Fire Assay and Metallurgical Analysis

Vocabulary flashcards covering the key terms, chemical processes, and materials involved in fire assay, smelting, cupellation, and bullion analysis.

Fire assay

A high-temperature analytical technique used to separate and measure precious metals, especially gold ($Au$) and silver ($Ag$), from ores and metallurgical samples at temperatures usually around $1000–1200\,^\circ\text{C}$.

Reduction

The chemical process of converting metal oxides into pure metal by using a reducing agent (like flour or carbon) to remove oxygen, producing molten lead in fire assay.

Smelting

The process of melting the charge so the sample separates into two liquid layers: a metal layer (lead and precious metals) and a slag layer (waste minerals).

Charge

The complete mixture of materials placed into the crucible before heating, usually including the ore sample, fluxes, litharge ($PbO$), and reducing agents.

Litharge ($PbO$)

An oxide of lead included in the fire assay charge to produce the lead collector; it is also the product formed when lead oxidizes during cupellation.

Lead button

A solid piece obtained after smelting and cooling that contains almost all the gold and silver concentrated into one small mass, separating them from the bulk ore.

Cupellation

The process of heating the lead button in a porous cup called a cupel with a strong air supply to oxidize lead and base metals, leaving behind precious metals.

Cupel

A porous ceramic dish used during cupellation to absorb molten lead oxide ($PbO$) while leaving the unoxidized gold and silver behind.

Doré bead

A small shiny metallic bead remaining after cupellation that contains mainly gold ($Au$) and silver ($Ag$).

Parting

The chemical process of separating gold ($Au$) from silver ($Ag$) by treating the flattened doré bead with hot nitric acid ($HNO_3$), which dissolves the silver but not the gold.

Inquartation

A metallurgical process where silver is intentionally added to a gold-containing alloy to reach a ratio of at least $3\text{ parts silver}$ to $1\text{ part gold}$ to facilitate proper acid parting.

Annealing

The process of heating the washed and dried gold residue to remove moisture and impurities, resulting in pure metallic gold for final weighing.

Slag

The molten glass-like waste material formed during smelting that contains unwanted gangue minerals and impurities from the ore.

Acidic Slag

A slag dominated by silica ($SiO_2$), usually viscous, produced when the ore sample is rich in silica or quartz.

Basic Slag

A slag containing excess basic oxides such as $CaO$, $FeO$, $PbO$, or $Na_2O$, which can be aggressive toward crucible walls if not balanced.

Basicity ratio

A concept used to describe the chemical balance between acidic oxides and basic oxides, helping the assayer select proper fluxes to ensure efficient fusion.

Scorification Assay

A fire assay method using an open dish called a scorifier, particularly useful for samples with high base metal content like concentrates, matte, or bullion.

Scoria

The slag-like layer formed during scorification into which oxidized base metals are absorbed.

Bullion Assay

The analytical process used to determine the exact purity and precious metal concentration in metallic products like gold bars, silver bars, or doré bullion.

Solution Assay

An analytical method used to determine metal concentrations in liquid streams, such as cyanide leach solutions, pregnant leach solutions ($PLS$), or electrolyte solutions.