Miller confronts all three definitions of crucible in Webster's dictionary: A vessel for melting and calcining a substance using a high degree of heat; a severe test; or a place or situation in which concentrated forces interact to cause or influence change or development.
In Act 3, Deputy Governor Danforth says, "We burn a hot fire here; it melts down all concealment," meaning things will be very heated, emotional and stressful, but his viewpoint will uncover the truth.
Several characters are severely tested, none more than John Proctor who refuses to put his name to a lie.
The events in Salem, and especially the court, fuel and heat the people's emotions and affect their actions.
In his autobiography, Miller says "the same spiritual nugget lay folded" within both the witch trials and HUAC. He knew he was possibly writing himself into the political and personal wilderness but ,like his character, John Proctor, he believed he had no choice.