Fire Assay Procedure (General)

Classification of ores:

Using a 100 X shop Microscope, carefully examine and
classify the ore according to the amount of silicates, sulfides, and lead
that it contains.

Class I: High in silicates and carbonates:

Has no oxidizing or reducing power. If an appreciable amount
of silicate, such as quartz, is present giving the ore a glassy or vitreous
luster, do not add more silica or glass (borax glass) to the basic flux
used. Quartz (SiO2) is the strongest acid in its molten state.

Class 2: High in sulfides, carbonates and lead:

If the ore is rich in sulfides (pyrite, galena, tellurides, sulfur,
etc.), it should first be ground, then roasted. Place it in a roasting dish
and roast in an oven until all of the sulfide fumes are
driven off. If the ore contains only a small amount of sulfides, these
can be removed by placing a 16d (penny) steel nail in the crucible
with the assay. If low in lead, add an additional 12g of litharge to the
standard flux.

Class 3: Iron, manganese, etc.                                    

Assay Procedure


  • Examine ore and adjust flux as explained, then pulverize as finely as possible, at least 80 mesh.   
    The finer the better.

  • Weigh out one assay ton (1/2 AT or 14.58g) of ore, mix thoroughly with appropriate amount
           of flux and place in a 20g crucible. This is your assay.

  • If sulfides are present, roast ore and place a 16d nail in the crucible.

  • Place the crucible in the furnace and turn to high heat. When the furnace
          reaches 1093°C. Note the time or set a timer. The assay must
          remain in the oven at this temperature for 60 minutes.

  • Remove the crucible from the oven, remove the nail from the crucible,
          and pour the entire molten assay into a pouring mold and allow it to cool.

  • When the assay has cooled, remove the lead button from the bottom of
          the mold.

  • Pound the button into a cube breaking away the slag.it's all right if a little slag adheres to the
    button. Brush off the button with a button brush.

  • Put the button in a cupel, and place it in the furnace at 850 °C until the cupel has absorbed all
           the lead. The remaining bead contains all of the gold, silver, and PGM's in the assay.

  • Examine the bead carefully with a magnifier. Its color and surface
          appearance will tell you something about the ratio of goldlsilver/PGM's
          present and whether these elements should be parted as described below.

  • Weigh the bead. Each milligram (mg) in weight represents 1 troy ounce of precious metals in       
    each ton of the original ore, or:    1 mg = 1 oz/T.

Parting

        If your bead appears to be relatively pure gold, no parting is necessary. If the presence
       a significant amount of silver and/or PGM's is indicated, proceed as follows:

       To part the silver from the gold, the button must contain at least 3
        times as much silver as gold. If it does not, add sufficient assay silver to
        make this ratio. This is called INQUARTATION. Then proceed to
        ANNEAL the button as follows:

Annealing

Anneal by repeatedly hammering the bead flat and heating it in a
flame until it's red hot. Repeat heating after every couple of blows with the hammer.
       
The annealed strip is then rolled into a loose coil or Coronet, placed in a  parting cup and covered with a
parting solution, a mixture of chemically pure nitric acid and distilled water, 6 parts water to I part acid, or
a 6/1 ratio.

Heat the flask gently. The coronet will blacken as the hydrogen bubbles off. Do not allow it to boil                
too fast. If the coronet becomes immediately black, add a little more distilled water to slow the                     
action.

Finish   

Gravimetric
Instrumental: GA-AAS    


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