Fire Assay

Fire Assay is a technique which analyzes the amount of precious metal (gold, silver, palladium and platinum) in a sample of ore or scrap. It is generally regarded as the most accurate (7-10 ppb or 0.1g/t), economical, and consistent method for gold analysis, though it is time consuming. It works with all forms of gold, and there is no such thing "un-assayable" gold, provided the method is done correctly

It is mentioned in the Doomsday Book and was invented as far back as 2000 BC, when very pure silver was produced from lead ore in Asia Minor. The earliest detailed written record known is De Rey Metallica by Georgius Agricola in 1556, which served as the standard book on the subject for about two centuries.

The process can be split into three stages: fusion, cupellation, and analysis.

Fusion: A sample of 10-30 g is blended into a fine powder. The most common sample is 30 g, which is roughly equivalent to a traditional "assay tonne". Up to 150 g can be used for particularly low concentrations.

This is added into a crucible along with lithard (PbO), borax (sodium borate), silica (sodium dioxide) and soda ash (sodium carbonate) to form a melt, and heated to 1000-1200° C. The exact temperature and composition of the ingredients depends on what trace elements are present in the sample. For instance, a siliceous ore requires a basic flux, and a basic ore requires an acid flux; soda ash is good at removing sulfur; etc. The details are adjusted to suit the ore, using an approach based on long empirical experience. The whole process from start to finish has a high degree of skill and benefits from having an assayer with knowledge and intuition.

The main purpose of these extra ingredients are to create a "flux" which mixes with the sample. This flux lowers the melting point of the mixture (mainly required for the metal oxide impurities) and imparts a homogeneous fluidity.

A small amount of carbon is added causing some of the lithard to be oxidized to lead and sink to the bottom of the melt along with any silver. Gold platinum and palladium in the melt will then drift into the lead-silver alloy at the bottom, while the impurities are collected into the liquid slag. The reactions here are complex and much of it has not been theoretically mapped out, but the basic principle is that the hot PbO is very oxidizing, and the slag is very soluble to metal oxides, so anything not a precious metal will tend to be oxidized and drift into the slag. In addition, most precious metals exhibit a good affinity for lead by themselves, and even the ones which do not will be dense enough to sink into the pool at the bottom, forming a mechanical mixture.

The melt is added to a mold and allowed to cool, then the glassy slag separates cleanly by tapping with a small hammer, leaving a lead button.

Cupellation:  The lead button is placed inside a cupel, which is a crucible made of pourous bone ash or magnesia. When heated to about 800-1000° C in air, the molten lead is oxidized back to PbO which then melts and is drawn into the cupel by capilliary action, leaving a small bead (called a dore bead) of precious metal. Cupels are rated by the grams of lead oxide they will absorb.

Analysis: The dore bead can be analyzed directly, or the silver can first be separated using nitric acid, dissolving the silver as silver nitrate. Separating the metals allows gravimetric analysis. The most common analytical method used today is AA (Atomic absorption spectroscropy). Other methods used are ICP (Inductively coupled plasma atomic emission spectroscopy), ICP/MS (Inductively coupled plasma mass spectroscopy) or for very low concentrations, INAA (Instrumental Neutron Activation Analysis).