In my opinion, the crucible is used as a title on purpose for the understanding of the definition. A crucible is a container which can get extremely hot. In fact many precious metals will melt in a crucible, but the walls or boundaries of the crucible will not be affected by the heat.
The courtroom gets hotter and hotter... figuratively, just like a crucible. In fact, as the emotions are heated, people are melted and molded into different shapes than they were before they had come into the room. This happens to the girls as they are motivated by Abigail to follow her pretense. They also probably fear telling the truth because peoples' lives have been taken as a result of their lies.
In fact Danforth specifically says to John Proctor,
"We burn a very hot fire in here; it melts down all concealment."
In this sense, the courtroom can be seen as a crucible because as a crucible heats a metal, it removes impurities. In this regard, this court is asking so many questions and is so determined to get to the truth that it actually does pursue someone completely and fully until the court believes they have the truth (whether it is the truth or not).