The primary purpose of flux is to prevent oxidation of the base and filler materials. Tin-lead solder, for example, attaches very well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures. This applies to lead and brass too. Flux is a substance which is nearly inert at room temperature, but it becomes strongly reducing at elevated temperatures, preventing the formation of metal oxides.
Secondarily, flux acts as a wetting agent in soldering processes for lead, copper and brass.
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