Fire Assay for Gold: Precision, Robustness and Traceability at Cotecna

Gold is a unique and valuable metal which requires precise analytical methods for its determination and is essential for the mining industry. Among various analytical methods, Fire Assay continues to be the world standard for both accuracy and robustness in various matrices. At Cotecna Chile, this method is a fundamental part of our analytical services, supported by international accreditations and advanced technology. 

Gold holds a unique place in both human history and the contemporary mining industry. Its geological rarity, chemical stability, and extraordinary malleability have made it one of the most valuable and sought-after metals in the world. With atomic number 79 and belonging to group 11 of the periodic table, gold (Au) stands out for its resistance to alteration, its unmistakable yellow shine and its ability to be found in a native state in nature. 

Its origin is even exceptional: various scientific studies propose that the gold present on Earth was formed in extreme conditions during the explosion of supernovae or by neutron star collisions, accounting for its true uniqueness in the universe. 

Gold and its chemical behaviour: the basis for reliable analysis 

The chemical behaviour of gold explains why certain methods are more suitable than others for its determination. It is a very low reactive metal: it does not oxidise easily, does not react with most acids and remains stable against chemical conditions that affect other metals. Only compounds such as aqua regia, cyanide, mercury, chlorine and lye can dissolve it or form complexes. 

This feature, which makes it ideal for jewellery and industrial applications, also demands specialised methods capable of separating and concentrating gold even in trace amounts. Of all of them, the Fire Assay is the most effective and accurate. 

Fire Assay: The Most Reliable Method for Au and Ag 

Assay and Testing is a classic but unsurpassed procedure. Fire Assay consists of melting the sample together with chemical fluxes to obtain two immiscible liquid phases: 

1. Slag: rich in complex silicates
2. Metallic phase: composed mainly of lead, responsible for collecting the noble metals (Au and Ag) 

Thanks to their difference in densities and physical properties, both phases are separated during fusion. When cooled, the lead regulus, which contains the precious metals, is extracted. 

Main stages of the Fire Assay 

1. Fusion 

The pulverised sample is mixed with lithargy, carbonates, borax, silica, and reductants. The mixture is heated above 1,000 °C, generating the separation between slag and collecting metallic lead. 

2. Cupellation 

The lead regulator obtained is placed on a magnesite cuping and subjected to hot oxidation. The cupel absorbs lead oxide, leaving a button of gold and silver on its surface. 

3. Completion 

The button is parsed by: 

1. Gravimetric method, ideal for high-grade samples
2. Atomic Absorption Spectroscopy (AAS), suitable for precise determinations at lower concentrations 

This process allows for highly reproducible results, with very low detection limits and internationally recognized reliability. 

Fire Assay at Cotecna Chile 

In our laboratory in Coquimbo, accredited under ISO/IEC 17025:2017 by A2LA and INN LE1422, we perform gold and silver determinations by fire assay with gravimetric or AAS completion. These accreditations guarantee reliable and traceable results, supported by international methods. 

Cotecna Chile also has: 

• Mechanical preparation laboratories in Coquimbo and Antofagasta
• Laboratory specialized in concentrates inaugurated in 2025 (Coquimbo)
• Advanced AAS, ICP-OES, ICP-MS, and humidity control capabilities 

In addition to Fire Assay, our services include: 

• AAS, ICP-OES, ICP-MS
• Gravimetry, Titrimetry, IR Combustion
• Determination of Cu, Ag, Au, As, Hg in final products
• Humidity control, sampling, and weight monitoring
• On-site laboratories for mining operations 

All of this makes Cotecna a reliable partner for mineral exploration, production and export operations. 

Our approach combines technology, quality control and analytical expertise to support all stages of the mining chain, from exploration to export. 

Global Gold Analysis Capabilities: Shiva Analyticals, India 

Complementing Chile's regional capabilities, Cotecna extends its gold analysis expertise globally through Shiva Analyticals, part of the Cotecna Group, with its specialised minerals and metals laboratory in Bangalore, India. 

Shiva Analyticals is recognised as a leader in Gold Analysis, offering high-precision services for bulk ores, concentrates, and complex matrices. The laboratory has advanced capabilities for the analysis of major and minor oxides, trace elements, rare earths and precious metals, using methodologies aligned with the highest international standards. 

Key capabilities at Precious Metal Analysis include: 

• Gold by Fire Assay with 50 g of lead, with AAS, ICP-OES or ICP-MS finishing, reaching limits of quantification (LOQ) from 10 ppb to 1 ppb
• Precious metals (Au, Pt, Pd) by Fire Assay with 50 g of lead and ICP-MS finishing, with LOQ of 1 ppb 

These capabilities allow Cotecna to offer an integrated analytical network, combining Chile's proven operational strength with Shiva's specialisation in India, to meet the needs of mining clients operating across multiple regions and requiring consistent, comparable, and technically robust results. 

An ancient method at the service of modern mining 

Fire Assay remains valid because it offers the essentials for the mining industry: accuracy, reliability and globally comparable results. At Cotecna, this method is executed under the highest standards, both in Chile and through our specialized capabilities in India. 

We are committed to delivering reliable, transparent and technically sound analytical results, supporting informed decision-making at every stage of the mining chain, from exploration to international markets. 

For more information on Gold Testing and our other Metals and Minerals Services, please contact us. 

About Cotecna

For over 50 years, we have been supporting agriculture, food and pharma, minerals and metals, and consumer products supply chains all over the world. We provide testing, inspection, and certification services to help our clients navigate evolving regulations, ensure product compliance, and support their sustainability journey.