A crucible is a cup-shaped piece of laboratory equipment used to contain chemical compounds when heating them to very high temperatures. They are generally made of a refractory substance such as graphite or porcelain and vary in many different sizes depending on the its purpose. The materials must have a melting point higher than the metals it is used to melt and the structure must be strong enough not to collapse even in intense heat. Some can be as small as a cup or bowl for uses such as smelting metals and creating beads or jewelry and others can be big enough to melt high volumes of metals at places like steel factories or refineries which require melting lots of metals at high temperatures. They are shaped like bowls or cups with a wide bottom where the metals are placed to be melted. After placing the metal in the crucible, it is placed in a furnace where it can be heated to a couple thousand degrees. Throughout history, crucibles have been used for scientific purposes even in the 1600's, but back then, they were used to melt metals for the purpose of alchemy.

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An active crucible at high temperature.

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Here is a cool video with explosions and you can see that the crucible becomes red hot but does not break.
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The Crucible Metaphor
1)Arthur Miller uses the title "The Crucible" as a metaphor for the events that occur in Salem. The exterior crucible is the town of Salem , and the fire that burns under it is the accumulating accusations. Following this metaphor the people of Salem are the metal that is inside the crucible. On the atomical level this similarity is much more indentifiable. When the metal is first put into a hot crucible the atoms are pretty inactive and do not move around as much. Then as the metal gets hotter the atoms become more active eventually becoming active enough to become liquid. In Salem the accusations begin to fire up the people when the accusations lead to hanging. Eventually people who where good Catholics were pushed to accuse anyone the had to just to save themselves.
2)Another way the crucible is similar to Arthur Miller's "The Crucible" is the way that a crucible can make things pure. John Proctor exemplifies this in the way he transforms from a man who can not forgive himself but in the end he shows his true self when he tears up his confession. This is is similar to when alloys like bronze are placed into the fire and break up into their separate metals. As the situation in Salem becomes worse the people show what they are truly made of. Another view using the same idea, is that the witch trials were conducted to purify the town. It was meant to vanquish all the evil people and witches from Salem and leave only the people pure of heart and soul, even if it turned out to be a failure. To remove the ambiguities of the metal was to remove the evil persons from Salem.
3)The metaphor of the crucible also applies in the form of transformation. When metal is placed into a crucible the metal is heated and even if the metal is simply taken out of the heat it is changed. Metals also undergo tranformations when they are mixed with other metals to create alloys (Such as when Reverand Hale and Judge Hathorn come to Salem). Similarly the people of Salem are transformed by the events of the Salem Witch Trials. The town goes from a calm and peaceful place to a town under the rule of marshall law.