An alternative to fire assay: confirming fineness by X-ray fluorescence

For a long time, determining the fineness of a precious metal meant removing part of it. X-ray fluorescence changed that: in a few seconds, it analyses a piece directly, without altering it.

That is the role of X-ray fluorescence.

On a homogeneous, uncoated alloy, a properly calibrated XRF instrument can confirm the declared fineness of a precious metal. This use is now governed by the standard SN EN ISO 23345, which covers the non-destructive confirmation of fineness by ED-XRF.

XRF is therefore no longer just a quick sorting tool. Under the right conditions, it is a genuine control method.

Checking without sacrificing the piece

The difference from fire assay is immediate.

Cupellation requires a sample to be taken. X-ray fluorescence, by contrast, measures the object itself: a piece of jewellery, a watch component, an alloy, a semi-finished part or metal bound for refining.

A few seconds are enough to identify and quantify the main elements present: gold, silver, copper, platinum, palladium, nickel or other constituents of the alloy.

No shaving is removed. The piece stays intact.

Above all, it makes it possible to multiply measurements: check several objects in a batch, compare different areas of the same piece of jewellery, or verify a material before it is worked.

The value of XRF is not only speed. It is that it makes control far easier to apply systematically.

Confirmation of fineness, now standardised

ISO 23345 sets out precisely how energy-dispersive X-ray fluorescence, or ED-XRF, is used to confirm non-destructively the fineness of finished or semi-finished jewellery articles considered homogeneous.

In Switzerland the standard has been adopted as SN EN ISO 23345.

The principle matters: when the material is homogeneous and the surface is representative of the whole, the composition measured by XRF can be used to confirm the stated fineness.

The method does, however, require a properly calibrated instrument, appropriate reference materials and control of measurement uncertainty.

In other words, performance does not depend on the spectrometer alone. Calibration is part of the measurement.

A few microns are enough — provided you know what you are measuring

The main limit of X-ray fluorescence is well known: it essentially analyses the surface.

In gold, the useful analysis depth is generally less than 10 microns. X-rays emitted deeper are largely absorbed before they leave the metal.

On a homogeneous alloy, this shallow depth is no difficulty: surface and core have the same composition.

On a plated piece, or one whose surface has been enriched, the situation changes. The instrument can measure the outer layer very precisely without seeing what lies beneath.

That is why the standard excludes coated articles.

This limit is not specific to any particular model: it comes from the very physics of X-ray fluorescence.

Where XRF has the edge

For a workshop, then, the question is not to replace every fire assay as a matter of course.

It is rather how many checks can be carried out directly, without destruction.

On a homogeneous alloy, XRF makes it possible in particular to:

• verify a declared fineness quickly;
• identify the composition of an alloy;
• check batches without multiplying samples;
• compare several points on the same piece;
• spot a solder joint or an atypical composition;
• obtain a result that can be used in the workshop straight away.

Fire assay keeps its value when a destructive reference analysis is needed, when the internal composition is uncertain, or when an official procedure requires it.

But it no longer needs to be the first reflex for every piece.

In Switzerland, technical control and official control are not the same thing

This distinction is particularly important.

XRF can technically confirm the fineness of an alloy within the framework defined by the standard. It does not, however, replace the obligations laid down by Swiss precious metals legislation.

Watch cases in gold, silver, platinum or palladium placed on the Swiss market are, in particular, subject to official testing and hallmarking. For other articles made of precious metals, official control is in principle optional.

XRF therefore comes in upstream or alongside: goods-in inspection, manufacturing, quality control, purchasing, sorting or internal verification.

For a watchmaker or a jeweller, that is precisely where it comes into its own.

The instrument matters. Calibration more than you might think.

Two XRF instruments can display the same gold content without offering the same measurement quality.

The detector, the geometry, the spot size, the measuring time and the analysis conditions all play a part. But the reference standards and the calibration are just as decisive.

An instrument designed for general alloys is not necessarily optimised to distinguish accurately between two close gold finenesses, or to work on alloys containing palladium, platinum or silver.

The choice of an XRF system should therefore be made on the basis of the actual application — and, ideally, on the user's own samples.

Test before you choose

YSPEKH distributes and configures portable and benchtop XRF solutions in Switzerland for the analysis of metals and precious metals.

We also carry out calibrations and adapt measurement methods to the alloys our customers actually work with.

Jewellery, gold alloys, silver, platinum, palladium or watch components can be tested directly before the instrument is chosen.

The aim is not simply to know whether a spectrometer "detects gold".

It is to know how precisely it answers your control problem.