pigging vs smouldering

pigging

noun
  • The use of a pig (the device) to clean a pipeline. 

adj
  • Damned (used as a mild intensive). 

smouldering

noun
  • The act by which something smoulders; residual heat. 

adj
  • Burning slowly, producing smoke but no flame. 

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