Eggshells, seashells and bone ash In his account of his travels in China, Marco Polo wrote: "They collect a certain kind of earth, as it were from a mine, and laying it in a great heap, suffer it to be exposed to the wind, the rain and the sun, for thirty or forty years, during which time it is never disturbed.
馬可波羅遊記中記載：中國人收集一種特別的泥土，似乎是一種礦土，然後堆疊成山，讓它風吹日曬雨淋，經三、四十年，不可動它。