Gold acid testing kits: what they tell you, what they miss, and when to skip straight to XRF

What is in the kit

A standard UK gold testing kit contains four small bottles of nitric acid at graduated strengths, one bottle of aqua regia for 22ct and higher, one bottle for silver, a black basalt touchstone the size of a domino, and usually a needle set of known-carat samples. That is the entire toolkit. Every bottle is a liquid acid; the touchstone is inert; the needles are reference samples you scratch alongside the unknown piece for comparison. If all you want is the purity settled, you do not need a kit at all: an XRF assay reads the actual alloy without touching the piece, and how we value gold sets out how weight and purity drive an offer.

• 9ct solution: a mild nitric acid. Anything below 9ct fizzes and darkens within a couple of seconds.
• 14ct solution: stronger nitric acid. 9ct dissolves; 14ct and above hold.
• 18ct solution: stronger again. 14ct fades slowly; 18ct and above hold.
• 22ct solution: aqua regia (nitric plus hydrochloric). Attacks 18ct; 22ct and 24ct hold for a few seconds before the aqua regia eventually wins.
• Silver solution: nitric acid diluted with potassium dichromate. Sterling gives a blood-red result; low silver gives brown; nickel or steel gives green.

How the test actually works

1. Rub a discreet part of the piece (an inside band, a clasp back) firmly across the touchstone until you leave a bright metallic streak two or three millimetres wide.
2. Place a single drop of the lowest-carat solution beside the streak, not on top of it, and tilt so the drop just contacts the metal line.
3. Watch the streak for five to ten seconds. If it dissolves or turns dark and green, the piece is below that carat.
4. If it holds, wipe the stone, re-streak, and repeat with the next strength up.
5. Stop at the solution that finally attacks the streak. The piece sits between that solution and the previous one.

The needle samples are there so you can run the unknown streak next to a known 9ct or 18ct streak and compare the reactions side by side. That side-by-side comparison is the part that separates a useful reading from a guess.

What acid cannot tell you

• Exact fineness. Acid places a piece in a band. It cannot tell 585 (14ct) from 600, or 750 (18ct) from 760. A refinery is paid on the actual number, not the band.
• Plated over a lower carat. A 22ct wash over a 9ct core will pass the 22ct acid on the surface streak. Cut deeper, and the reaction changes.
• Heavy plating over base metal. A thick gold-plated chain will read as gold on a shallow streak and as brass on a firmer one. If two rubs give two answers, the piece is plated.
• Hidden solder joints. Rings and chains are often soldered with a carat below the main piece. Acid on the joint reads the solder, not the ring.
• Stone and hollow content. A ring can be 18ct and half air. Acid says nothing about payable weight.

None of this makes the kit useless. It makes the kit a triage tool. See gold-plated versus solid gold, how to tell for the non-acid checks that catch most plated pieces before any liquid comes out of a bottle.

Safety, plainly

• Nitrile gloves. Not latex, which nitric acid rots through.
• Wraparound safety goggles. A single splash is enough for a hospital visit.
• Ventilated space. Open window, or better, outdoors. Do not test over a kitchen worktop where food is prepared.
• Baking soda solution in a bowl beside you. Neutralises spills and drips.
• Store the bottles upright in the plastic tray they came in, out of reach of children, out of sunlight, capped tight. Nitric acid degrades plastic slowly; check the caps every few months.
• Dispose of used droppers and paper towels in a sealed bag. Do not tip acid down the sink.

Is it legal to own?

Not automatically, and this is worth reading carefully. Under the Poisons Act 1972 as amended (implemented via the Control of Poisons and Explosives Precursors Regulations 2023), nitric acid at concentrations above 3% w/w is a regulated explosives precursor. Members of the public need a Home Office Explosives Precursors and Poisons (EPP) licence to acquire, possess, import or use nitric acid above that threshold. The current guidance for individuals is on gov.uk at licensing for home users of poisons and explosive precursors.

Some consumer gold-testing kits are supplied at concentrations at or below 3% (which do not require an individual licence), and some are supplied under the reportable-supply regime that applies to sellers rather than buyers. The strengths needed to test 18ct and 22ct gold (the aqua regia solution, in particular) sit at concentrations that are firmly in licensed territory for a home user. If a kit does not state the concentration on the bottle, treat it as regulated until proven otherwise. Do not assume any nitric-acid-based kit is licence-free.

Separate from the licensing question: it is also an offence to test someone else’s property without their permission, the test marks the piece and the mark does not come out. In practical terms that means don’t "test" a friend’s ring across a pub table. Nothing on this page is legal advice; the definitive current position is on the gov.uk pages linked above.

When a home kit is genuinely useful

• Sorting a jumbled tin of jewellery into gold, silver, plated and costume before you commit to posting anything.
• A quick sanity check on a car-boot or house-clearance buy before you pay.
• Teaching yourself the difference between gold and gold-tone. After a dozen tests, the eye gets faster.
• Confirming that an unmarked chain is at least in the gold family, so you know it is worth sending for XRF.

When to skip the kit and just get XRF

Anything you intend to sell should be assayed, not scratched. XRF (X-ray fluorescence) reads the actual composition of the alloy in seconds without cutting, scratching or removing material. It reports fineness as a number (say 587 rather than "somewhere between 14ct and 18ct"), and it does that per item, not averaged across a bag. The full method is on XRF testing explained.

A parcel of a dozen mixed rings tested by acid is a dozen scratches and a rough band each. The same parcel tested by XRF is a dozen exact readings, no damage, and one price sheet you can check. For scrap decisions the second is the one that pays.

Hallmarks help too. If a piece already carries a UK punch of 375, 585, 750 or 916, acid tells you nothing the mark hasn’t already said. See how to read a gold hallmark in 60 seconds for the sixty-second version.

What we do at GoldPaid

Every parcel is signed for on arrival and opened at our Bristol office, where each item is weighed and tested by in-house XRF. No acid, no scratching, no melting before you have accepted the offer in writing. If you decline, the parcel goes back tracked and free. That is the whole procedure. For the pricing side see how we value gold.