How Dealers Test Gold Purity: Acid Test vs XRF Analyzer
Before a dealer quotes you a number, someone has to confirm what your gold actually is. The karat stamped on a ring is a claim, not proof, and the payout math runs on verified purity, not on the stamp. Two tools do most of this work: the traditional acid test and the XRF analyzer. Knowing what each one measures, and where each one can mislead, tells you whether the karat in your quote is the karat the dealer really checked.
Every gold payout is the same short equation: weight, times purity, times the current spot price, minus the dealer's margin. Weight is easy; a calibrated scale settles it in seconds. Spot price is public. The one variable a seller cannot see with the naked eye is purity, and that is exactly the number a testing method exists to pin down. If the purity used in the math is wrong, everything downstream is wrong, so it pays to understand how the two common tests work and what they can and cannot see.
The stamp on a piece is a starting point and nothing more. Stamps get worn flat, applied to plated goods, or faked outright on imported items. A responsible dealer treats the marking as a hypothesis and then tests to confirm it. The disagreement worth watching is not acid versus XRF as rivals; it is surface versus core, because that gap is where sellers most often lose money without realizing it.
The acid test: cheap, fast, and only skin deep
The acid test, sometimes called the scratch test, is the oldest tool still in daily use. The dealer rubs your piece against a dark testing stone to leave a small streak of metal, then applies acid solutions mixed to specific karat strengths. If the streak survives the 14k acid but dissolves under the 18k acid, the metal reads somewhere in that range. It is quick, it costs almost nothing, and an experienced hand can sort a tray of mixed jewelry with it in minutes.
The limitation is right there in the name: it is a scratch. The acid only ever touches the outermost layer of metal, so it reports the surface and assumes the rest matches. On solid, honestly stamped gold that assumption holds fine. On plated or filled pieces it fails badly. A heavy gold electroplate can pass a surface acid test while the body underneath is brass or silver. Gold-filled items, where a thin gold layer is mechanically bonded over a base core, are the classic trap; the streak can read like solid karat gold until you file deeper or cut the piece open.
Acid testing is also destructive in a small way. It leaves a mark on the stone and a faint abraded spot on your item, which matters more on an intact collectible than on scrap headed for the melt. For a dealer buying to melt, that is a non-issue. For anyone hoping to sell a piece as-is, it is worth noting before someone drags it across a stone.
The XRF analyzer: reads the alloy, still reads the surface
An XRF analyzer, short for X-ray fluorescence, is the modern counterpart. The unit fires low-energy X-rays at the metal, which excites the atoms into emitting their own signature energy. The device reads those signatures and returns a full breakdown: not just how much gold, but the copper, silver, nickel, zinc, and other metals in the alloy, usually within a percent or two. It is non-destructive, it takes seconds, and it hands the dealer a printed or on-screen composition rather than a judgment call. For confirming exact karat and catching odd alloys, it is the stronger instrument.
Here is the catch most sellers never hear: XRF is also a surface reading. The X-rays penetrate only a very shallow depth of metal, often just microns. So a well-made gold-plated item can fool an XRF analyzer the same way it fools acid, because the machine is honestly reporting the gold layer sitting on top. The analyzer is more precise and more informative, but precision on the wrong layer is still the wrong number. This is why a careful buyer pairs the XRF read with a physical check, filing a small notch on an inconspicuous edge and re-testing the freshly exposed core, or weighing against expected density. The tool is excellent; the technique around it is what catches the fakes.
There is also the matter of calibration. An XRF unit that has drifted, or a cheap handheld running loose settings, can report karat a point or two off in either direction. That difference is small per gram but real across a heavy lot, and it always seems to drift in the house's favor when nobody checks. A reputable operation calibrates against known standards and does not mind you knowing it.
So what should a seller actually ask for? First, ask which method was used and to see the result yourself. On an XRF read, ask to look at the composition screen, not just to hear a karat called out. Second, on anything thick or suspiciously light for its size, ask whether the dealer tested the surface only or filed through to check the core. Filled and plated pieces earn a core check every time. Third, ask to see the number that goes into the math and confirm it matches what the test showed; the karat that gets verified and the karat that gets paid should be identical, and any gap between them is a conversation to have before money changes hands, not after.
Neither tool is a gimmick and neither is a magic wand. Acid is fast and cheap but shallow and rough. XRF is precise and detailed but still reads the top layer and depends on honest calibration. The seller who understands the difference between a surface test and a verified core walks in able to ask the one question that protects the payout: is the karat you are paying me for the karat you actually confirmed all the way through?