XRF testing: what it can and cannot tell you about gold and silver

01

What an XRF analyser actually measures

Whether an XRF number can be trusted depends on how it is made. An analyser directs a narrow beam of X-rays at a small spot on the item. The atoms in that spot give off X-rays of their own, at energies characteristic of each element, and the analyser counts them and estimates the proportion of each element in the layer it reached. For a gold alloy the result is a list such as gold 58.6 per cent, silver 8.1, copper 30.9 and zinc 2.4, which the operator then interprets as a fineness.

The test is quick, leaves no mark and needs nothing cut from the piece. Those are real advantages over an acid test, which takes a streak of metal, and fire assay, which destroys a sample. They also make it easy to trust a reading more than it deserves.

How deep it reads

XRF reads only the near surface: a matter of microns, varying by metal and instrument. One analyser maker gives about 15 microns for gold. Metal deeper than that does not contribute to the reading.

How wide it reads

The area the beam covers, from a fraction of a millimetre on some benchtop instruments to several millimetres on a handheld one. A reading describes that patch, not the whole item.

Reading

The percentages of each element reported for one spot. Taking several readings on different parts of a piece is normal practice and does not mean something is wrong.

Fineness recorded

The parts-per-thousand figure finally recorded for the item once the readings have been checked against the hallmark and the other evidence.

02

What XRF cannot prove on its own

Because the signal comes from the surface, any item whose outside differs from its inside can give a reading that is true of the surface and wrong about the piece.

• Plating. A very thin coat over brass usually lets the copper and zinc underneath show in the reading, because the beam reaches them. A heavier coat does not: once the coating is thicker than the depth the analyser reads, the reading describes the coat and says nothing about the base metal beneath it.
• Gold-filled and rolled gold. A bonded sheet of carat gold over brass is thicker than plating and can read as solid carat gold across the whole surface. Weight, density and the marks usually tell the difference.
• Solder. Joints and repairs are made with lower-fineness solder. A reading on a joint under-reads the item; a reading away from the joint does not reveal that a large repair is there.
• Hollow construction. A hollow 9ct bangle or rope chain reads like a solid one. XRF says nothing about how much metal there is, only what the surface is made of.
• Internal fillings. Weighted silver, pitch-filled handles and cement under stones are outside the beam altogether.
• Surface enrichment. Some manufacturing and finishing processes leave the surface richer in gold than the body of the alloy, so the surface reads a little high.
• Geometry and size. Curved, very small or very thin pieces lose signal, and the readings can drift from the true composition.
• Contamination. Polish residue, skin oils, dirt and adhesive all distort a reading. Cleaning a spot changes the piece, so it is done only if you agree in writing.

03

How a reading is turned into a fineness

A reading is a percentage; a fineness is a standard, and the two are checked against each other rather than treated as the same thing. If a ring hallmarked 585 reads 58.4, 58.7 and 58.5 per cent gold at three spots away from the shank joint, the fineness recorded would be 585: the readings agree with the mark within the analyser's normal variation.

If an unmarked ring reads 58.4 per cent and its weight and density fit, the same fineness may be recorded, and the item list says it rests on the tests rather than a mark. If a ring marked 750 reads 58.4 per cent everywhere, the mark would be treated as unreliable, further checks made, and the discrepancy written down.

The order of checks below is the one used for any piece. While no analyser is in use, the XRF step is left out and the item list says so.

1. Read the hallmark firstThe sponsor's mark, fineness mark and assay office mark are recorded before any test, so the test checks a claim rather than making one.
2. Weigh itEvery piece is weighed and the weight recorded on the item list. A piece that is lighter than its size suggests may be hollow or filled.
3. Magnet checkGold and silver alloys are not drawn to a magnet. A pull means steel or nickel somewhere: a clasp spring, a plated base or a loaded interior. No pull rules out steel, but not brass, copper or other non-magnetic cores.
4. XRF at more than one spot, where an analyser is usedReadings would be taken on a clean area of the main body and, where present, at joints, clasps and any area that looks different in colour or wear. If a spot has to be cleaned first, that is asked for in writing.
5. Check them against each otherIf the mark, the weight and the other checks agree, the marked fineness is recorded. If they do not, the disagreement is written down and a further check is proposed.
6. Further tests: what needs your permissionA density check does not mark the piece. It is used on unmarked solid pieces, on bars, and whenever the other checks disagree, where the shape allows, and never on pieces with pearls, opals, glued stones, a movement or anything else water could harm without asking you first. An acid test takes a small streak of metal and is never done without your written agreement; you can have the piece back untested instead. Fire assay would settle what the rest cannot, but there is no arrangement with a laboratory, so it is not offered.

