crucible vs water boiler

crucible

noun
  • A cup-shaped piece of laboratory equipment used to contain chemical compounds when heating them to very high temperatures. 

  • A heat-resistant container in which metals are melted, usually at temperatures above 500°C, commonly made of graphite with clay as a binder. 

  • A very difficult and trying experience, that acts as a refining or hardening process. 

  • The bottom and hottest part of a blast furnace; the hearth. 

water boiler

noun
  • An electric device which heats water up to boiling temperature. 

How often have the words crucible and water boiler occurred in a corpus of books? (source: Google Ngram Viewer )