The main symbol in The Crucible is the crucible. A crucible is a receptacle for heating materials to high temperatures. It can be used in chemical reactions to separate valuable metals out from impurities or base metals. The idea of a crucible is metaphorically used in context of severe human test or trial.
There are two historical periods associated with Miller's play from the time he wrote it. The first was the Salem witch trials. In 1949 Marion Starkey's book The Witch Trials in Massachusetts launched widespread interest in this up until then little known moment in Massachusetts history. During this time of social-pathology in the form of group hysteria spurred by lies cover-ups, twenty people were killed for being witches. At their trials they were given a chance to confess their crimes of witchcraft, recant their ways, and name other people who were involved in witch practices and ceremonies.
Arthur Miller began writing The Crucible after The Witch Trials in Massachusetts was published. The Crucible first appeared in 1953. The second historical event related to Miller's play is the Joe McCarthy hearings in the U.S. House Committee on Un-American Activities. Communism was feared and McCarthy and the House Committee felt that Communism was being spread in the U.S. among organization and through the media. People who fell under suspicion were called to the committee. At their hearings they were given a chance to confess their Communist activities, recant their beliefs and give names of other people who were involved in Communist practices and propaganda.
The primary level of historical focus in Miller's work is an analysis of the Salem Witchcraft Trials and their impact on a group of people. I think that one can find a similar level of application to the Red Scare that took place in America after the Second World War, where the fear of Communism was manipulated by men like Joseph McCarthy. The accusation setting of Salem and America in the 1950s possessed striking similarities in that both displayed the results of government unchecked. In this really bad vision of "governments gone wild," institutional checks that seek to limit the encroaching power of government are tossed out in favor of a vision of harmony and symmetry that are advanced by those who feature duplicitous motives. Abigail's accusations have less to do with witchcraft and more to do with people seeking to consolidate their power and gain greater control of life in Salem. In much the same way, the fear of Communists was used by those in the position of power to keep the public in fear and ensure that obedience was maximized. In both settings, the cost of silence and fear is that government loses its accountability to the people and is allowed to run amok.