Open-pit mining
After mining finishes, the mine area may undergo land rehabilitation. Waste dumps are contoured to flatten them out, to further stabilise them. If the ore contains sulfides it is usually covered with a layer of clay to prevent access of rain and oxygen from the air, which can oxidise the sulfides to produce sulfuric acid, a phenomenon known as acid mine drainage. This is then generally covered with soil, and vegetation is planted to help consolidate the material. Eventually this layer will erode, but it is generally hoped that the rate of leaching or acid will be slowed by the cover such that the environment can handle the load of acid and associated heavy metals. There are no long term studies on the success of these covers due to the relatively short time in which large scale open pit mining has existed. It may take hundreds to thousands of years for some waste dumps to become "acid neutral" and stop leaching to the environment. The dumps are usually fenced off to prevent livestock denuding them of vegetation. The open pit is then surrounded with a fence, to prevent access, and it generally eventually fills up with ground water. In arid areas it may not fill due to deep groundwater levels. Instead of returning the land to its former natural state, it may also be reused, converting it into recreational parks or even residential/mixed communities.
Japanese pottery and porcelain
Clay is chosen largely based on local materials available. There is an abundance of most basic types of clay in Japan. Due to naturally occurring kaolin deposits, many porcelain clays are found in Kyushu. Kilns were traditionally built at the sites of clay deposits, and most potters still use local clays, having developed a range of glazes and decoration techniques especially suited to that clay. The pottery clays found in the Japanese archipelago range from fusible earthenwares to refractory kaolins. From the Jōmon period to the Yayoi period, Japanese potters relied on high plastic iron-bearing shale and alluvial clays. Organic materials appear in much of the early Jōmon period work, but sand or crushed stone predominates thereafter. Further refinements came about under the Chinese influence in the 8th and 9th centuries AD, when creators of Nara three-color wares and Heian ash glazed wares sought out white, refractory clays and enhanced their fineness through levigation. In Kyoto, where demand makes it both practical and profitable, the clay is crushed, blunged (made into slip), and filtered commercially. To use the clay, you must first break it up into small pieces, pour a small amount of water over it, and beat it with a "kine", a wooden mallet, until you obtain the plasticity and uniformity of texture you want. Then you put it through an "aramomi" or "rough wedging" process, a kneading movement, after which the clay is stored for two or three days, or sometimes up to a week. Before the clay is ready to be thrown, it must pass through the "nejimomi" ("screw-wedge") process, which produces a bullet-shaped mass from which all air bubbles have been removed and in which the granular structure is arranged so that it radiates outwards from the center of the mass.