What can be used as a flux?

Could you elaborate on the concept of "flux" in the context of materials science or engineering? Specifically, I'm interested in understanding what substances or materials can be utilized as a flux during various processes such as welding, soldering, or smelting. What properties make a substance suitable as a flux? Are there specific chemicals or minerals that are commonly used as fluxes, and if so, why are they chosen? I'm curious to know if the choice of flux has a significant impact on the quality or efficiency of the process.