• Clay is a type of fine-grained natural soil material containing clay minerals.
  • Clay is the oldest known ceramic material.
  • Prehistoric humans discovered the useful properties of clay and used it for making pottery.
  • Between one-half and two-thirds of the world's population live or work in buildings made with clay, often baked into brick, as an essential part of its load-bearing structure.
  • Shale, formed largely from clay, is the most common sedimentary rock.
  • Although many naturally occurring deposits include both silts and clay, clays are distinguished from other fine-grained soils by differences in size and mineralogy.
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  • The defining mechanical property of clay is its plasticity when wet and its ability to harden when dried or fired.
  • Clay has a high content of clay minerals that give it its plasticity.
  • When the clay is dried, most of the water molecules are removed, and the plates hydrogen bond directly to each other, so that the dried clay is rigid but still fragile.
  • The tiny size and plate form of clay particles give clay minerals a high surface area.
  • In some clay minerals, the plates carry a negative electrical charge that is balanced by a surrounding layer of positive ions, such as sodium, potassium, or calcium
  • .Some clay minerals are described as swelling clay minerals, because they have a great capacity to take up water, and they increase greatly in volume when they do so.
  • When dried, they shrink back to their original volume.
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  • The main groups of clays include kaolinite, montmorillonite, smectite, and illite.
  • Chlorite, vermiculite, talc, and pyrophyllite are sometimes also classified as clay minerals.
  • There are approximately 30 different types of "pure" clays in these categories, but most "natural" clay deposits are mixtures of these different types, along with other weathered minerals.
  • Varve is clay with visible annual layers that are formed by seasonal deposition of those layers and are marked by differences in erosion and organic content.
  • Quick clay is a unique type of marine clay indigenous to the glaciated terrains of Norway, North America, Northern Ireland, and Sweden.
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Use As Medicines

  • Traditional uses of clay as medicines go back to prehistoric times.
  • An example is an Armenian bole, which is used to soothe an upset stomach.
  • Some animals such as parrots and pigs ingest clay for similar reasons.
  • Kaolin clay and attapulgite have been used as anti-diarrheal medicine.
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