Ceramic (alumina) cups are the standard — inexpensive, heat-resistant and available in a range of sizes from #4 to #12. Larger cups give wider gas coverage and are used for higher amperage and for materials sensitive to oxidation. Smaller cups give better visibility in tight joints. Fused quartz (Pyrex) cups let you see the arc and puddle from the side, which is useful for tube and pipe work where the joint is hard to see.
Cup size must match gas flow. A large cup with insufficient flow gives poor coverage; a small cup with too much flow creates turbulence that pulls air in. Gas lens setups allow larger cups at the same flow because the flow is laminar rather than turbulent.
Specialty cups include lava (machinable ceramic) for custom shapes, and stubby cups for access into confined joints. Cups crack from thermal shock and from being knocked against the work; a cracked cup changes gas flow and should be replaced.
