XRF gold testing: what it shows and what it cannot

In short

Every piece is identified by its marks first, then weighed on a calibrated balance, checked with a magnet and, where the shape allows, measured for density. X-ray fluorescence reads the composition of a surface without marking it, but it cannot see through plating, solder or a filled base, so it is never used alone. No analyser has been named yet, and your offer says item by item which checks were used.

What XRF readsThe outer few tens of microns of a surface, not the core
Where it goes wrongPlating, solder, worn edges, curves, hollow and filled work
Never used aloneMarks, calibrated weight, magnet and, where the shape allows, density
Anything that marks a pieceOnly with your written consent, item by item (clause 4.2)
AnalyserNot yet named; your offer lists the checks used on each piece

What the analyser measures

An analyser fires X-rays at a small area of a piece. Atoms in that area are excited and give off X-rays of their own, at energies unique to each element, and the detector counts them. Out comes a list of percentages: gold, silver, copper, zinc, nickel, palladium, whatever is there. Nothing is removed and nothing is marked.

The catch is depth. On a gold alloy those X-rays escape from roughly the outer tens of microns, thinner than a sheet of kitchen foil. The reading describes that skin and says nothing about what lies beneath it. No single test settles a question beyond doubt, and none is described here as though it did.

Diagram: how far into a piece an XRF reading actually sees, and what sits below it.

Where a surface reading misleads

These are the situations that turn a confident number into a wrong one. Each is the reason for a habit at the bench rather than an abstract caveat.

Plating and vermeilA few microns of 24 carat over silver or brass can fill the analyser's window with gold. That is why a plated ring reads high on the outside and gives itself away the moment it is weighed in water.
Surface enrichmentPolishing, pickling and years of wear leave the outside of an old piece richer in gold than the metal underneath it. Reading a worn shank and a hidden inner face can bracket the true alloy.
Solder at the jointsChain links, clasps and ring sizings are joined with a lower-carat solder. A reading taken across a joint measures the solder as much as the piece, so joints are avoided or read separately and noted.
Filled and weighted constructionA loaded candlestick is a thin silver shell over pitch or plaster; a weighted base is a silver skin over a steel disc; gold-filled work is a bonded sheet over brass. The surface reads as the real metal because it is the real metal. Only the weight against the size gives away how little of it there is.
Curves, edges and tiny piecesThe measurement assumes a flat surface filling the window. A thin band, a wire earring or a domed cufflink returns less signal and a shakier number, so those pieces get more positions and more weight on density.
One object, several alloysA watch case, a bracelet with a replacement clasp, a brooch with a steel pin: each part can be a different metal. The parts are checked separately and recorded as such, never averaged into one answer.

What goes beside the reading

The hallmark comes first, under magnification, because a full UK mark names the sponsor, the standard, the assay office and often the year. Struck marks and laser-etched ones look different under the loupe, and a mark that has been added to a piece rarely sits right. A mark is strong evidence and still only evidence, so the metal is checked anyway.

Then weight. Each piece is weighed on its own on a calibrated balance reading to 0.01 g. A piece that is much lighter than it looks is telling you something before any analyser is switched on.

A magnet catches steel cores, spring bars and clasp mechanisms that are not the metal you are selling. Density does the rest: weigh a piece in air, weigh it again suspended in water, and the difference gives a specific gravity that plating cannot fake. A hollow chain, a filled bangle or a gold-plated brass ring is lighter for its size than solid metal, and the water shows it.

Diagram: the two weighings a density check is made from, and the figures it is read against.

Acid on a touchstone is still useful where the shape defeats everything else. It leaves a mark, so it is proposed only when it will change the answer, and only with your written consent for that piece (clause 4.2).

Anything that alters a piece is asked for first

Where a mark is too worn or a surface too dirty to read, a usable result can mean cleaning, degreasing or lightly abrading a small window. That alters the piece, so it is never done without your written consent for that item, with a photograph of the place (clause 4.2). Refusing is free: the piece is assessed without it and the gap is stated on your offer, or it comes back unassessed at our cost (clause 4.3).

