Metallurgical assay

A metallurgical assay is a compositional analysis of an ore, metal, or alloy, usually performed in order to test for purity or quality.

Silver is assayed by titration, gold by cupellation and platinum by inductively coupled plasma optical emission spectrometry (ICP OES).

[3] Differences in precious metal content as small as 10 to 20 parts per thousand can often be established with confidence by the test, using acids and gold samples both of a specific, known concentration.

One process for X-ray fluorescence assay involves melting the material in a furnace and stirring to make a homogeneous mix.

The principal advantage of fire assay is that large samples can be used, and these increase the accuracy in analyzing low-yield ores in the <1g/T range of concentration.

Fusion: the process requires a self-generating reducing atmosphere, and so the crushed ore sample is mixed with fluxes and a carbon source (e.g. coal dust, ground charcoal, flour, etc.)

The temperature required, and the type of flux used, are dependent on the composition of the rock in which the precious metals are concentrated, and in many laboratories an empirical approach based on long experience is used.

When fusion is complete, the sample is tipped into a mold (usually iron) where the slag floats to the top, and the lead, now alloyed with the precious metals, sinks to the bottom, forming a 'button'.

Method details for various fire assay procedures vary, but concentration and separation chemistry typically comply with traditions set by Bugby or Shepard & Dietrich in the early 20th century.

The standard traditions have a long history of reliability; "special" new methods frequently associate with reduced assay accuracy and fraud.

A flow of air through the muffle assists oxidation of the lead, and carries the fumes for safe collection outside the furnace unit.

Fire assay, as applied to ores, concentrates, or less pure metals, adds a fusion or scorification step before cupellation.

In the UK, the Trial of the Pyx is a ceremonial procedure for ensuring that newly minted coins conform to required standards.