One definition of a crucible is a vessel, often ceramic or porcelain, used for melting down and purifying metal. Another definition is that a crucible is a time or trial of great severity, in which different elements react and something new is formed.
In this chemistry experiment, magnesium is heated. When it is allowed to react with oxygen, it turns into Magnesium Oxide. It not only is transformed chemically but it also has a higher mass. More importantly, it is irrevocably changed.
Arthur Miller wrote, The Crucible, to show how a person is forged in times of stress. In this story, the main characters are irrevocably changed by the trial in which they have to decide between good and bad.
Each time we take a belt test, we are placing ourselves in the crucible as well. We are changed by the tested effort of the day and we mark the change with a change in the belt. We are irrevocably changed and we cannot go back to what we were before.
Another “fiery” example is the failed flight of Apollo 13. 170,000 miles from earth, the crew had an explosion in their oxygen tank. The ground crew and spacecraft crew worked on a solution to rescue themselves using duct tape, cardboard and slide rule calculations.