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1A4 and 1A4-1000 - Au Fire Assay-Metallic Screen

Metallic Screen

A representative 500 g split (1,000 g for Code 1A4-1000) is sieved at 100 mesh (149 micron) with fire assays performed on the entire +100 mesh and 2 splits on the -100 mesh fraction. The total amount of sample and the +100 mesh and -100 mesh fraction is weighed for assay reconciliation. Measured amounts of cleaner sand are used between samples and saved to test for possible plating out of gold on the mill. Alternative sieving mesh sizes are available but the user is warned that the finer the grind the more likelihood of gold loss by plating out on the mill.


Fire Assay

A sample size of 5 to 50 grams can be used but the routine size is 30 g for rock pulps, soils or sediments (exploration samples). The sample is mixed with fire assay fluxes (borax, soda ash, silica, litharge) and with Ag added as a collector and the mixture is placed in a fire clay crucible. The mixture is then preheated at 850°C, intermediate 950°C and finish 1060°C with the entire fusion process lasting 60 minutes. The crucibles are then removed from the assay furnace and the molten slag (lighter material) is carefully poured from the crucible into a mould, leaving a lead button at the base of the mould. The lead button is then placed in a preheated cupel which absorbs the lead when cupelled at 950°C to recover the Ag (doré bead) + Au.

Au is separated from the Ag in the doré bead by parting with nitric acid. The gold (roasting) flake remaining is weighed gravimetrically on a microbalance. Two splits on the -150 micron fraction are weighted and analyzed by fire assay with a gravimetric finish. A final assay is calculated based on the weight of each separated fraction and obtained Au values.



Code 1A4 (Fire Assay-Metallic Screen) Detection Limits (g/T)


Detection Limit






Hoffman, E.L. , Clark, J.R. and Yeager, J.R. 1998. Gold analysis-Fire Assaying and alternative methods. Exploration and Mining Geology, Volume 7, pp. 155-160.




Activation Laboratories Ltd. | 41 Bittern Street, Ancaster, Ontario, L9G 4V5, Canada | TF: +1.888.228.5227
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