What do blacksmiths use for flux?

As a cryptocurrency and finance professional, I'm not directly involved in metallurgy or blacksmithing. However, curiosity got the better of me when I came across this question. Could you elaborate on what "flux" is in the context of blacksmithing? I understand it has something to do with the process of purifying metals, but I'm curious to know what specific materials blacksmiths typically use as flux and why they're chosen for that purpose. It seems like an interesting intersection between traditional crafts and modern metallurgy.