Fire Assaying: An Overview

Prepared by Shawna Baker, Senior Fire Assayer
Florin Analytical Services, LLC

10 March 2008

It is uncertain when the fire assaying method was first utilized by man. The first explicit records of assaying were contained in an anonymous booklet published in German in the sixteenth century. Detailed procedures to perform basic fire assaying were published by Georgius Agricola in De Re Metallica in 1556. The process of fire assaying has changed little since that time. The technique continues to be the most accurate and economical method for the determination of gold and silver.

Fire assaying is a method used for the quantitative analyses of precious metal content in exploration samples, ores and mining processing products such as concentrates, activated carbon and zinc precipitates from Merrill Crowe collection. Fire assaying may also be used to determine gold and silver content in process solutions.

Sample Preparation

Geological samples from the mine or exploration sampling program are sampled using a statistically accurate method fitting a desired accuracy to obtain a representative sample (30 grams) for the fire assay. The sample is prepared by coning and quartering, crushing, splitting and pulverizing (80% minus 0.075 millimeters). This is a process referred to as sample preparation.

Fluxing and Fusion

Geological samples and mining process products vary widely in characteristics and care must be taken to match the flux to the material being assayed if a true quantitative analysis is to be achieved. Therefore, no one single flux is suitable for the testing of all materials. The fire assayer must be able to determine the nature of the sample or mine product and then adjust the flux accordingly.

Ores may be classified as acidic, neutral or basic. Furthermore, ores may also be classified as reducing, neutral or oxidizing. A simple acid test of the ore can be used to determine if an ore is acidic or basic. Test assays where the reducing power of the flux is known, may be run to determine the oxidizing or reducing potential of an ore. A seasoned assayer has a basic knowledge of ore types and therefore has an advantage when addressing a new ore to be assayed. The most common oxide impurities are silica, lime and various metal oxides such as iron. Sulfides of copper, zinc and iron present special conditions.

Assay fluxes are composed of litharge, borax, silica sand, sodium carbonate and wheat flour. Litharge or lead oxide is added as the collector. Wheat flour is added as a carbon source which acts as a reducing agent forming metallic lead. Borax and silica sand are added for the purpose of lowering the melting point and for controlling the viscosity of the melted oxide impurities. Sodium carbonate is added for de-sulfurization.

These flux components may be classified as acidic or basic. By manipulating the amount of the various flux reagents the assayer is able to compensate for an acidic or basic ore. With the addition of other reagents, such as, potassium nitrate, additional flour and fluorspar the assayer is able to balance the flux to accommodate ores that are reducing (sulfide ores), oxidizing (ores high in iron oxide) or ores that tend to be high in alumina. For ores that contain high quantities of base metals such as zinc, copper or selenium the sample size to be assayed and/or the flux must be adjusted so that these metals report to the slag and not the lead button. High amounts of base metals reporting to the lead button will cause adverse effects in the cupellation process.

A known weight of the sample is then taken. For the fire assay, a special system of weights is used in which the assay ton (AT), equal to 29.167 grams, has the same relationship to a milligram as the avoirdupois short ton (2000 lbs) has to the troy ounce. Therefore, when a 1 AT sample is used in the fusion the resulting precious metals, weighed in milligrams, are equivalent to the troy ounces of precious metal per ton of ore. Each assay set contains a silver blank used for determining a correction factor for silver loss in the cupellation, a certified standard ore with a known gold content and two duplicate samples for quality control.

The ore sample is thoroughly mixed with the flux charge in a crucible. A pure silver inquart is added to the charge to provide the high silver to gold ratio necessary in the parting process and to aid in the handling of the doré bead. The crucible is then placed in a kiln at 1950°F for a predetermined amount of time. During the fusion the lead oxide is reduced to metallic lead. Small globules of metallic lead form during the fusion. As these globules of lead move through the fusion they collect precious metals. By the end of the fusion the precious metals have been collected by the lead fall and are separated from the gangue material which has been dissolved in the slag.

The fusion is then carefully poured into a conical mold where the heavier lead settles to the bottom of the mold with the slag on top. After cooling the slag is broken and the lead button is retrieved. The lead button is hammered to remove any remaining slag.

Cupellation

The lead buttons are then placed into a cupel. Cupels are made of bone ash and cement or magnesium oxide. The cupels are loaded into the cupellation kiln at 1750°F. The lead buttons are allowed to "open", that is to become molten. When the cupellation is "driving" or the lead is being absorbed into the cupel or driven off as lead oxide, the furnace is opened slightly to provide a draft. The lead is absorbed leaving behind a tiny doré bead containing the gold and silver. The cupels are allowed to cool. The doré is then removed, brushed to remove any adhering cupel material and flattened using an anvil and bead hammer. At this point the doré is weighed. The doré is then finished gravimetrically or by wet chemical digestion and analysis using Atomic Absorption Spectroscopy.

Parting and Annealing

If the analysis is to be finished gravimetrically the flattened doré is placed in a porcelain parting cup. Nitric acid (15% v/v) is added to the parting cup and heated on a hotplate. The nitric acid dissolves the silver leaving behind a gold sponge. The sponge is washed three times with de-ionized water and allowed to thoroughly dry. The delicate gold sponge is then annealed. Annealed gold beads are then weighed using a microbalance.

Calculations

Silver correction factor (c.f.) = blank inquart wgt. (mg.) /blank doré wgt. (mg.)

Gold (oz/st) = gold (mg.) x (29.167/sample wgt.)

Silver (oz/st) = (((doré (mg.) - gold (mg.)) x c.f.)- inquart wgt.) x (29.167/sample wgt.)