Alloy Analysis of Precious Metals

Measurement technology for buying and trading in gold. With instruments from FISCHER you can test the authenticity of gold and other precious metals – reliably and non-destructively. The devices detect the slightest differences in alloys and are thus also suited for the authentication of valuable specie or the analysis of complex decorative coatings. Rely on more than 60 years' experience.

Alloy analysis of precious metals

Application notes

Determination of the Silver Content of Silver Plated or Blanched Silver Alloys

For finishing purposes silverware is often plated or blanched. In the plating process, a pure silver coating is applied to a silver/copper alloy. This produces an attractive, white satin sheen. Blanching achieves the same effect by chemically removing copper from the topmost layer of the silver/copper alloy. The result of both methods is a veneer of much higher silver content than the substrate, which complicates the determination of the fineness, and therefore, of the value of the silverware.

The only foolproof method for correctly determining the silver content of silver-plated or blanched silver/copper alloys is to test the base material directly, for example, by grinding or sawing down into it, since non-destructive standard analysis with X-ray fluorescence (XRF) shows erroneously high silver content due to the increased concentrations at the surface.

Using a measuring application which looks for a silver coating atop a silver/copper alloy, FISCHER X-ray fluorescence instruments allow for the non-destructive determination of both the thickness of the silver coating as well as the fineness of the base material. This works accurately for silver-plated items because the silver content in the plating is consistent. However, with blanching the silver content decreases steadily as the distance from the surface increases which makes the determination much more difficult.

Figure 1: Blanched silver coin and enlarged image of the cross-section. The coin was cut in half to gather a depth-dependant profile for verification.

The blanched silver coin shown in Fig.1 was made from a 625 silver alloy (i.e. 62.5% nominal silver content).

A simple material analysis using XRF would show a silver content of 85%, a value significantly greater than the nominal silver content.

Figure 2: Depth dependant silver content of a halved coin. The nominal silver content is 62.5%. The measurement was verified using a FISCHERSCOPE® X-RAY XDV®-µ instrument with high local resolution.

Fig. 2 shows how the silver content increases as a function of the closeness to the coin surface, where the silver concentration approaches 100%. Using the FISCHERSCOPE® X-RAY XAN® 220 and an application created specifically for the measurement of blanched silver, “silver on silver/copper”, a concentration of 61.4% is obtained, which is very close to the nominal silver content. To verify the results, this coin was then cut in half, allowing direct measurements in the cross-section using an XDV-µ instrument with an extremely small spot size due to a focusing optics.

Using specialised applications with the FISCHERSCOPE® X-RAY XAN® 220 makes it possible to determine the fineness of the substrate and the thickness of the covering layers of both silver-plated and blanched silverware – quickly, accurately and non-destructively. Your local FISCHER representative will be glad to answer additional questions you may have. 

Authenticity testing of gold bullion using electrical conductivity

Gold and precious metals have always been popular investment objects. However, due to the sky­rocketing prices of precious metals in recent years, the production of counterfeit gold bars has become a booming industry; more and more sophisticated fakes emerge ever more frequently, leading to much uncertainty in the market and investor wariness. This is why the demand for fast, reliable and non-destructive testing has also been on the rise.

To determine the authenticity of gold bullion – non-destructively and therefore without loss of value – its electrical conductivity is measured using the eddy current method according to ASTM E 1004. This method utilizes the physical fact that fine gold, various alloys and foreign material will all differ in their conductivity, there­fore allowing forgeries to be detected.

Fig.1: SIGMASCOPE® GOLD B for testing gold bullion and ingots – even through a plastic cover

For this purpose FISCHER has developed the SIGMASCOPE® GOLD B, which can determine the electrical conductivity of gold bars up to about 1 kg in weight. By taking measurements from both sides toward the middle, the entire depth of the bullion can be measured, testing the genuineness of the fine gold. Hidden enclosures of non-precious metals with a comparable density (e.g. tungsten) can be detected unambiguously with the SIGMASCOPE® GOLD B – and the piece revealed as counterfeit. The phase-sensitive evaluation of the measurement signal enables determination of the electrical conductivity without surface contact – even through non-conductive protec­tive layers such as plastic packaging.

Fig.2: Schematic drawing of a counterfeit gold bullion showing a core of tungsten sticks hidden by a real gold skin

The SIGMASCOPE® GOLD B is designed to operate with three measurement ranges for different penetration depths of the electrical field. A penetration depth at least to the very centre of the bullion is necessary in order to detect even tiny inclusions at its core. For small ingots (e.g. 1 oz / 50 g) the SIGMASCOPE® GOLD B also provides thickness compensation; that is, the thickness of the bar is entered into the instrument and accounted for in the measurement results. Any influence of the conductivity value by various thicknesses is avoided by operating with the correct measuring range and the thickness compensation.

Fig.3: The electrical field to determine the conductivity penetrates through the gold bullion, forgeries show different conductivity values and can therefore be easily detected, wherever they are located.

