What is an assay?
The analytical determination of metal content in an ore, concentrate or refined product. Buyer and seller assays commonly differ slightly; contracts usually set a tolerance and appoint an umpire laboratory to settle disputes beyond it.
Cargo surveyors explained: sampling, the assay, the draught survey and the umpire lab
A bulk shipload is worth millions of dollars, and its price depends on what is inside. Samples are taken from the conveyor as the ship loads, and the assay — metal, moisture and impurities — sets the final price. The draught survey weighs the cargo by how deep the ship sits: on a big bulk carrier, one centimetre of draught is more than 100 tonnes. Buyer and seller each send samples to their own lab; if the results are close they split the difference, and if not, a third, independent umpire lab decides. Moisture is a safety matter too: wet ore can liquefy and shift in heavy seas, as when the Bulk Jupiter sank off Vietnam in January 2015 carrying bauxite from Malaysia, with 18 of its 19 crew lost. In bulk commodities, the scale and the lab matter as much as the price.
Key points
- Payment is for contained metal, so the assay builds the invoice.
- Buyer and seller results are expected to differ slightly.
- No sealed reserve sample means no umpire, and a commercial argument instead.
At a glance
| What it measures | Contained metal in ore, concentrate or refined product |
|---|---|
| Weighed | Wet |
| Paid | Dry |
| Typical splitting limit | About 0.3% Cu on a copper concentrate |
| Beyond the limit | Umpire laboratory decides, usually finally |
What an assay measures
An assay is the analytical determination of how much of a given metal a material contains — copper in a concentrate, gold in a doré bar, iron in an ore. It is the number the invoice is built on, because payment is for contained metal rather than gross tonnes.
For concentrates the assay is inseparable from moisture: material is weighed wet and paid dry, so a wet tonne and a dry tonne are different quantities and the contract prices the dry one.
Assay exchange and splitting limits
Buyer and seller each have the parcel analysed, then exchange results. Because sampling and analysis both carry variability, the two rarely match exactly. Contracts therefore set a splitting limit — a tolerance within which the two results are simply averaged.
A typical copper concentrate contract might split within 0.3% copper. Inside that band the mean is used and the matter closes. Outside it, the dispute mechanism starts.
Umpire laboratories
Where results differ beyond the splitting limit, an umpire laboratory chosen from an agreed list analyses a sealed reserve sample. The umpire result is normally final, and commonly the party whose figure is further from it pays the cost.
This is why sample preparation and sealing matter as much as the analysis. Without a properly witnessed and sealed reserve sample there is nothing for an umpire to test, and the dispute becomes a commercial argument rather than a technical one.
Frequently asked questions
- What is assay exchange?
- Buyer and seller each analyse the same parcel and exchange results. If they agree within the contractual splitting limit the two are averaged; if not, an umpire laboratory decides.
- What is a splitting limit?
- The tolerance within which buyer and seller assays are treated as agreed and simply averaged — for example 0.3% copper on a concentrate contract. Beyond it, the umpire procedure applies.
- Why do buyer and seller assays differ?
- Because both sampling and analysis have inherent variability, especially on heterogeneous material like concentrate. Small differences are expected, which is why contracts anticipate them rather than treating them as a breach.
Related terms
The laboratory document stating the measured composition of a specific parcel — purity, contaminants, moisture. It refers to the lot actually shipped, unlike a typical or datasheet analysis which describes what a plant usually produces.
The largest independent inspection, testing and certification company in the world, founded in 1878 and headquartered in Geneva. In commodity trade its certificates of quality and weight are what a buyer, a bank and an insurer all rely on instead of taking the seller’s word.
The reference iron content for seaborne iron ore fines and the basis of the main published index. Cargoes above or below the reference settle against published premiums and discounts rather than a flat renegotiation.
Zinc of 99.995% minimum purity, the standard grade for galvanising and die-casting, specified under ASTM B6. Anything below this is a lower commercial grade and prices differently.
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