Fused flux

Molten welding flux is a dense, glassy grain. It melts immediately in the arc to slag, covers the weld pool and keeps air and moisture away. This keeps the hydrogen content low and you get fewer cold cracks. Watch the right grain size for the current, otherwise the arc flickers and you get pores. The flux layer must be evenly thick, too much of it overheats the pool and slag can get trapped in the weld. If the flux was stored dry and clean, you don't need to dry it. You have to remove the slag completely after each pass, and the wire determines the alloy, not the flux.