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.:
* 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
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