Assay: what the number represents
An assay reports elemental content — for copper, typically as % Cu — determined by a defined analytical method on a prepared sample. The number is only as reliable as the chain behind it: how the sample was taken, how it was prepared, which laboratory ran it and which method was applied. A single grab-sample certificate tells you far less than a representative composite analyzed by an accredited laboratory.
When reviewing an assay report, ask: sampling protocol, sample preparation, analytical method, laboratory accreditation, and whether the result refers to the shipped lot or an earlier campaign.
Dry basis vs. wet basis
Mineral materials carry moisture, and moisture is weight. A lot reported at 25% Cu on a dry basis contains 25 kg of copper per 100 kg of dry material — but if the same material ships at 10% moisture, 100 tonnes of shipped wet weight contain only 90 tonnes of dry material, hence 22.5 tonnes of copper, not 25.
This is why commercial lots are typically settled on a dry-weight basis: contract tonnage is corrected for moisture, then multiplied by the dry-basis grade. If a quotation does not state its basis, the basis is unknown — and the effective price per unit of copper is unknown too.
Moisture: measurement and disputes
Moisture percentage is measured by drying a sample under defined conditions. Results vary with sampling point (stockpile vs. load port), weather exposure, material fineness and method. In bulk trades it is standard to agree where moisture will be determined — often at load port by an independent inspector — because the figure directly changes payable weight.
Fine materials retain more moisture and lose it less predictably; granulometry is therefore not just a handling question but a settlement variable.
Impurities and penalty elements
Copper content alone does not define value. Smelters and processors penalize or reject loads carrying deleterious elements above thresholds — arsenic, antimony, bismuth, lead, zinc, fluorine, chlorine, mercury and others, depending on the buyer's process. A high-grade lot with disqualifying impurities can be worth less than a moderate-grade clean lot. Serious quotations address the full analysis, not only the headline Cu figure.
Checklist for buyers
Bottom line
Contained copper is a calculation — dry tonnage times verified dry-basis grade — while payable copper is defined by the contract, not by a universal percentage. Specification clarity protects both sides: the buyer pays for metal received, and the supplier is judged on evidence rather than suspicion. REPT Minerals structures transactions around documented lots, and flags the missing pieces instead of papering over them.