How a cupellation gold or fire assay works ?

The method is more accurate, with an accuracy of 2 to 3 parts per ten thousand (0.02%), is a method sometimes called cupellation to test feu.La Cupellation involves taking a small sample of object, usually a hole made in a crude bullion.

It is best to know the approximate purity of this ingot, whether of gold 22 carat African native or a mixture of various assays jewelry.

The metal sample is about 200 mg, with the weighing accuracy of thousandths grams more preferably at or near one hundred thousandths of grams.

These 200 mg were wrapped in a sheet of lead of about 5 grams with a little extra money (known as ‘Inquartation or quartation’).

Cupellation is made in a furnace at about 1100 ° C to remove any metal base. This is specific type of furnace to evacuate fumes from molten lead. They are often of a large mass with a rather small chamber in order to have a very good temperature stability and inertia.

There is placed a cup magnesia, it once preheated exited the furnace and the golden sample enclosed in its lead foil is positioned then put back in the oven.

Lead and the sample will melt and formed amalgam and at some point a flash will occur and lead has been absorbed by the cup, taking with him all the oxidizable metals.

Then the alloy is rolled pearl gold-silver in nitric acid to dissolve the money (called « separation ») and weighing the resulting pure gold, one can know the exact content by a rule of three with initial weight before cupellation alloy.

This is the standard reference technique used by the laboratories of foundries and national analysis worldwide and is governed by the international standard, ISO 11426: 1997.

In the ISO standard, on a raw gold bullion, it is necessary to take a sample from each side of plus or minus 1 gr to keep a second sample to remake another Cupellation for errors or mismatched result between the bottom and top of the raw gold bullion.

Is therefore produced two cups for each side, one with the addition of silver to the analysis of the gold content and without allowing once the known gold content, establish three rules by the content money.

It is also necessary to put two more cups, one with a metal calibrated sample of the desired assay and fine gold, each with + – 200 mg and 5 gram of lead. Both sample will calculated the percentage of vaporization of the metal during its passage in the oven and cupellation, this percentage, once known and calculated using these two samples calibrated allows 4 corrected the initial samples. This vaporization is often very small but necessary for reliable results required by the ISO standard.