Fineness, karats and fire assay, explained for sellers

Two bars of the same weight can differ in value by a third or more depending on how much fine gold each contains. Purity is the single most important number in any transaction, and the method used to establish it deserves attention.

Fineness and karats

Fineness gives gold content in parts per thousand. Investment bars are 999.9, the London good-delivery minimum is 995, and artisanal doré commonly runs between 700 and 950. Karats say the same thing in twenty-fourths, so 24 karat is pure and 22 karat is 916 fine. In trade, fineness is the working unit. Karats belong to jewellery.

Two ways to measure it

X-ray fluorescence

A handheld XRF analyser reads the surface composition of a metal in seconds without damaging it. It is the standard field-screening tool and it is good enough to catch obviously misrepresented material. Its weakness is that it reads only the surface, so a plated or uneven bar can fool it.

Fire assay

Fire assay is the reference method used by refineries and accredited laboratories. A weighed sample is fused with lead and fluxes, the base metals are absorbed into a bone-ash cupel, and the remaining gold-silver bead is parted with nitric acid and weighed. It is destructive, takes hours and is accurate to a fraction of a part per thousand. Settlement should always rest on it.

The split sample

When a bar is melted and sampled, one portion should go to the seller, one to the buyer and a third should be sealed for an umpire. If the two assays differ beyond an agreed tolerance, the umpire sample is tested by an independent laboratory and its result governs. This is standard practice in the international market. A buyer who refuses it should be treated with caution.