ISO 11426:2021

Overview - ISO 11426:2021 (Cupellation method / fire assay)

ISO 11426:2021 specifies a cupellation (fire assay) method for the determination of gold in homogeneous materials, primarily used for jewellery and precious-metal alloys. The standard covers the full assay workflow - from sampling and method selection to cupellation, parting and calculation - and is the recommended method for fineness determination in jewellery alloys covered by ISO 9202. It is intended for gold contents preferably between 100 ‰ and 999.5 ‰; fineness above 999.5 ‰ is measured by spectroscopy by difference (e.g., ISO 15093).

Key topics and technical requirements

• Scope and limits
  • Applicable to most gold samples; not suitable if iridium, rhodium and ruthenium exceed 0.25 ‰ (sum).
  • Modifications described for samples with large amounts of base metals, platinum/palladium, or high silver content.
• Principle
  • Inquartation with silver, addition of lead, cupellation in a furnace to produce precious-metal buttons, parting (nitric acid) to remove silver, then weighing of gold; proof samples assayed in parallel to correct systematic errors.
• Reagents and materials
  • Nitric acid (~33 % and ~49 %) with low halide content; assay-grade lead; pure silver (min. 999.9 ‰) for inquartation; pure gold (min. 999.90 ‰) for proof samples.
• Apparatus and conditions
  • Cupellation furnace able to maintain 1 050–1 150 °C (oxidizing atmosphere), magnesia cupels, annealing crucibles, jewellers' rolls, polished anvil/hammer or press, and an analytical balance (reading accuracy ≥ 0.01 mg, or 0.001 mg preferred for fineness >995 ‰).
• Procedure essentials
  • Sample masses commonly ≈ 250 mg (range 125–500 mg), lead mass 4–6 g, silver added to reach 2.3–3× the mass of fine gold; cupellation typically 20–40 min; parting involves boiling in 33 % HNO3 then 49 % HNO3 and annealing at 500–900 °C before final weighing.
• Quality controls
  • Use of proof samples, parallel assays, and guidance on repeatability and calculation of gold content.

Applications - who uses ISO 11426:2021

• Assay laboratories and precious-metals testing facilities performing gold determination and fineness certification.
• Jewellery manufacturers, hallmarking authorities and quality control teams needing traceable, standardized fire assay results.
• Regulatory bodies and trade organizations requiring a reference method for gold fineness in alloys.

Related standards

• ISO 9202 - jewellery alloy fineness (related application)
• ISO 15093 - spectroscopy by difference for ultra-high fineness (>999.5 ‰)
• ISO 11596 - sampling procedure for jewellery gold alloys

ISO 11426:2021 remains a practical, authoritative method for accurate gold assays using cupellation (fire assay), balancing laboratory controls, reagent and apparatus specifications, and procedural steps for reliable fineness determination.