Measuring the electrical conductivity is a precise yet quick method for testing – non-destructively – the genuineness of valuable items made of precious metals. X-ray fluorescence analysis (XRFA) is a suitable complement for accurately determining their composition. For further information please contact your local FISCHER representative.

Using X-ray fluorescence for fast, reliable gold analysis in the gold-buying industry

The economic crisis has coincided with historically high gold prices to boost the importance of so-called “cash for gold” businesses. Because the buyer usually has only a few minutes to estimate the value of gold items presented for sale, methods like touchstone analysis are often used: Although this test severely scratches the piece it is still not 100% reliable. The industry demands a precise, quick and foolproof method for testing gold content that is, above all, non-destructive.

Fig.1: Various items potentially presented for sale to gold buyers

Besides the weight, the gold content of an object determines its value. The commonly-used analysis methods pose different disadvantages for retail buyers of personal gold items: Fire assaying is time consuming and relies on chemical processes, and despite the fact that touchstone analysis also requires acids for testing, the results are still not always reliable. Because both methods damage the item in varying degrees, neither really meets the needs of the industry.

However, X-ray fluorescence analysis (XRF) offers a non-destructive method for quick material analysis and coating thickness measurement on jewelery, watches and other precious metal products; FISCHER’s XRF instruments are simple to use, even for non-technicians.

The FISCHERSCOPE® X-RAY XAN® 220 enables precise and reliable determination of gold and platinum content even under challenging circumstances. Fast and easy-to-use, the XAN 220 features excellent long-term stability and rarely ever requires calibration. Operation is as simple as opening the hood, placing the item on the inspection window, closing the hood and starting the inspection with the press of a button. In less than a minute the exact content of gold and various other elements is presented on the screen.

Functionality and the minimization of running costs took high priority during development of the XAN 220. Designed without moving parts – thus eliminating wear and tear – the XAN 220 is truly a precision instrument meant for real-life, daily use.

Advantages:

  • Fast: results in less than a minute
  • Easy: single-button functions
  • Reliable and accurate: intelligent WinFTM® software prevents measurement errors
  • Robust and stable: factory calibrated, no time-consuming re-adjustments

Fig.2: Fast and non-destructive measurement of a gold wristband with the FISCHERSCOPE® X-RAY XAN® 220

The innovative FISCHERSCOPE® X-RAY XAN® 220 uniquely meets the needs of professional end-users in the gold buying market. With this measurement system, jewelery and other gold items can be analysed quickly, accurately and non-destructively. Your local FISCHER dealer will be happy to provide you with more information.

Precious metal analysis via X-ray fluorescence for assaying offices and precious metals refineries

For assayers and refiners of precious metals, material analysis instruments must fulfill exacting requirements. Just determining the precise gold content is not enough: It is important to ascertain the complete composition of the alloy, including elements like platinum or silver. In addition to reliably generating comprehensive, accurate, and reproducible results, the ideal analysis procedure should also be fast, easy to use, and – of course! – non-destructive.

Fire assaying (cupellation) is the traditional method for determining gold content, whether for gold bars, coins or valuable jewellery. However, serious handling issues outweigh this method’s good precision, as it is time consuming, requires the use of acids and, worst of all, damages the item in the process.

Fig.1: FISCHERSCOPE® X-RAY XAN® 250 provides the highest precision in precious metal analysis.

Fortunately, the well-established X-ray fluorescence method (XRF) provides an excellent alternative for analysing the content of precious metals – without damaging the object. FISCHER’s high-end XRF instrument, the FISCHERSCOPE® X-RAY XAN® 250, is designed especially to determine precious metal content. Providing precision rates of 0.3‰ or better, the XAN 250 can help assayers and refiners significantly reduce the number of cupellation tests required, saving them valuable time and resources

The XAN 250 both determines the material content and assesses coatings all in one analysis cycle, with extra-ordinary repeatability: Table 1 shows the results from ten measurements of a 14-carat gold object coated with 400 nm of rhodium, taken specifically to verify the instrument’s repeatability precision.


Rh

[µm]

Au

[‰]

Ag

[‰]

Mean

0.402

583.3

31.02

Standard
Deviation

0.001

0.150

0.188

Coefficient of
Variation [%]

0.24

0.03

0.61

Range

0.003

0.518

0.550

Tab. 1: Repeat measurements of coating thickness and base material composition with the FISCHERSCOPE® X-RAY XAN® 250. Ten readings of 120 seconds each were taken on a rhodium-coated gold sample.

At the same time, further measurement tasks can be covered like determination of the exact content of platinum or testing for the presence of prohibited elements such as Ni, Cd or others.

XAN 250’s new silicon-drift-detector combines with advanced electronics and FISCHER’s superior analytical software, WinFTM®, to give exceptional performance. And whatever the task at hand, interchangeable filters and collimators (for variously sized measurement spots) are available for truly perfect, flexible instrument set-up.

FISCHERSCOPE® X-RAY XAN® 250 is the leading-edge measurement system for fast and non-destructive precious metal analysis. With its outstanding trueness and superb repeatability precision, the XAN 250’s measurement results are comparable to cupellation – but far easier to obtain. Your local contact person for FISCHER products will gladly provide further information.

