Assay Sampling Protocols for Industrial Metal Scrap Valuation
Assay sampling protocols establish true dry weight and payable metal content, directly governing settlement values, working capital allocations, and lender borrowing base limits.
Cupellation
Meaning
High-temperature separation represents the analytical stage in fire assaying where precious metals are isolated from base metals. The cupellation process involves heating a lead alloy in a porous, magnesium oxide or bone ash dish under oxidizing conditions. This treatment burns off the lead and other impurities, leaving behind a small bead of gold and silver.
Thermal Separation
A furnace heats the sample to around nine hundred degrees Celsius in a stream of air. Lead oxidises to lead oxide, which melts and is drawn into the porous walls of the cupel by capillary action. This selective absorption separates the non-oxidising precious metals from the base metal matrix.
The process is complete when the shiny bead of precious metal remains alone. If the temperature is too low, the lead will freeze and halt the separation.
Absorbent Bed
The porous dish, known as a cupel, must withstand high thermal shock and chemical attack. Its composition allows it to hold many times its own weight in molten lead oxide without cracking or leaking. If the cupel fails, the precious metal bead is lost in the furnace floor.
This requires careful selection of cupel materials.
Precious Metal Recovery
Once the lead has been absorbed, the bead is cooled and weighed. This bead is then treated with nitric acid to separate gold from silver. Nitric acid dissolves the silver, leaving pure gold behind.
The accuracy of the result depends on the completeness of the lead removal.