How Is Gold Purity Measured?

The karat system divides gold content into twenty-four parts: 24K describes gold with no intentional alloying, while 18K describes eighteen parts gold to six parts other metals.

Fineness expresses the same idea in parts per thousand. A fineness of 999 means 999 parts gold per thousand. Fineness is the standard in bullion trading because it is more precise than karat grades.

X-ray fluorescence measures the elemental composition of a surface layer without damaging the item. It is fast and useful for screening, but because it reads the surface it can be misled by plating or surface enrichment.

Ultrasonic testing and specific-gravity measurement provide additional checks on whether an object is homogeneous and consistent with the density expected of the claimed metal.

Fire assay remains the reference method for determining gold content. A sample is taken, fluxed and melted, and the resulting gold bead is weighed to establish content with high accuracy.

Because fire assay consumes the sample, it is applied to a representative sample rather than the whole lot, which is why sampling procedure matters as much as the assay itself.

Screening methods and reference methods answer different questions. A surface reading can confirm that an item is broadly consistent with expectations; a laboratory assay establishes content to a standard suitable for settlement.

The appropriate method depends on the product, the quantity and what the parties require before proceeding.