There are two basic definitions of the word "crucible" which make it an excellent choice for the title of the play. The first one is : n. A severe test, as of patience or belief; a trial. The second definition is also a noun : a vessel made of material that does not melt easily; used for high temperature chemical reactions.
The military (Marine Corps, I think) also has something they call the crucible to test the will and strength of bootcamp members. It's meant to be both mentally, emotionally, and physically taxing.
Since the play deals with both a trial of patience and belief (and a literal trial with judge and jury) as well as a situation that is destined to explode with hot tempers and reactions, there is definitely something to use here for your hook.
You could begin your essay with something like this:
Arthur Miller's play, The Crucible, has a most appropriate title. A "crucible" is either a container meant to house chemicals at high temperature or a trial of patience and belief. The play continually raises heated debates of people who discuss the themes of the moral choice, justice and injustice, truth and lie, trials of the innocently accused person, witchcraft, evil powers vs. good ones. There is much explosion of emotion and just as much trial of patience and belief taking place in this aptly named play.
Webster's defines the word "crucible" as:
1 : a vessel of a very refractory material (as porcelain) used for melting...a substance that requires a high degree of heat 2 : a severe test 3 : a place or situation in which concentrated forces interact to cause or influence change or development
You could use any one of these definitions as a "hook" for your essay. For instance, if you chose the first definition, you might compare the trials and punishments as the vessel in which the people accused of witchcraft are tested. Using the second definition to explain the title is obvious, again through the trials and sentencing. The third option might be the most intriguing theme on which to build your essay. What are those concentrated forces that interact to cause change in Salem? And what does that change ultimately turn out to be: the routing out of witches in the town or the exposure and ridicule of superstition and paranoia? Remember that Arthur Miller wrote The Crucible in the aftermath of the McCarthy hearings, whose goal was to uncover Communists in America. Isn't it interesting that those trials were called "witch hunts"?