Clay

Clay is a type of fine-grained natural <a href="/facts/Soil/5Ddpr3QM">soil</a> material containing <a href="/facts/Clay_mineral/A1Sod7Hp">clay minerals</a> (hydrous aluminium phyllosilicates, e.g. <a href="/facts/Kaolinite/N7D6K65q">kaolinite</a>, <a href="/facts/Aluminium/7wmR8jYE">Al</a>2<a href="/facts/Silicon/cgWx0hdr">Si</a>2<a href="/facts/Oxygen/acyn6o8r">O</a>5(<a href="/facts/Hydroxide/zW0wxpWD">OH</a>)4). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impurities, such as a reddish or brownish colour from small amounts of <a href="/facts/Iron_oxide/KDo8hLWn">iron oxide</a>.
Clays develop <a href="/facts/Plasticity_(physics)/2VKo6keA">plasticity</a> when wet but can be hardened through <a href="/facts/Pottery/XLcUT1mX">firing</a>. Clay is the longest-known <a href="/facts/Ceramic/9JKudpqX">ceramic</a> material. Prehistoric humans discovered the useful properties of clay and used it for making <a href="/facts/Pottery/XLcUT1mX">pottery</a>. Some of the earliest pottery shards have been <a href="/facts/Radiocarbon_dating/qjNglhKP">dated</a> to around 14,000 BCE, and <a href="/facts/Clay_tablet/9mbScsAn">clay tablets</a> were the first known writing medium. Clay is used in many modern industrial processes, such as <a href="/facts/Paper/FUETwwKV">paper</a> making, <a href="/facts/Cement/VfDMkk12">cement</a> production, and chemical <a href="/facts/Filtration/x4no9XQy">filtering</a>. 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. In agriculture, clay content is a major factor in determining land <a href="/facts/Arable_land/pTSyNmdg">arability</a>. Clay soils are generally less suitable for crops due to poor natural drainage; however, clay soils are more fertile, due to higher <a href="/facts/Cation-exchange_capacity/4DoXx9Jk">cation-exchange capacity</a>.
Clay is a very common substance. <a href="/facts/Shale/bsIhqY5F">Shale</a>, 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. <a href="/facts/Silt/HAxJkhDP">Silts</a>, which are fine-grained soils that do not include clay minerals, tend to have larger particle sizes than clays. Mixtures of <a href="/facts/Sand/MdTIt0M8">sand</a>, <a href="/facts/Silt/HAxJkhDP">silt</a> and less than 40% clay are called <a href="/facts/Loam/R8parQXe">loam</a>. Soils high in swelling clays (<a href="/facts/Expansive_clay/JsaySWsq">expansive clay</a>), which are clay minerals that readily expand in volume when they absorb water, are a major challenge in <a href="/facts/Civil_engineering/avh5kEq2">civil engineering</a>.

Clay open-in-new

Clay