rusk vs zwieback

rusk

noun
  • a twice-baked bread, slices of bread baked until they are hard and crisp (also called a zwieback) 

  • a rectangular, hard, dry biscuit 

  • a cereal binder used in meat product manufacture 

  • a weaning food for children 

zwieback

noun
  • A usually sweetened bread enriched with eggs that is baked and then sliced and toasted until dry and crisp; considered easy to digest and therefore given to the ill and used as a teething food for toddlers. 

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