The fire assay is the preferred method of determining the mineral grading in a deposit because it is the most accurate. If the assay is performed on the ore materials using fusion and followed by cupellation separation, detection of mineralization may be in parts per billion. However, accuracy on ore material is typically limited to 3 to 5 percent of reported value. The downfall of fire assays is that it is very disruptive; however, if performed on exploration-level properties this does not pose a problem.Once the weight is pinned down, a grade is calculated.
The first step in the fire assay process is to fuse or dissolve the rock sample in question in a crucible using a lead glass flux. The flux is a mixture of materials, usually- sodium bicarbonate, potassium carbonate, borax, litharge (lead oxide) and flour that is selected by the laboratory chemist. The appropriate flux is mixed with the crushed ore (rock sample). The mixture is then heated to encourage a reaction. The reaction is allowed to go to completion, and then the crucible is removed. The flux mixture and rock sample have now reacted to form two parts- a molten glass that is on top of metal at the bottom of the crucible. The molten glass is poured off, as this part of the material does not contain any valuable metals. The remaining metal is poured into a mold, and allowed to cool until it solidifies. Once cooled, the solid metal is removed from the mold. This metal generally contains lead, gold and silver. The mass of lead metal is then placed on a cupel made of bone ash- a material that readily absorbs lead oxide, but does not absorb metal. The metal is then heated; the lead metal is oxidized to lead oxide. The oxide then is either absorbed into the cupel, or volatized off into the atmosphere. What is left is a tiny bead containing all of the gold and silver that was in the original sample. The bead is then weighed, and the total weight of the gold and silver together is known.
This article has more detail, and it serves as a good guide that explains what goes on in assaying as well as the significance of the results. It's well worth a read.