Bars Are Not Perfect Solids
Although Good Delivery bars are produced under tightly controlled conditions, they are not metallurgically perfect monoliths. Casting or manufacturing involves molten metal, moulds, atmospheric control and solidification — all of which can influence the internal structure.
Investigations have shown that microscopic air pockets or porosity can form during solidification, even when best‑practice procedures are followed. These features are typically metallurgically benign and have no impact on purity, mass or market acceptability. However, from an ultrasonic perspective, the contrast between gold and trapped gas is extreme. Even very small voids or hairline cracks can reflect sound strongly, attenuate the backwall signal or locally alter depth readings.
As a result, PAUT scans of otherwise acceptable bars may show signal loss, reduced backwall response or variations in depth‑related parameters. These responses do not necessarily indicate foreign material or fraud — they reflect physical realities of casting. But it could result in a batch of Good Delivery Bars to be rejected since it failed the ultrasonic test, which is a frustration for refiners, vaults and customers.
Understanding PAUT Parameters
To accurately interpret PAUT data, certain parameters are used during ultrasonic inspection of Good Delivery bars. The probe applied in these tests features 64 piezoelectric elements operating together, and proper result interpretation is crucial. Factors such as peak depth and signal amplitude provide details that should closely match the actual height of the bar. The scan image should clearly show both the top and bottom lines, indicating the corresponding surfaces of the Good Delivery bar. Because these measurements are linked and offer a comprehensive overview, any failed result requires careful evaluation of all parameters to determine the cause.
Consistency is just as vital. For results to be reliably compared between different bars, operators, and locations, items like probe type, ultrasonic velocity settings, calibration blocks, gain, gate positions, and scan plans must all be standardised.
Introducing Foreign Materials: What Testwork Revealed
In the beginning when PAUT was introduced as part of the final quality control of the Good Delivery bars, a number of bars were rejected (either onsite or at vaults). This led to the question: how does PAUT respond to different internal features? Controlled testwork was undertaken in which foreign materials were deliberately introduced into gold bars during casting under monitored conditions. The aim was to explore how different materials –which could accidently be introduced during the normal production process – could influence ultrasonic response. Certain tests were performed to simulate fraudulent situations; however, the findings are too sensitive to be disclosed.
Materials introduced included slag and crucible fragments representing realistic production risks, as well as iron and other high‑density metals. Parallel testwork involved adding silver and copper to assess alloying behaviour and its impact on ultrasonic signals.
The results were instructive. Silver and copper alloyed fully with the gold in the respective areas where it was introduced, producing no visible surface features and generating ultrasonic responses consistent with homogeneous material. Despite the presence of additional metals, the acoustic contrast within the alloyed regions was insufficient to disrupt the scan profile.
Slag and crucible fragments, by contrast, were detected when present as discrete inclusions. Their size and acoustic mismatch disrupted sound transmission, resulting in localised loss of backwall signal and reduced amplitude and signal parameters.
Gold bar appearance after the introduction of foreign material like slag, iron and steel parts
The results of these tests indicate that if foreign material is inadvertently present in the bars, its detection through visual inspection will result in a failure during the quality control process and therefore the risk of such a bar being shipped to the vaults are minimal.
Introducing Atmospheric Variation: Insights from Testwork
Understanding ultrasound results in the context of foreign materials prompted further inquiry into the reasons behind Good Delivery bars failing PAUT testing. Several failed bars were sectioned for internal evaluation, with initial analyses indicating the presence of microscopic air pockets, often located near the surface.
These observations aligned with the amplitude parameter results. To determine the origin of these air pockets, multiple scenarios were simulated by altering the furnace’s atmospheric conditions. The results varied from the identification of microscopic holes or cracks with and without the detection of oxygen and nitrogen gases – with each outcome directly associated with specific atmospheric conditions introduced into the furnace.
Gold bar with microscopic air pockets which resulted in a failed ultrasound report
From Detection to Understanding
Perhaps the most important lesson from both operational experience and testwork is that PAUT does not deliver simple pass‑or‑fail answers. It delivers information. It is a highly sensitive inspection tool which can even detect the presence of microscopic holes.
This information must be interpreted within a broader framework that includes metallurgical knowledge, manufacturing context and complementary inspection methods. Visual inspection, mass verification, elemental analysis and microstructural analysis where necessary all play a role in building a coherent assessment.
PAUT should therefore be seen as part of a layered assurance strategy rather than a standalone arbiter of acceptability. Over‑reliance on numerical thresholds without context risks false positives, unnecessary escalation and erosion of confidence in an otherwise sound process.
Implications for the Industry
With the increasing adoption of PAUT, the bullion industry faces a pivotal decision. The technology can be incorporated simply as routine practice, or it can be purposefully woven into inspection frameworks that feature comprehensive training, a shared understanding, and well‑defined guidelines for all stakeholders.
Establishing unified standards regarding what is acceptable and what raises concerns is vital. These standards should be supported by scientific validation from other non‑destructive testing methods, which can work together within an inclusive testing package or matrix, rather than relying on any single indicator.
PAUT offers the bullion market a robust new way to assess physical gold. It improves transparency, enhances record‑keeping, and bolsters confidence — provided it is implemented with proper knowledge, clear procedures, and as part of a broader testing strategy for both refiners and vault operators.
Recent investigations underscore a crucial point: technology alone cannot provide certainty. Interpretation, context, and oversight are equally important. By recognising both the advantages and limitations of PAUT, the gold industry can sustain trust while meeting ever‑evolving inspection standards.