How a set of readings becomes a fineness

Readings are interpreted against the standards a piece could plausibly be, not rounded to whatever is nearest. An alloy sold as 14 carat is 585 parts gold per thousand; a reading a few tenths either side of that, agreeing across two positions and supported by density, is recorded as 585.

Item 04, an unmarked yellow chain (illustrative)

Gross weight 6.40 g; steel spring in the clasp noted separately at 0.10 g
Reading, mid-chain link: Au 58.9 / Ag 5.1 / Cu 35.6
Reading, second link away from the joint: Au 58.4 / Ag 5.4 / Cu 35.9
Weighed in water: consistent with a 14 carat yellow alloy, not with plate

Recorded as 585, with both positions, the steel spring and the density result written down beside it.

When the checks and the mark disagree, nobody guesses. A shank marked 750 that reads 41% gold is either plated or not what the mark says. You are told before anything else happens: what was measured, where, what it suggests, and the choice between a further test and having the piece back. A disagreement is a conversation, not a quiet adjustment.

What you are shown for each item

Your offer carries the list of everything that arrived (clause 5.1). For every piece it records:

The item number, a description and the photographs taken on arrival.
The weight recorded, and any weight left out of the assessment, with the reason.
Every check used and what it showed, with the position of each reading.
Any test you asked us not to carry out.
The fineness concluded, and a plain sentence about what the checks could not establish.
Whether the piece was set aside as worth more whole than melted, and why.

The offer is one amount for the lot, set out with that list, and accepting or declining it is up to you. Offers, returns and payment explains what you can do with it.

The bench itself

Not yet: The make and model of the analyser, the reference standards it is checked against, the date of its last verification and a photograph of the unit go in this paragraph once it is in the building. A laboratory that will not name its instrument is asking for trust it has not earned, and so would we. What else is blank, and why

Fire assay is the reference method for gold, and it consumes part of the sample.

Not yet: No arrangement with an assay laboratory exists, so we cannot say whether fire assay can be had for a disputed piece or on what terms. Today a dispute you are not satisfied with ends with your items coming home rather than with a guess; the terms of any arrangement appear here when there is one. What else is blank, and why

Recording the opening, the weighing and the testing is designed into the process and is not switched on, so it is not a protection you have. What is captured, who may watch it, how long it is kept and how to ask for a copy or its deletion are set out here on the day it starts running.

Questions sellers ask

Will the X-ray damage my jewellery?

No. The beam is aimed at a few millimetres of surface for a few seconds and marks nothing; nothing is removed or heated. What can alter a piece is preparing a surface so it can be read at all, such as lightly abrading a small window on a worn mark. That never happens without your written consent for that item (clause 4.2).

Can you tell plated from solid without cutting it?

Usually. Plating gives itself away on weight: a plated piece is too light for its size when it is weighed in water, and where an analyser is used its surface reads suspiciously pure. Where the checks are still ambiguous we say so rather than treat the piece as solid. See plated, rolled and gold-filled.

My pieces have no hallmark. Is that a problem?

Not at all. Plenty of older, imported and lightweight jewellery was never marked, and below the exemption weights a mark was never required. The metal is checked, and the fineness found is what goes on the record. See the hallmark guide.

What happens if two readings disagree?

More positions, then density, then a note on your record explaining the spread. If only a test that marks the piece would settle it, we ask your written consent first. Say no and we either price the piece conservatively or leave it out of the offer, and tell you which (clause 4.4). Or you can have it back unassessed, at our cost (clause 4.3).

Do you ever cut or scrape an item?

Nothing is cut, filed, polished or melted, and no stone is taken out, before you have accepted an offer for it (clause 4.4). Before then, the only things that can mark a piece are an acid touchstone test and preparing a small window on a worn mark, and each needs your written consent for that item.

Further reading: the physics and the limits in full · UK hallmarks and fineness · weighted and loaded silver

XRF gold testing: what it can settle, and what it cannot