04

The tests used alongside XRF and what each adds

Complementary tests and the question each one answers

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Density is the non-destructive check that catches much of what XRF misses. The piece is weighed in air and again in water, and the two figures give a density that a thin plating barely changes. Pure gold is about 19.3 g per cubic centimetre, a 9ct alloy roughly 11 to 12 depending on the other metals in it, and sterling silver about 10.3 to 10.4. A piece whose surface reads 18ct but whose density comes out near brass is not 18ct through.

Density has its own blind spots, and it is never decisive on its own. A hollow rope chain or a bangle with a sealed air space reads too light, a set stone skews the figure, and on a very small piece the measurement error can swamp the result. Some alloys overlap: 9ct white gold and palladium can give similar figures. On those pieces the weight against the size and the marks count for more, which is another reason no single test is enough.

05

Three readings on one ring, and what they are read against

This is an illustration of how readings are checked when an analyser is used, on an invented ring rather than a real case. Three readings rarely land on the same decimal. What matters is whether they fall within the range the alloy would be expected to give, and whether they agree with the mark.

Illustration only: an invented hallmarked 585 ring, read at three spots on a clean surface

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The mark, the three readings, the magnet and the weight all point the same way, so 585 is the fineness recorded, and the record says it rests on the hallmark and the readings together. Nothing more is done to the ring.

Now change one number. Had the shank joint read 45 per cent against the 58.4 on the body, the joint would be noted as solder rather than the whole ring being described by its lowest reading. Recording where each reading came from stops one small area of lower-grade metal from misdescribing the whole piece.

Had all three readings come in near 45 per cent on a ring marked 750, the mark would be treated as unreliable, the disagreement written down, and a density check made before anything else happened. The readings help decide what the item is; the offer is made afterwards, for the lot as a whole.

06

How test results appear on your item list

Every item Assaypost receives gets its own line on the item list, and the test evidence sits on that line rather than in a summary at the bottom. Each line shows what the piece is, its weight, any weight left out and why, which tests were done, any test you asked us not to do, the fineness recorded, and anything the tests could not establish. Coins each get their own line, with the date and type, and coins of the same standard may be weighed and checked together as a group, with the coins tested named. The list says which checks were used on each piece. One written offer for the whole lot follows the list, as one amount with nothing taken off it afterwards. The terms set this out in full.

07

Seller risks and exceptions

• A piece that looks like carat gold at the surface but is plated or filled may only be found out by density or, with your consent, an acid test. Where that happens the item list says so. You can say no and have everything back, or take that piece back and receive a new offer for the rest; the first offer stays open on its original terms for as long as it would have done.
• Antique and hand-made pieces are often built from more than one alloy, with a different fineness in the shank, the setting and the decorative work. Where the parts can be told apart by their marks, or by tests you agree to, they are listed separately; where they cannot, the item list says the fineness recorded is for the piece as a whole and why.
• Very small items, such as single earring backs or thin wire, are hard to test one by one. They are usually grouped and recorded at the lowest fineness the evidence supports, and the list says so.
• White gold is usually rhodium plated to make it whiter. The layer is thin and often wears through at the shank, which affects any surface test; a heavily re-plated piece is noted on the item list.
• Dental alloys vary widely and often contain platinum-group metals, which cannot be established without an analyser or a laboratory assay. While no analyser is in use, the item list says what could and could not be established for dental work. The dental gold guide covers it.
• No acid test is carried out unless you agree to it, and fire assay is not available here. If you say no to a test, you choose: the piece is described on the non-destructive evidence with its limits stated, or it is sent back at our cost.

08

When not to use the scrap route

The testing page sets out the methods and what each one can settle, and the how it works page shows where testing sits between your photographs and your payment. Selling gold by post puts the same checks in the context of a whole drawer.