slacken vs unthaw

slacken

verb
  • To gradually decrease in intensity or tautness; to become slack. 

  • To make slack, less taut, or less intense. 

  • To deprive of cohesion by combining chemically with water; to slake. 

unthaw

verb
  • To thaw out, to unfreeze; to become soft (of something which had been frozen). 

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