The main operation of a fire assay is melting. At that, for the extraction of precious metals in a fire assay, the property of metallic lead in the molten state is used to quantitatively dissolve gold and silver to obtain a low-melting alloy (the melting point of lead is 327 °C). Such substances are called collectors of precious metals. For the subsequent separation of the amount of precious metals from the collector, another property of lead is used – the ability to quickly oxidize with air oxygen at elevated temperatures with the formation of easy – melting lead oxide – PbO (the melting point of PbO is 883 °C). The affinity of lead to oxygen (which is understood as the isobaric-isothermal reaction potential for the formation of oxide from elements in the standard state, attributed to one gram atom of gas) is188.5 kJ/mol. Gold and silver are not oxidized. When using porous thick-walled vessels made of oxidized refractory material, well wetted with molten oxide, but not wetted with molten metal, lead oxide formed during the oxidative melting of lead is absorbed into the pores of this vessel, and the alloy of gold and silver remains in the form of a spherical bead on its bottom surface.
To separate the remaining amount of precious metals, the property of silver to dissolve in dilute nitric acid is used when gold is completely unaffected in this process (the parting operation). The grade value of precious metals in the analyzed material is determined by weighing beads before and after parting. A special assay balance used for weighing precious metals has an accuracy of 0.001 mg.
As in ancient times, the fire assay method remains the main one in the analysis of gold samples, due to the possibility of analyzing relatively large subsamples – weighing 50 g or more.