ANALYSIS

Anaytical scale | Cupellation furnace | Cartridge Filter for Lead Oxide
Assay bench | Partition hoods | Laboratory hoods

Precious metals assay

Precious metals are assayed in various ways to accurately and precisely evaluate each metal’s percentage of purity. The most ancient and most used method is the Fire Assay (Cupellation) method.

It involves 6 steps:

Step N. 1: Sampling and weightening

A small scraping from the article, typically about 250 milligrammes, is taken and weighed using an high precision analytical scale.

Step N. 2: Inquartation

Some additional silver is added to dilute the gold content to 25% or less and the sample is then wrapped in lead foil.

Step N. 3: Cupellation

The sample is placed in a special porous crucible called cupel and put into a furnace. The lead melts and all base metals are oxidised to form a liquid slag which is absporbed into the cupel to leave a small gold-silver alloy bead

Step N. 4: Crushing, rolling and annealing

On cooling, this bead is hammered flat and rolled to a thin strip to increase its surface area.

Step N. 5: Parting

The sample is then put into a flask and processed with two stages of nitric acid to dissolve out the silver

Step N. 6: Determination of the fineness

The remaining gold cornet is pure and can be washed, dried and weighed to confirm its carat rating.