retardant vs wet blanket

retardant

noun
  • Something that serves to retard (slow down) the action of something 

adj
  • Serving to impede (slow down) the action of something 

wet blanket

noun
  • A person who takes the fun out of a situation or activity, as by pessimism, demands, dullness, etc. 

  • A timid person; one who lacks confidence; a wimp. 

  • Used other than figuratively or idiomatically: see wet, blanket. 

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