cooking pot vs firkin

cooking pot

noun
  • Any vessel (such as a saucepan), with or without a lid, used to cook food 

firkin

noun
  • A small wooden vessel or cask of indeterminate size, used for butter, lard, etc. 

  • A varying measure of capacity, usually being a quarter of a barrel; specifically, a measure equal to nine imperial gallons. 

  • A weight measure for butter, equalling 56 pounds. 

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