Electrical conductivity testing of precious metal coins

In recent years gold and other precious metals have drastically increased in value, making the purity of the base components the most important attribute of any precious metal product. Therefore, precisely determining alloy composition and monitoring the trace and minor elements they contain is critical. Testing high-value coins and identifying fakes requires a reliable, accurate and non-destructive measurement technique.

When dealing with high-value coins, adulterated or counterfeit alloys can cause considerable losses and trading in fakes can entail legal ramifications. To avoid these risks, reliable analytical methods are necessary – but destructive analytical methods damage the coin’s material and value, which is unacceptable. Thus, non-destructive methods have become the standard for material testing valuable objects made of precious metals. In short, these methods must be predictable, reproducible and able to:

· Identify the alloy and the precious metal content

· Detect ignoble inclusions and forgeries

The various alloys and fine gold all differ in their electrical conductivity, making this particular physical dimension optimally suited for the quick analysis of precious metal bullion and coins.

Fig.1: Schematic drawing of counterfeit coins filled with a powdered tungsten alloy and covered with gold

The FISCHER SIGMASCOPE® SMP10 measures electrical conductivity using the eddy-current method according to DIN EN 2004-1 and ASTM E 1004. The phase-sensitive evaluation of the measurement signal enables determination of the electrical conductivity in a contactless way – even through non-conducting protective layers such as plastic packaging.

The SIGMASCOPE® SMP10 uses a measurement frequency of 60 kHz (or alternatively 120, 240, 480 kHz) and is therefore optimised for the testing of coins and thin ingots.

Fig.2: Measurement of the electrical conductivity of coins – fast and reliable

Gold coins such as the Kruegerrand consist of a precisely defined alloy which, itself, features an exactly defined electrical conductivity value. Authentication based on this characteristic is therefore quick and easily verifiable. Matching conductivity values can only be achieved by using a metal alloy with a dramatically different density, which is then evident in the visibly different coin thickness.


Electrical conductivity

Mean value

9.68 MS/m

Standard deviation

0.07 MS/m

Coefficient of variation

0.71%

Tab.1: Authentication of a Kruegerrand coin which should exhibit a nominal electrical conductivity of 9.69 ± 0.32 MS/m

The SIGMASCOPE® SMP10 can reliably determine the alloy composition of gold coins and thin ingots via electrical conductivity, making it possible to test their authenticity without actually touching the items. For further information please contact your local FISCHER representative.

Analysis of Tarnish-Resistant Silver Alloys

Sterling Silver is often used for making jewelery and decorative items. However, one common problem with silver is surface oxidation, or "tarnishing". To prevent or at least reduce this effect, various alloy elements are added to the silver.

Silver is used in the production of numerous decorative items ranging from jewelery to accessories to musical instruments and fine cutlery; its bright color makes this metal particularly attractive for such applications. The most popular alloys consist of silver and copper with typical millesimal fineness of 825, 925 (Sterling), or even 935 parts silver per thousand by mass. Cu is added to improve the strength and workability of the material.

Though silver is a precious metal, it has a strong tendency to oxidize on the surface, discoloring its bright, glossy face with a dark, dull stain. This effect, called tarnishing, also occurs in copper-alloyed silver.

Fig 1: Tarnished silver, the tarnish may appear either uniform or spotty.

Therefore, the focus of much R&D has been to improve silver’s tarnish resistance through the addition of small amounts of a variety of elements, including precious metals such as Palladium and semimetals like Germanium, among others. While this vast range of possible alloys makes analyzing the silver challenging, X-ray fluorescence instruments allow for quick, accurate, non-destructive analysis. Fig. 2 shows sections of spectra of various such alloys.

Fig 2: Spectra of various tarnish-resistant silver alloys. The signals of the more exotic elements Ge (blue lines) and Ga (red) are particularly noticeable.

For such special alloys reference materials for the calibration of XRF instruments are not always available. Therefore it is particularly important to be able to perform uncalibrated (standard-free) measurements that lead to high-quality, reliable results.

Ag

Cu

Ga

Ge

Pd

Sn

In

932

65.8

0.1

0.2

n.d.

1.3

0.3

913

59.5

0.4

0.2

n.d.

3.3

n.d.

898

37.3

6.0

0.2

31.5

n.d.

n.d.

928

56.7

0.1

12.1

n.d.

2.1

0.1

926

44.9

5.6

0.2

n.d.

6.6

5.4

922

39.5

0.1

0.1

n.d.

0.1

5.0

Table 1: Several chemical compositions of tarnish-resistant silver alloys (not listed are concentrations of Zn, Fe, Ni; in per mil).

FISCHERSCOPE® X-RAY XAN® 250 instruments are optimally suited for visualizing even the subtlest differences in silver alloys. They allow precise purity measurements as well as the exact determination of tiny amounts of tarnish-resistant materials and other alloy components. For more information please contact your local FISCHER representative.