What Is Fire Assay? Step-by-Step Process and Equipment Overview for Precious Metal Analysis

What Is Fire Assay?

Fire assay is a traditional and widely used analytical method for determining precious metals such as gold and silver in ores, concentrates and other mineral samples. The method separates and concentrates precious metals from the sample through high-temperature fusion and cupellation, and it is commonly applied in mining laboratories, mineral testing laboratories and precious-metal analysis.

How Does Fire Assay Work?

A typical fire assay process mainly includes sample preparation, flux mixing, fusion, cupellation and final determination. Each stage serves a specific purpose and relies on dedicated laboratory equipment.

1. Sample Preparation

Before analysis, the original sample is crushed, ground and divided to obtain a representative laboratory sample. Common equipment used at this stage includes:

• Laboratory jaw crusher
• Laboratory pulverizer or grinder
• Sample divider
• Vibrating screen

2. Flux Mixing

A measured sample is mixed with suitable fluxes. Common materials may include litharge, borax, silica and sodium carbonate. The flux helps the sample melt properly and allows precious metals to be collected during the fusion process.

3. Fusion

The prepared sample and flux mixture are placed in a fire assay crucible and heated in a fusion furnace, commonly at approximately 1,000–1,200 °C. During fusion, the material separates into slag and a lead button, and gold, silver and other precious metals are collected in the lead button.

4. Cupellation

After cooling, the lead button is separated from the slag and placed in a cupel. The cupel is then heated in a cupellation furnace. During this process, lead is oxidized and absorbed by the porous cupel, leaving a small precious-metal bead.

5. Final Determination

The remaining precious-metal bead is further processed and analyzed to determine the gold, silver or other precious-metal content of the original sample. Depending on the analytical method, weighing, parting or instrumental analysis may be used for final determination.

What Equipment Is Needed for Fire Assay?

A basic fire assay laboratory generally requires equipment for sample preparation, fusion, cupellation and final analysis. The table below groups the most common items by process stage:

Typical equipment in a fire assay laboratory therefore includes:

• Laboratory crusher
• Laboratory grinder or pulverizer
• Sample divider
• Flux mixing equipment
• Fire assay crucibles
• Fire assay fusion furnace
• Cupellation furnace
• Cupels
• Pouring molds
• Crucible and cupel tongs
• Analytical balance
• Ventilation and fume extraction equipment

The specific equipment configuration can vary according to the sample type, laboratory capacity and testing requirements.

Summary

Fire assay is an important method for precious-metal analysis, especially for gold and silver samples. The basic process moves from sample preparation → flux mixing → fusion → cupellation → final determination.