Gold
How gold is refined: from mine doré to 999.9 bars
Gold leaves a mine as doré, an alloy typically 85–90% gold. A refinery weighs, melts and assays it, then removes the silver and base metals. The Miller process (chlorine gas blown through molten gold) reaches about 99.5%; electrolytic Wohlwill refining reaches 99.99% or higher. The pure gold is then cast into kilobars, Good Delivery bars or grain.
The refining process, step by step
| Stage | What happens | Why it matters to a buyer |
|---|---|---|
| 1. Receipt | The refinery receives the doré or scrap, checks the documents and weighs each lot | The receipt weight is the starting point for settlement |
| 2. Melting and sampling | The lot is melted to make it uniform and sampled; the sample is split between refiner, supplier and an umpire | A fair sample decides the price of the whole lot |
| 3. Assay | The samples are fire-assayed for gold and silver content | Payment is based on the assayed fine gold, not the gross weight |
| 4. Refining | Silver and base metals are removed by the Miller chlorine process (to about 99.5%) and, for higher purity, by electrolytic Wohlwill refining (to 99.99% or above) | Decides the fineness of the finished gold |
| 5. Casting and marking | The gold is cast into bars or grain and stamped with the refiner's mark, weight, fineness and serial number | The refiner's mark and accreditation determine where the bar can be sold without re-assay |
| 6. Settlement | The supplier is credited with an agreed share of the assayed gold, less refining charges | In filed refining contracts, 99.85% to 99.95% of the gold measured was credited |
Miller and Wohlwill: the two main methods
| Method | How it works | Purity reached |
|---|---|---|
| Miller process | Chlorine gas is blown through the molten impure gold; silver and base metals form chlorides that are skimmed off | About 99.5% |
| Wohlwill process | Electrolysis in a chloroauric acid electrolyte: a doré or Miller-gold anode dissolves and pure gold plates onto the cathode | Up to 99.999% |
Many refiners use Miller refining first, for speed, and Wohlwill refining when the customer needs 999.9 gold. See gold fineness.
What refining means when you buy
- The refiner matters. Bars from refiners on the LBMA Good Delivery List or the UAE Good Delivery list trade without re-assay; others are assayed on arrival. See the gold bar specification and 400 oz Good Delivery bars.
- Doré is priced on refining terms. A doré buyer pays for the assayed gold less refining charges and a payable percentage. See how doré is valued.
- Assay protects both sides. Split samples and an umpire settle any difference. See gold assay.
Refining and gold from Uganda
Uganda's ministry has said since December 2024 that exported gold must be refined to 99.9% (reported). Whether doré may still be exported is unresolved: see gold doré. No Ugandan refinery is on the LBMA Good Delivery List, so Ugandan bars are assayed, and often re-refined, by the buying refinery. Read 1 kg gold bars and how gold from Uganda is priced, or request a gold quote.
Quotes
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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 |
|---|---|---|---|
| The Miller process blows chlorine gas through molten, slightly impure gold and yields gold of about 99.5% purity; the Wohlwill process is electrolytic, uses chloroauric acid as electrolyte with a doré anode, and produces gold of up to 99.999% purity. | Reported | Wikipedia: Miller process; Wohlwill process | |
| Gold doré is normally 85–90% pure; refined gold is 99.5% or better. | Verified | LBMA Responsible Gold Guidance v9, Definitions | |
| In refining contracts filed publicly, the melt sample is split between the parties and an umpire, both sides fire-assay, and the mine is credited with 99.85%–99.95% of the gold measured. | Verified | Refining agreements filed on SEC EDGAR | |
| Ministry guidelines of 21 Dec 2024 require proof that exported gold has been refined to 99.9%. | Reported | The Independent (Uganda), 21 Dec 2024 | |
| No Ugandan refinery appears on LBMA's Good Delivery List for gold. | Official | LBMA Good Delivery List, gold |