Fire Assaying

Fire Assaying Today (cont'd)

The technique of fire assaying still uses the same principles that were reported by Agricola.

The gold and silver must still be separated and removed from the other parts of the ore. A flux is added so the gold and silver can be collected by the lead and the other parts of the rock can be made into a readily separating glass slag. The standard flux to be added depends on the type of rock, but generally is defined by whether the rock is silicate or carbonate. If the rock is silicate, litharge (lead oxide, PbO), soda ash (sodium carbonate, Na₂CO₃) and borax (Na₂B₄O₇) are added. The sodium carbonate and borax combine with the silicate in the rock to form a glass that floats on top of the molten lead, which contains the gold, silver, and other precious metals. If the rock is carbonate, silica (SiO₂) in the form of sand must be added too in order to provide the silicate so the glass can form from the sodium carbonate and borax to create a good glass for the parts of the rock (the slag) other than the precious metals.
A reducing agent is added so the lead is changed (reduced) into lead metal. Again, the amount and type of reducing agent depends on the rock, but generally flour (source of carbon) is added. The reaction can be simplified to: 2PbO + C = CO₂ + 2Pb.

Fire Assaying Subject 2