How Does a Gold Refinery Assay Work?
A gold refinery assay is the process by which a refinery determines the precise gold content of a delivered lot. The material is weighed, melted into a homogeneous bar, sampled, and analysed — classically by fire assay, the most accurate method for gold. The assay result establishes the fine gold content on which the seller is paid, which is why the assay, not the shipped weight, is the commercial heart of every doré transaction.
Why the Assay Matters
Doré bars arrive at a refinery with purity that is only estimated. Mine assay certificates accompany the shipment, but no reputable refinery settles on them. Settlement is based on the refinery's own (or a mutually appointed laboratory's) determination of purity. For the seller, the difference of even half a percent in assayed purity on a large lot is a significant sum — so understanding and contractually governing the assay process is essential.
The Assay Process Step by Step
- Receipt and gross weighing. The lot is received under chain-of-custody procedures, checked against shipping documents, and weighed on calibrated scales, often with the seller's representative present.
- Melt and homogenisation. The entire lot is melted together in a furnace. Melting matters because a doré bar is not uniform — gold content can vary within a single bar. A full melt produces a homogeneous mass whose samples genuinely represent the whole lot. Weight is recorded again after melt (small losses on melting are normal and contractually defined).
- Sampling. Samples are taken from the molten metal — typically by vacuum tube, dip sampling or drilling the cast bar at specified points. Samples are usually split: one set for the refinery's lab, one for the seller, and a sealed set retained for an umpire in case of dispute.
- Analysis. The samples are tested. The reference method is fire assay (cupellation): the sample is fused with lead, the base metals are absorbed into a cupel at high temperature, silver is separated by acid parting, and the remaining pure gold is weighed. Fire assay is accurate to within fractions of a part per thousand. Instrumental methods such as ICP-OES support the analysis of by-metals, while XRF is used for fast non-destructive screening rather than final settlement.
- Assay report and settlement. The lab issues the final purity figure. Fine gold content = net melt weight × assayed purity. Settlement follows under the contract: payable percentage applied, price fixed against the LBMA benchmark or spot, refining charges deducted.
Example
A 30 kg doré lot arrives in Dubai under a CIF contract. After melt, net weight is 29.94 kg. Fire assay returns 89.7% gold. Fine content is 26.86 kg. At a 99% payable rate, the seller is settled on 26.59 kg of fine gold at the agreed fixing, less refining and treatment charges. The seller's split sample confirms the result within the contractual tolerance, so no umpire is needed.
What Protects the Seller
- Witnessing rights — the contract should allow the seller or their agent to witness weighing, melting and sampling.
- Split samples — the seller should receive a sealed sample tested by an independent lab.
- Umpire clause — if the refinery and seller assays differ beyond a defined tolerance (commonly a fraction of a percent), a named independent umpire laboratory decides, with costs borne by the party whose figure was further out.
- Defined melt-loss allowance — legitimate melting losses are small; the contract should cap them.
Key Takeaways
- Settlement in doré transactions is based on the refinery assay, not the mine's certificate or shipped weight.
- The full lot is melted and homogenised before sampling so that the samples genuinely represent the whole delivery.
- Fire assay (cupellation) remains the reference method for gold purity, accurate to fractions of a part per thousand.
- Well-drafted contracts give the seller witnessing rights, split samples and an umpire mechanism for disputed results.
- Fine gold content = net melt weight × assayed purity; payable percentage and refining charges are then applied.
Frequently Asked Questions
How long does a refinery assay take?
Receipt, melt and sampling typically happen within a day or two of arrival; laboratory results commonly follow within one to several working days, depending on the refinery's schedule and the analysis required.
What is an umpire assay?
An independent third laboratory named in the contract. If the refinery's and the seller's assay results differ by more than the agreed tolerance, the umpire's result is binding.
What happens after the assay?
The fine gold content is calculated, the price is fixed per the contract, the payable percentage and refining charges are applied, and settlement is paid — by bank transfer or under the letter of credit governing the transaction.
Why does the refinery melt the whole lot before sampling?
Because doré is not homogeneous. Gold content varies within and between bars, so only a full melt produces samples that accurately represent the entire lot.
Can a seller watch the assay process?
Reputable refineries allow the seller or an appointed representative to witness weighing, melting and sampling, and provide sealed split samples. These rights should be written into the contract.