fire-assay
Quick Summary
The fire assay is a chemical analysis process used to determine the proportion of precious metals, particularly gold and silver, in ores and metallurgical products. This method involves subjecting a sample to high heat through several steps, including sampling, fusion, cupellation, weighing, parting, and final weighing.12
The process begins with taking a representative sample. This sample is then melted with fluxes and other agents in a crucible, allowing lead to collect the precious metals. After cooling, the resulting lead alloy "button" is further heated to oxidize impurities. The remaining bead, containing gold and silver, is weighed. Hot nitric acid is used to dissolve the silver, leaving the gold to be weighed separately.1
If platinum-group metals are present, they are collected in the gold-silver bead after cupellation. Their concentration can be determined using arc spectrography. Fire assaying is considered the most reliable method for accurately determining these precious metal contents in ores and concentrates.13
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