Fire Assay Procedure
by Jensen Technologies, LLC

Classification of ores:

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

Class 1: 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 PGMs in the assay.

* Examine the bead carefully with a magnifier. Its color and surface appearance will tell you something about the ratio of goldlsilver/PGMs 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.


If your bead appears to be relatively pure gold, no parting is necessary. If the presence a significant amount of silver and/or PGMs 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:


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.


Gravimetric Instrumental: GA-AAS

This article was contributed by Jensen Technologies, LLC whose laboratory is located at 15985 S. Golden Rd, Unit H, Golden, CO 80401. Their phone number is (303) 478-3929 and their FAX number is (720) 542-3498. You may find out about their Laboratory Services by contacting by e-mail. Their website is where all the information is available.

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