Gold
Gold assay: how gold is tested and what the certificate must show
A gold assay measures fineness: how many parts per thousand of a bar are gold. Wholesale gold is priced on a fire assay of a sample, not on a surface scan, and in trade both sides assay and an independent umpire settles any difference.
What an assay measures
Fineness is expressed in parts per thousand: 999.9 means 99.99% gold, 995.0 means 99.5%. A bar's value is its weight × its fineness × the gold price, so a difference of a few parts per thousand on a kilogram is worth hundreds of dollars, and on a shipment much more.
Assay methods: fire assay, XRF and density checks
| Method | What it does | Used for |
|---|---|---|
| Fire assay (cupellation) | A weighed sample is melted with lead and cupelled; the remaining gold is weighed. Destructive, and the reference method. Standardised in ISO 11426:2021 for gold contents between 100 and 999.5 parts per thousand | Settlement of refined bars and doré between refiners, miners and traders |
| X-ray fluorescence (XRF) | Reads the elements at the surface, to a depth reported at 10 to 50 microns. Fast and non-destructive | Screening. It cannot see inside a bar, so a plated bar or one with a tungsten core can read as pure gold |
| Density and ultrasound checks | Compare a bar's weight and dimensions with pure gold, or look for internal voids and inserts | Detecting cored or filled bars before assay |
What an assay certificate should show
- The laboratory's name and address, and its accreditation
- The sample or bar identification: serial number, weight and marks
- The method used (fire assay for settlement)
- Fineness, to at least one decimal place in parts per thousand
- The date, and the signature of the responsible assayer
A certificate is only as good as the sample behind it. A certificate from a laboratory the seller chose, on a sample the buyer never saw, proves little. Fraudsters in the Ugandan gold trade have used genuine test results on one sample to sell different material. See Uganda gold scams and supplier checks and how fake gold bars are detected.
How assay works in a gold trade
In refining and purchase contracts filed publicly, the gold is sampled at melting, the sample is split and sealed, and each side assays its part. If the two results are within an agreed splitting limit (0.05% to 0.10% for gold in the contracts we read), they are averaged. Otherwise an independent umpire laboratory, which "shall not be the representative of either party", decides, and the party whose result is furthest from the umpire's pays for it. Payment follows the final weight and assay.
Assay and gold from Uganda
Uganda's Mining and Minerals Act requires a certificate of analysis authenticated by the country of origin for each export consignment. Because no Ugandan refinery is on the LBMA Good Delivery List, the buying refinery's assay on arrival is the reference for price. Any lot we quote will require an assay on arrival, with an independent umpire if results differ beyond the agreed tolerance; payment terms are agreed in a signed contract. Read the gold bar specification or how doré is assayed and valued, or request a gold quote.
Quotes
Tell us what you are buying.
Send the gold form, fineness, quantity, delivery point and the documents you require. We reply by email with what can and cannot be quoted at this stage, and on what basis.
Sources for the figures and rules on this page
Evidence for this page
Each statement is labelled by the strength of its evidence and dated. How we label evidence.
| Statement | Evidence | Source | Checked |
|---|---|---|---|
| ISO 11426:2021 specifies a cupellation (fire assay) method for the determination of gold, for gold contents preferably between 100 and 999.5 parts per thousand. | Official | ISO 11426:2021, Jewellery and precious metals — Determination of gold — Cupellation method (fire assay) | |
| XRF penetrates only the outer surface of a gold bar, typically 10 to 50 microns, and cannot reliably detect a tungsten core under thick gold plating. | Reported | XRF instrument makers' technical guides (VRAY XRF; Evident Scientific) | |
| In filed refining contracts, the melt sample is split, both parties assay by fire assay, results within the splitting limit are averaged, and otherwise an umpire who 'shall not be the representative of either party' decides; splitting limits for gold were 0.05%–0.10%; final payment follows the final weight and assay certificates. | Verified | Refining contracts filed on SEC EDGAR | |
| An export permit does not exempt the exporter from requirements including a certificate of analysis authenticated by the country of origin. | Verified | Mining and Minerals Act 2022 (Uganda), assented copy, s.150 | |
| No Ugandan refinery appears on LBMA's Good Delivery List for gold. | Official | LBMA Good Delivery List, gold |