when does the hard calcium shell form around the egg
Eggshell The fibrous chicken shell membranes are added in the proximal (white) isthmus of the oviduct.[1] In the distal (red) isthmus mammillae or mammillary knobs are deposited on the surface of the outer membrane in a regular array pattern.[7][8] The mammillae are proteoglycan-rich and are thought to control calcification. In the shell gland (similar to a mammalian uterus), mineralization starts at the mammillae. The shell gland fluid contains very high levels of calcium and hydrogen carbonate. The thick calcified layer of the eggshell forms in columns from the mammillae structures, and is known as the palisade layer. Between these palisade columns are narrow pores that traverse the eggshell and allow gaseous exchange. The cuticle forms the final, outer layer of the eggshell.[9]
Century egg Century egg or Pidan (Chinese: 皮蛋; pinyin: pídàn), also known as preserved egg, hundred-year egg, thousand-year egg, thousand-year-old egg, millennium egg, skin 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.[1]