mezzotint vs photogravure

mezzotint

noun
  • An etching or print made using this method. 

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

verb
  • To make such etchings 

photogravure

noun
  • A print so made. 

  • An intaglio process for printing photographic reproductions in newspapers and books. 

verb
  • To print by this process. 

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