Oct 18

Fire Assay Question

1 comment

Hello Everyone, I received a question from a person new to fire assay. He was asking about fluxes and procedures for the fire assay of materials that are range in the 40-60% copper and 10-40% Iron collectively. This is the set up he has now:

Sample size: 5-10g, pulverized and screened, 125um mesh


Lead Oxide (Precious Metal Free) 61%

Soda Ash 19.5%, Borax (anhydrous) 9.8%

Silica 9.8%

Each fusion is run with 60g of flux and 5g of flour to 2000F for 2 hrs. Then cupel and dissolve the bead for ICP analysis.

Any suggestions for flux variations to handle these types of samples?

Oct 28

In large part, this should depend on the type of material at-hand. If it isn’t a sulfide or matte material, I would increase the soda ash a bit (to 25%) due to the iron. If it is a sulfide or matte, I would drop the soda ash to 15% and add about 6% K2CO3 to aid the soda ash in fixing the iron and sulfur.


I do believe that bumping up to 100g of flux would be ideal, as well, but that depends on crucible capacity and reactivity considerations during the fusion (even a 40g crucible can easily hold that plus room for a salt cover, if needed). At 61% PbO anyway, the 5g of flour seems excessive, since it is far more than enough to reduce all PbO to Pb in 60g of your flux. I do believe that keeping a fair amount of PbO unreduced throughout the fusion to carry into the slag matrix is a good thing (since residual PbO during the fusion acts as a good desulfurizing agent, prevents regulus in silicates, aids in the physical formation of slag, etc.), so I would calculate flour for maybe a 30g Pb button, adjusting for the reducing power of the ore itself.


Please excuse me if some of this lacks relevance to your current operation, though, since my experience is in a high-grade system that uses relatively small sample charges, QC-corrections, and trial inquartation. Best of luck!

New Posts
  • atn5612
    Feb 11

    Just wondering how many out there make their own fluxes from scratch and how many purchase flux that is pre-blended to their own specs?

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