crucible

Short definition from Wiktionary

Noun

1. 1
   A cup-shaped piece of laboratory equipment used to contain chemical compounds when heating them to very high temperatures.
2. 2
   A heat-resistant container in which metals are melted, usually at temperatures above 500°C, commonly made of graphite with clay as a binder.
3. 3
   The bottom and hottest part of a blast furnace; the hearth.
4. 4
   A very difficult and trying experience, that acts as a refining or hardening process.