wool grease vs yolk

wool grease

noun

  • Lanolin. 

yolk

noun

  • The grease in a sheep's fleece; lanolin. 

  • The yellow, spherical part of an egg that is surrounded by the white albumen, and serves as nutriment for the growing young. 

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