The fire assay reloaded.

Fire assay remains the most accurate and precise method for measuring the gold content in gold alloys. Scanning and transmission electron microscopy were used to study the changes in the microstructure of samples during the fire assay process. The results showed that the microstructure of the bead obtained during the cupellation stage was not a perfect Au-Ag binary alloy as expected, but also contained Cu-Ag oxides and other residual compounds. The cornet-shaped product obtained by treating the bead by hammering, flame annealing and rolling showed the presence of surface nano-voids created by dislocation coalescence during annealing. The parting stage in which the Au and Ag are separated to produce a cornet made only of Au appeared to involve a de-alloying process leading to the formation of a nanoporous Au nanostructure. The application of the results to improve the fire assay process is discussed, in particular to avoid cracks inside the cornet which may result in the loss of small pieces of material and errors in Au content measurement.
Fire assay remains the most accurate and precise method for measuring the gold content in gold alloys. Scanning and transmission electron microscopy were used to study the changes in the microstructure of samples during the fire assay process. The results showed that the microstructure of the bead obtained during the cupellation stage was not a perfect Au-Ag binary alloy as expected, but also contained Cu-Ag oxides and other residual compounds. The cornet-shaped product obtained by treating the bead by hammering, flame annealing and rolling showed the presence of surface nano-voids created by dislocation coalescence during annealing. The parting stage in which the Au and Ag are separated to produce a cornet made only of Au appeared to involve a de-alloying process leading to the formation of a nanoporous Au nanostructure. The application of the results to improve the fire assay process is discussed, in particular to avoid cracks inside the cornet which may result in the loss of small pieces of material and errors in Au content measurement.