How Is Gold Tested? XRF, Touchstone, Acid and Fire Assay

Every claim about gold, the 916 stamp, the seller's assurance, the family story about a bangle's purity, eventually meets the same arbiter: a test. Gold testing is quick, inexpensive and widely available, yet many buyers have never watched one performed. Knowing what each method does, and where each can be fooled, turns testing from ritual into protection.

XRF: The Modern Standard

X-ray fluorescence (XRF) spectrometry is today's workhorse. The instrument irradiates a small spot and reads the characteristic energies the alloy emits, reporting the composition, gold, silver, copper and the rest, in seconds, without touching the piece. Benchtop and handheld units are now common in serious shops, buying counters and laboratories.

XRF's limitation is depth: it reads the surface layers, so heavy plating over base metal can deceive a single reading. Competent testers therefore measure several spots, including clasps, posts and worn edges, and treat inconsistent readings as a reason to dig further.

Touchstone: The Oldest Test

The touchstone method is thousands of years old and still in daily use. The tester draws the item across a fine-grained black stone, leaving a streak of metal, then compares its behaviour, often under acid, against streaks from needles of known fineness. In experienced hands it separates the common grades reliably; in inexperienced hands it is guesswork. It removes only a trace of metal from an inconspicuous edge.

Acid Testing

Acid testing extends the touchstone principle: test acids of graded strength are applied to a streak or a small filed spot, and the reaction, dissolving, discolouring or standing unchanged, brackets the karat. Kits are cheap and common at buying counters. The method is serviceable for sorting, but it reads only the surface, involves corrosive chemicals and offers nothing like XRF's precision.

Fire Assay: The Definitive Answer

Fire assay, or cupellation, is the ancient laboratory method against which the others are judged. A weighed sample is melted with fluxes and lead, the base metals are driven off, and the remaining precious metal is weighed to determine gold content with great accuracy. Its precision comes at an obvious cost, the sample is destroyed, so fire assay belongs to refining, bullion and formal assay work rather than finished jewellery. Formal results may be documented in an assay report.

When a Buyer Should Insist on Testing

Test before you rely: on any significant purchase, on any piece bought without documentation, and on anything you are selling, where the buyer's test sets the price, watch it done and ask to see the reading. Remember that a stamp such as 916 is a claim, and hallmarking practices vary between countries and makers. A sixty-second XRF reading, with the result written on the invoice, settles what no mark can.