kaolin vs potter's clay

kaolin

noun
  • A fine clay, rich in kaolinite, used in ceramics, paper-making, etc. 

potter's clay

noun
  • The clay used by a potter. 

  • A person or people created and shaped by God. 

  • A nation or kingdom. 

How often have the words kaolin and potter's clay occurred in a corpus of books? (source: Google Ngram Viewer )