"Fluxes are very frequently required in cases of chemical action amongst metallic compounds at high temperatures, and often can not be dispensed with. Their use is to protect the substance from the air; to dissolve impuries which would otherwise be infusible; and to convey active agents, as charcoal and reducing matter, into contact with the substance operated upon." - Faraday.
In the large way, limestone and fluor spar are used as fluxes. On the small scale, the fluxes chiefly used are black flux, white flux, crude flux, and glass.
Nitre, 1 part; crude tartar or cream of tartar, 2 parts; mix, and deflagrate, by small quantities at a time, in a crucible, heated to dull redness. The product consists of carbonate of potassa, mixed with charcoal in a finely divided state. Used for smelting metallic ores. It exercises a reducing action, as well as promotes the fusion. It must be kept in a dry corked bottle.
Into a large earthen crucible, heated to redness, throw successive portions of a mixture of 2 parts of nitre and 1 of tartar. Keep it as the last.
Crude Flux is the mixture of nitre and tartar before deflagration.
Mix crystallized carbonate of soda with one eighth of charcoal, and heat gradually to redness.
Dry carbonate of potash 3 parts, cyanide of potassium 1 part.
Cream of tartar 10 parts, nitre 3 1/2, borax 3. Triturate well together.
Pulverized glass (free from lead) 8 parts, calcined borax 1/2 part, charcoal 1/2 part. Used as black flux.
Saturate a solution of tartaric acid with carbonate of soda, evaporate to dryness, and calcine in a covered platinum crucible.