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