butter up vs richen

butter up

verb
  • To flatter, especially with the intent of personal gain. 

richen

verb
  • To gain richness; become heightened or intensified in brilliancy. 

  • To make or render rich or richer. 

  • To become rich or richer; become superior in quality, condition or effectiveness. 

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