HOW WE TEST METALS: KEE, ACID, XRF AND WHY ANTIQUE GOLD ISN'T ALWAYS EXACT

Metal testing sits at the heart of responsible jewellery buying. Whether you are collecting Victorian watch chains or mid century charms, knowing what the metal actually is rather than what it claims to be is essential. The tools we use today make that process clear, fast and far more reliable than anything available when most antique jewellery was made. Below is a straightforward run through of the main testing methods used in the trade, followed by a fully accurate explanation of why older pieces sometimes read a touch above or below their stamped carat.

KEE Testing

KEE TESTER

KEE testers use electrical conductivity to estimate the gold content. Gold conducts electricity in a very specific way, and the machine reads that response to give a carat range.

The advantages are speed, portability and the fact you do not need to mark the piece. The limitations are equally important. KEE testers measure surface response only, so readings can be affected by cleaning residue, wear or plating. It is a quick indicator rather than a definitive result, which is why it is usually paired with a second method.

Acid Testing

Acid testing remains a standard because it gives an immediate indication of metal content. It works by streaking a tiny line of metal on a test stone and applying different strengths of acid. The way the streak reacts shows whether the gold behaves as nine, fourteen, eighteen carat and so on.

It is more reliable than KEE because you are testing the metal itself rather than just the surface, but it cannot provide a breakdown of alloys. It also cannot identify whether different components on a chain all match the same alloy. It simply confirms whether the streaked metal behaves as the carat it should.

XRF Testing

XRF TESTER

XRF sits at the top of the accuracy scale. The machine fires low energy X rays into the metal and measures the wavelengths that return. Each metal responds differently, so XRF produces a full breakdown of gold, silver, copper, zinc and other trace metals.

This level of detail makes XRF invaluable for vintage and antique jewellery. It can show whether a clasp, jump ring or tag matches the same alloy profile as the chain, and it can identify the presence of metals such as nickel at extremely low levels. XRF is non destructive and precise, but the barrier is cost. Even handheld units usually start well above twenty thousand pounds, and the larger models used by assay offices cost significantly more. Because of this, many jewellers rely on trusted workshops or assay offices when XRF confirmation is required.

I will always use XRF testing if I am unsure of the purity of any piece. It is the only method that gives a complete and accurate alloy breakdown when something needs to be confirmed beyond surface or streak testing. I have also used it when a client has needed absolute clarity about the metals present. One recent example was a client with a severe nickel allergy who wanted to purchase a vintage chain but could only wear gold with zero nickel content. Her piece passed KEE and acid, but neither method can identify exact alloy percentages. The XRF scan gave the full breakdown and confirmed the chain was safe for her to wear.

Why Antique Gold Does Not Always Match Its Stamped Carat

When a genuinely antique piece tests slightly above or below the hallmark, it is not a flaw and it is not evidence of forgery. It reflects the historical reality of how gold was alloyed, refined and hallmarked at the time it was made.

Hand mixed alloys

For most of jewellery history, alloys were mixed by hand at the bench. Goldsmiths weighed out gold, copper, silver and later zinc on balance scales, not precision electronic equipment. A variation of a few tenths of a carat was completely normal. A stamped fifteen carat piece may XRF at fourteen point three or fifteen point six and still be entirely correct for its period.

Different purity levels in historic metals

Copper, silver and zinc used in earlier alloys were not refined to modern purity standards. Natural impurities slightly shifted the overall percentages, even when a goldsmith aimed for a particular carat.

Workshop recipes and colour preferences

There was no universal standard. Some workshops added more copper for strength or a deeper colour, others added more silver for a paler yellow, and many melted down scrap gold to reuse. Two Victorian nine carat chains can look identical yet test slightly differently because of the workshop that made them.

Historic hallmarking tolerance

Assay offices tested a sample rather than the entire item. The accepted tolerances were wider than they are today because hand alloying naturally varied. If the sample passed, the whole piece was marked, even if different areas of the item sat fractionally above or below when tested with modern equipment.

Wear, solder and long life repairs

Antique jewellery has often been repaired, resoldered or fitted with replacement components over generations. Solder contains different metal mixes and will test differently under XRF. Replaced parts may visually blend perfectly but come from a different gold batch. Multiple test points help distinguish what is original from what was added later.

Recycled and remelted gold

Victorian and Edwardian goldsmiths frequently remelted scrap, offcuts and old jewellery. Mixing batches always creates slight shifts in the final composition, which become clear when tested with modern XRF.

What This Means For Modern Collectors

When an antique piece tests at eight point seven carat instead of nine, or fourteen point four instead of fifteen, the reading is identifying history rather than a fault. These small variations are the fingerprints of early goldsmithing and one of the clearest differences between true antique alloys and modern reproductions, which tend to test with precision that did not exist when the originals were made.

Using KEE, acid and XRF together provides an honest and complete picture of what you are buying or selling. For antique jewellery especially, understanding these slight carat quirks adds another layer to the story of the piece.