The high precision classical Pb-fire assay technique is used for the quantitative determination of gold, and the PGMs in samples. It is important to note that the presence of minerals or elements like chromite, base metal sulphides, graphite, As, Te, Se, Ni, Cu etc can interfere with the process of fire assay. The analysis can be improved by correct fluxing ratios taking into consideration the presence of those deleterious minerals in the particular ore. Therefore, different ores are assayed with different flux formulations.
The prepared sample is weighed and fluxed with a lead-containing flux. A collector is added to assist the collection of the precious metals. Fusion is then done by heating the fluxed sample in a furnace until it is molten. The purpose of sample fusion is to concentrate and collect the precious metals particles of the ores into a lead button. The precious metal is concentrated through formation of two phases, the slag phase and the metal phase. The metal phase settles at the bottom in a lead button while the slag phase floats on top and is slagged off. The lead button is cubed and taken for cupellation.
The process of cupellation takes place in a muffle furnace which operates at a temperature of about 600 to 980◦C. The hammered lead button is placed in a pre-heated cupel which has attained the muffle furnace temperature. The muffle furnace must be kept above the melting point of lead (Pb) and below the melting point of the precious metal. The reasons for preheating the cupels are that since sugar water is used as a binding agent in the manufacture of cupels if they are not preheated the burning off of the sugar and the steaming off of water will cause the lead to spatter thus contaminating other samples. It also leads to the loss of gold and silver from the assay concerned. Most of the litharge formed is absorbed by the cupel as it has high affinity for lead (Pb) and 90% is absorbed by the cupels while the rest passes off as vapour. Cupellation continues until all the lead has been oxidised and removed. At this this stage the precious metal prill remains in the cupel. The cupel is removed from the muffle furnace and allowed to cool. The resultant prill is then analysed with alternative finishes (AAS, ICP-OES, gravimetric).