Major Fire Assay Procedure

In this very article we will explain about the major fire assay procedure. As we know together that the fire assay method is a long method. The method really need full attention in order to get a good result. Better for you to know the history behind fire assay. The history is as long as the method itself. While if you need to know the assay definition and fire assay definition, you can find on each link above. The method use by many commercial assay laboratory as well as the big mining company.

We know that the fire assay need a very high temperature, at least 1100 degree of Celcius for gold. So before you go to the major fire assay procedure, make sure you have the equipment setting as need.

Fluxing and fusion the sample

First Major fire assays procedure is fluxing and fusion the sample. Due to a long explanation, I will divide the explanation about fire assay flux in to another article. The main function of fluxing is to break down the rock matrix. While you can find the full explanation on the article about fluxing.

assay definition, example of fire assay furnace
fire assay furnace

Separation of slag from the button

The next major fire assay procedure is separation of slag from the button. When we have finished with the first, we need to pour the fused material. The fused material is poured into a conical mold where the molten Pb sinks to the bottom and the slag floats on top. The cooled Pb button and slag are taken from the mold and separated by usually hammering the slag off the button.


The next major fire assay procedure is cupellation. In a short, the pounded Pb button is placed into the cup of a cupel in a furnace and the Pb is oxidised to PbO, most of which is absorbed into the cupel. The Au, Ag and other precious metals do not oxidize and are left as a small “prill” of metal at the bottom of the cupel.

major fire assay procedure, cupellation step
Cupellation result. One from major fire assay procedure


The next major fire assay procedure is parting. There are two different parting. First  parting for gravimetric or weighing finish and parting for instrumental finish.

Parting is the process of dissolving the Ag in the prill from the cupellation step to leave the Au behind for weighing. If Ag is to be determined gravimetrically as well, then the prill is weighed before parting.

Weighing finish

The next major fire assay procedure is weighing. The (usually) very small prills are weighed using sensitive micro-balances. I recommend to use micro-balances in order to determine how much Au and/or Ag was in the original sample. So you can get a very high accurate result.

Instrumental finish

The next major fire assay procedure is instrumental finish. In the case of AAS or ICP/OES measurement of Au, the entire prill is dissolved (both Ag and Au) and the Au concentration is measured with the instrument. The choice of which instrumental going to use is depend on the requirement of the result. ICP of course can measure in ppb and very accurate result compare to AAS.



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