mezzotint vs soften

mezzotint

verb
  • To make such etchings 

noun
  • A form of intaglio etching in which a metal plate is roughened evenly and then smoothed to bring out an image. 

  • An etching or print made using this method. 

soften

verb
  • To make something soft or softer. 

  • To palatalize. 

  • To become or make (a consonant) more lenis, to lenite. 

  • To make less harsh 

  • To become soft or softer 

  • To undermine the morale of someone (often soften up). 

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