Question: Century eggs are made by preserving eggs in a mixture of clay, ash, salt and what kind of oxide?

Context paragraphs:
Eggs Sardou: Eggs Sardou is a Louisiana Creole cuisine dish made with poached eggs, artichoke bottoms, creamed spinach and Hollandaise sauce.  It is on the menu of many Creole restaurants in New Orleans, including Antoine's, where eggs Sardou was invented, and Brennan's.  Eggs Sardou is named for Victorien Sardou, a famous French dramatist of the 19th century, who was a guest in New Orleans when the dish was invented.

Custard pie: A custard pie is any type of uncooked custard mixture added to an uncooked or partially cooked crust and baked together.  In North America, custard pie commonly refers to a plain mixture of milk, eggs, sugar, salt, vanilla extract and sometimes nutmeg combined with a pie crust.  It is distinctly different from a cream pie, which contains cooked custard poured into a cooled, precooked crust.  In the United Kingdom, the comical or political act of pieing is conventionally done with a "custard pie".  Some common custard pies include pumpkin pie, lemon and buttermilk chess pie, and coconut custard.  True custard is defined as a liquid thickened with eggs.  Due to the often large number of whole eggs in custard pie it is a very rich pie.

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.

Raw animal food diets: Foods included in raw animal food diets include any food that can be eaten raw, so including raw, unprocessed meats/organ-meats/raw eggs, raw dairy, and aged, raw animal foods such as century eggs, fermenting meat/fish/shellfish/dairy(such as kefir), as well as, to a lesser extent, nuts/sprouts/plants/fruits, but generally not raw grains, raw beans, raw soy etc., because of digestibility and toxicity issues and also because paleolists tend to reject neolithic or domesticated foods.  Raw foods on such diets have not been heated at temperatures above 104 °F .  “Raw Animal Foodists” believe that foods cooked above this temperature have lost much of their nutritional value and are harmful to the body.  Smoked meats are frowned upon by many Raw-Omnivores.  Some make a distinction between hot-smoked and cold-smoked.

Calcium oxide: Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound.  It is a white, caustic, alkaline, crystalline solid at room temperature.  The broadly used term "lime" connotes calcium-containing inorganic materials, in which carbonates, oxides and hydroxides of calcium, silicon, magnesium, aluminium, and iron predominate.  By contrast, "quicklime" specifically applies to the single chemical compound calcium oxide.  Calcium oxide that survives processing without reacting in building products such as cement is called free lime.

Salt glaze pottery: Salt-glaze or Salt glaze pottery is stoneware with a glaze of glossy, translucent and slightly orange-peel-like texture which was formed by throwing common salt into the kiln during the higher temperature part of the firing process.  Sodium from the salt reacts with silica in the clay body to form a glassy coating of sodium silicate.  The glaze may be colourless or may be coloured various shades of brown (from iron oxide), blue (from cobalt oxide), or purple (from manganese oxide).

Scrambled eggs: Scrambled eggs is a dish made from whites and yolks of eggs (usually chicken eggs) stirred or beaten together in a pan while being gently heated, typically with salt and butter and variable other ingredients.

Salted duck egg: A salted duck egg is a Chinese preserved food product made by soaking duck eggs in brine, or packing each egg in damp, salted charcoal.  In Asian supermarkets, these eggs are sometimes sold covered in a thick layer of salted charcoal paste.  The eggs may also be sold with the salted paste removed, wrapped in plastic, and vacuum packed.  From the salt curing process, the salted duck eggs have a briny aroma, a gelatin-like egg white and a firm-textured, round yolk that is bright orange-red in color.

Oxide thin-film transistor: An oxide thin-film transistor (TFT) is a particular kind of field-effect transistor made by depositing thin films of a semiconductor active layer as well as the dielectric layer and metallic contacts over a supporting substrate.  The principal difference between amorphous silicon TFT and Oxide TFT is that the material of the electron channel is oxide or amorphous silicon.  A common substrate is glass, since the primary application of TFTs is in liquid crystal displays and organic light emitting displays (OLEDs).  This differs from the conventional transistor where the semiconductor material typically is the substrate, such as a silicon wafer.  TFT electrical performance is dramatically degraded if a zinc–tin–oxide TFT is covered with a dielectric layer and does not undergo both types of annealing.  In addition to silicon dioxide, successful passivation of zinc–tin–oxide TFTs is accomplished using thermally evaporated calcium fluoride, germanium oxide, strontium fluoride, or antimony oxide as passivation.

Borlengo: A borlengo (plural: "borlenghi"), also called a "burlengo" or "zampanelle", is a thin crepe now made with milk, eggs (sometimes omitted) flour and salt.  Originally a food eaten by the poor and made with flour and water, it is now often made outside in a frying pan the size of a cartwheel.  These are then rubbed with a mixture that can contain rosemary, garlic, salt pork, olive oil, or what is called "cunza", sauteed minced pancetta, sausage and rosemary, folded into quarters and sprinkled with Parmigiano.

Supporting paragraphs: Century egg Calcium oxide
Answer: Calcium oxide