The four methods side by side
| Method | Measures | Damage | Strength | Limit |
|---|---|---|---|---|
| XRF | Elemental composition of the surface layer | None | Fast, on site, many elements at once | Reads only the surface; plating and inclusions inside are missed |
| Acid (touchstone) | Fineness of a streak rubbed on a stone | A trace streak | Quick, cheap, portable | Surface only; reading by eye |
| Density (specific gravity) | Mass per volume of the whole piece | None | Looks through the whole piece | Some metals are close to gold in density |
| Fire assay | Gold content of a weighed sample | Destroys the sample | The reference method, very precise | Laboratory only; the sample must represent the lot |
XRF: fast, but only the surface
X-ray fluorescence excites the atoms at the surface of the metal and reads the energy they give back, which identifies each element and its proportion. A handheld analyser gives a result in seconds and does not mark the gold.
XRF reads only a thin surface layer. A gold-plated bar, a bar with a different metal inside, or a doré bar whose composition varies from surface to core will all read differently from what they are. XRF is the right first test and the wrong last one.
Acid testing on a touchstone
A streak of the metal is rubbed on a black touchstone and tested with acids of known strength. How the streak reacts gives an estimate of fineness in karats. It is quick and useful at the counter, but like XRF it tests only the surface.
Density: the whole piece
The piece is weighed in air and then in water. The difference gives its volume, and mass divided by volume gives its density. Pure gold has a density of about 19.32 g/cm³; alloys with silver or copper are lighter. A bar of the right weight that is too large for that weight shows that something else is inside.
Density has one important limit: tungsten, at about 19.25 g/cm³, is almost as dense as gold, so a tungsten core inside a gold shell can pass a density test. Drilling, ultrasonic thickness testing or melting is what finds it.
Fire assay: the reference method
Fire assay (cupellation) separates the precious metals from a weighed sample at high temperature, then parts silver from gold with acid and weighs the gold that remains. It is the benchmark for gold in bullion and jewellery alloys — ISO 11426 describes it for gold alloys — and in an accredited laboratory it measures gold content to a few tenths of a part per thousand.
The result is only as good as the sample. That is why the sample should be drawn by an independent person, from inside the bar or from a melt, and sealed before it goes to the laboratory. Choose a laboratory accredited to ISO/IEC 17025 for the method, and ask for the certificate to be issued to you.
Testing by form: bars, doré, nuggets and dust
| Form | Main question | How it is sampled |
|---|---|---|
| Refined bars | Is the bar what the stamp says, all the way through? | XRF, density, ultrasonic or drill sample; fire assay if needed |
| Doré bars | What is the gold content, given the bar is not uniform? | Drill samples from several points, or melt and sample the melt |
| Nuggets | Is it natural gold, and how pure? | XRF and density on each piece; melt a representative portion |
| Alluvial dust | How much is gold and how much is other material? | Clean, weigh, melt into a bar, sample the melt |
| Scrap and jewellery | What karat, and are any pieces plated? | Sort by karat, XRF and acid, melt and sample by batch |
Purity, karats and fineness
| Karat | Fineness (parts per thousand) | Gold % |
|---|---|---|
| 24K | 999 and above | 99.9%+ |
| 22K | 916 | 91.6% |
| 21K | 875 | 87.5% |
| 18K | 750 | 75.0% |
Bars traded in the London market must meet the LBMA Good Delivery rules: a minimum fineness of 995 parts per thousand and a gold content between 350 and 430 fine troy ounces. One troy ounce is 31.1035 grams.
From test to value
Once the weight and fineness are known, the value of the gold content follows from a simple calculation:
Our gold valuation sets the calculation out line by line, using the tested purity and the LBMA price on the day.
How our testers combine the methods
- Count and weigh the lot on calibrated scales, and photograph each piece.
- Take XRF readings on each piece.
- Measure density on each piece or a representative set.
- Drill, cut or melt to take a sample from inside, as the form requires.
- Seal the lot and the samples with numbered seals in front of both parties.
- Send the sample for fire assay at an ISO/IEC 17025-accredited laboratory, certificate in the client’s name.
- Report every reading, weight and seal number with photos and video.
Our testers work in Tanzania, the DR Congo, Zambia, Kenya, Uganda and West Africa, with bases in Ghana and Sierra Leone, and deploy anywhere in Africa.