List of Basic Alkaline Fluxes used in Fire Assay
Sodium carbonate (Na2CO3) or Soda ash: Sodium carbonate is a powerful basic flux that reacts with silica-based (acidic) minerals to form a fusible sodium silicate.
Na2CO3 + SiO2 = Na2SiO3 + CO2.
There can be problems with soda ash reactions. The CO2 gas that is produced can cause frothing if the temperature is too high. Soda ash is very fluid and can hold finely powdered materials such as carbon (flour) in suspension in the melt which can interfere with the formation of the button in the fusion process. Soda ash can also act as a desulphurizing and oxidizing agent. Soda ash is also very corrosive towards the crucible.
Sodium bicarbonate (NaHCO3) baking soda: This can be used in place of soda ash but it is less common. It has less soda, which is the active ingredient. It loses water and CO2 at low temperatures and can cause losses due to, cross contamination, frothing, and the sample boiling over. The end product of the use of sodium bicarbonate is the same as with soda.
Litharge (lead oxide – PbO) & Red lead (lead oxide – PbO:Pb2O3 or Pb3O4): Litharge (PbO) or red Lead (Pb3O4) is a basic flux that has many purposes. It helps to decompose silica to form fusible lead silicate.
PbO + SiO2 = PbSiO3
It supplies the lead to produce the lead button that collects the gold and silver.
PbO: 2PbO + C(flour) = 2Pb + CO2.
It also dissolves difficult-to-fuse metal oxides for example Copper. One should practice some caution because excessive litharge corrodes the crucible and destroys the furnace floor if there is a spill or boil over.
Red lead acts as an oxidizing agent as it decomposes to PbO and gaseous O2 on heating as oppose to CO2 and is sometimes preferred to litharge.
2Pb3O4 = 6PbO + O2.
Hematite (iron oxide – Fe2O3): Hematite is sometimes used as to replace litharge. It is combination of a basic flux and an oxidizing reagent. Ferric oxide is very difficult to fuse but is reduced to a more easily fused ferrous state during the fusion. It then combines with silica to yield ferrous silicate. Manganese oxides act similarly to iron oxides. These oxides act as oxidizing agents by consuming carbon (flour).
2 Fe2O3 + C = 4FeO + CO2
2 FeO + SiO2 = 2FeOSiO2
Lime (CaO): Lime also known as the mineral limestone (CaCO3). It is a powerful base for fluxing the silica. On heating, limestone loses CO2 and reacts with silica and produces a fusible silicate. Calcium also is found with magnesium in a carbonate mineral called dolomite (CaMg(CO3)2). Magnesium behaves very similarly to calcium in a fusion.