XRF testing explained: what it reads and where it stops

What X-ray fluorescence actually does

An XRF analyser aims a beam of X-rays at the metal. Some of that energy knocks electrons out of the inner shells of the atoms, and other electrons drop in to fill the gaps. As they do, the atoms give off small bursts of X-rays of their own. That returning glow is the fluorescence in the name.

Here is the clever bit. Each element gives off X-rays at its own set of energies, so gold has one signature, silver another, and copper, zinc, nickel and tungsten each their own. A detector inside the analyser sorts the returning signal by energy and counts how much of each signature turns up.

Software turns those counts into a breakdown of the metal. You place the piece over the measuring window, the machine runs for a few seconds, and a list of elements with percentages appears on the screen.

What an XRF reading looks like at the counter

A genuine 999.9 gold bar reads as essentially pure gold, at or very close to the fineness stamped on it, sometimes with a faint trace of something else at the edge of what the instrument can detect. A fine silver bar reads almost entirely as silver. That is what we expect from refiners such as PAMP, Valcambi, Perth Mint and Metalor.

Alloys are where XRF really proves useful. A 22 karat coin like a Krugerrand or American Gold Eagle should show roughly 91.6% gold, with the rest made up of copper and, in the Eagle's case, some silver as well. That is correct for those coins. So staff judge every reading against what the piece claims to be, and a bar stamped 999.9 that comes back with a few percent of copper or silver gets set aside.

Jewellery shows how much detail you get. A 916 bangle will usually show gold alongside silver and copper, sometimes zinc, and white gold often reveals palladium or nickel. We read the whole list, not just the gold figure.

Why XRF does no harm to your bar or coin

Older methods meant rubbing the gold on a touchstone and applying acid, or filing a small notch to see what lay underneath. Both leave marks. On a bar or coin where condition matters, nobody wants a scratch.

With XRF, the piece simply rests on the window. Nothing is removed or dissolved, and the X-rays do not change the metal. A coin goes back into its capsule and a bar back into its card looking exactly as it did before.

There is one practical catch. The beam has to reach the metal, so plastic, a sealed card, a coating or a layer of grime can weaken or distort the reading. A clean, bare surface gives the most dependable result.

The blind spot: XRF only reads the surface

The X-rays coming back out of gold can only travel a very short distance through the metal before they are absorbed. So the reading describes a thin outer layer, a tiny fraction of a millimetre deep. Anything further inside never reaches the detector.

For a bar made of one metal all the way through, that does not matter. For a plated or filled fake, it matters a great deal. A tungsten core wrapped in a thick enough layer of real gold will read as gold, because the outside really is gold. Tungsten is used for this because its density is very close to gold's, so the weight and size can look right too.

In short, XRF tells you what a piece is made of on the outside. On its own, it cannot tell you what is underneath.

• XRF picks up: the metals at the surface and their proportions, the wrong alloy, low-karat gold passed off as high-karat, base metals mixed into silver.
• XRF can miss: a thick gold or silver layer over a different core, such as a tungsten-filled or heavily plated bar.
• XRF readings suffer from: dirt, coatings, packaging between the beam and the metal, and very curved or uneven surfaces.

Why we pair XRF with PMV and other checks

To deal with the surface problem you need a test that reaches further in. A PMV, or Precious Metal Verifier, sends an electromagnetic signal into the piece and measures how the metal responds, which depends on how well it conducts electricity. Gold, silver and tungsten conduct quite differently, and the signal goes deeper than XRF can see, so a tungsten core under gold should show up as a mismatch.

Each test covers the other's weak spot. XRF gives a detailed breakdown, but only of the surface. PMV looks further in, but does not list the elements. A piece that passes both, and has the right weight and dimensions for its stated size, has been checked from several directions.

PAMP bars get a third check. Veriscan reads the surface pattern of the bar and matches it against PAMP's own registered record of that bar.

What this means when you buy or sell with us

Every piece we sell is tested first: PMV and XRF, plus Veriscan on PAMP bars. We also supply other dealers in Singapore on a wholesale basis, and the same testing applies to everything that goes out of our door.

Selling to us works the same way. Bring your bar or coin to the shop at #01-17 Centrium Square, 320 Serangoon Road; no appointment is needed. We test it at the counter, and the final amount is set after testing at that moment's price. If you still have the original packaging, receipt or certificate, bring those along too.

On larger transactions above S$20,000, we take your NRIC or passport details, as the PSPM Act requires of every registered dealer. That has nothing to do with testing, but it saves a trip home if you know beforehand.