Assay

An assay is a laboratory test that measures the exact metal or mineral content of an ore, concentrate, or bullion sample, used to determine grade and settle payment.

An assay is a laboratory measurement of exactly how much of a valuable metal or mineral a sample actually contains, run on a small portion taken to represent a larger batch of ore, concentrate, or refined bullion. It is the technical basis for both ore grade estimates at a mine and the payment terms in a concentrate sales contract.

Because so much money depends on the result, assay methods are standardized and often checked by more than one laboratory, with buyer and seller sometimes each commissioning their own assay of the same shipment and splitting any difference according to an agreed formula. Gold bars and other refined bullion carry an assay certifying their purity, which is what allows them to trade internationally without a buyer needing to independently verify the metal content of every bar.