double boiler vs potjie

double boiler

noun
  • A pair of interlocking pots, the lower one filled with water which when boiling heats the contents of the upper pot. 

  • A large pan containing hot water, into which other smaller pans are set in order to cook food at low heat (below the boiling point) or to keep food warm. 

potjie

noun
  • A lidded round-bottomed pot used for cooking in South Africa. 

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