potter's field vs urnfield

potter's field

noun
  • A public place where strangers, paupers, and criminals are buried. 

urnfield

noun
  • Ground used as a cemetery in Bronze Age Europe, in which the ashs of cremations were buried in cinerary urns. 

How often have the words potter's field and urnfield occurred in a corpus of books? (source: Google Ngram Viewer )