kaolin vs paper clay

kaolin

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

paper clay

noun
  • A kind of uniformly light-coloured clay composed of fine particles, used in the production of paper and wallpaper; kaolin. 

  • Clay mixed with shredded paper or other fibres, valued for its strength and light weight in the production of large ceramic works. 

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