Question: Century eggs are made by preserving eggs in a mixture of clay, ash, salt and what kind of oxide?
Sentence (from "Century egg"): Century egg or Pidan (), also known as preserved egg, hundred-year egg, thousand-year egg, thousand-year-old egg, millennium egg and black egg, is a Chinese preserved food product and delicacy made by preserving duck, chicken or quail eggs in a mixture of clay, ash, salt, quicklime, and rice hulls for several weeks to several months, depending on the method of processing.
Relevant evidence: Yes