What is the origin of the phrase fired in The Crucible?

The metaphor is drawn from metallurgy. A crucible is a vessel, often made of stone, and sometimes with a spout, in which metals are melted. It is used by metal workers and also by early alchemists who were searching for the 'philosopher's stone' a substance which they believed could turn base metals into gold.

When crude ore is melted in a crucible the impurities or 'dross' float to the surface and burn off, or the pure melted metal can be poured off from underneath.

To be 'fired in the crucible' then, would mean to undergo an intense and painful trial from which one would emerge purified or refined.