Fused flux is melted, quenched and crushed into glassy particles - chemically uniform, largely non-hygroscopic and tolerant of very high currents, but unable to carry alloy additions. Agglomerated flux binds its constituents and bakes them at lower temperature, so it can carry deoxidisers and alloys and reach higher basicity, at the cost of absorbing moisture.

Weld metal cleanliness and toughness. Higher basicity means lower oxygen content in the deposit and better low-temperature impact properties, which is why basic fluxes are specified for pressure equipment and offshore work. Acidic and neutral fluxes generally run faster with better bead appearance and slag release, so the choice follows the required properties.

Because moisture in the flux becomes hydrogen in the weld metal, and hydrogen in a hardenable steel weld causes delayed cracking that may appear days later. Agglomerated fluxes absorb moisture from the air, so they are supplied sealed, re-dried to the manufacturer's schedule if exposed, and held in heated hoppers while in use.

Yes, and it is normal practice - most of the flux laid down never melts. It is recovered by vacuum, screened to remove fused slag and crushed fines, and returned to the hopper, usually blended with a proportion of virgin flux. What must not be reused is fused slag, which has already given up its function and would change the chemistry.

A vacuum pickup trailing the welding head lifts loose flux and slag from the joint, a cyclone or settling chamber separates the fines and dust, and a screen removes fused slag lumps. The cleaned flux is returned to the hopper, often through a heated holding section. Units may be mounted on the tractor or boom, or run as a shop system.

As a pair, because the deposit chemistry results from both. Classification systems such as AWS A5.17 for carbon steel and A5.23 for low-alloy steel designate the flux-wire combination together with the mechanical properties it achieves in the as-welded or post-weld heat-treated condition. Substituting either one changes the qualification.

As a rule of thumb, roughly one kilogram of flux is melted for each kilogram of wire deposited, though it varies with parameters and joint geometry. That refers to flux actually consumed; the quantity laid on the joint is considerably greater, which is exactly why recovery is standard rather than optional.