Clay is a term used to describe a group of hydrous aluminium phyllosilicate (phyllosilicates being a subgroup of silicate minerals) minerals (see clay minerals), that are typically less than 2 μm (micrometres) in diameter. Clay consists of a variety of phyllosilicate minerals rich in silicon and aluminium oxides and hydroxides which include variable amounts of structural water. Clays are generally formed by the chemical weathering of silicate-bearing rocks by carbonic acid but some are formed by hydrothermal activity. Clays are distinguished from other small particles present in soils such as silt by their small size, flake or layered shape, affinity for water and tendency toward high plasticity.
Depending upon academic source, there are three or four main groups of clays: kaolinite, montmorillonite-smectite, illite, and chlorite (the latter group is not always considered a part of the clays and is sometimes classified as a separate group within the phyllosilicates). There are about thirty different types of "pure" clays in these categories but most "natural" clays are mixtures of these different types, along with other weathered minerals.
Historical and modern uses of clay
Clay is plastic when wet, which means it can be easily shaped. When dry, it becomes firm and when subject to high temperature, known as firing, permanent physical and chemical reactions occur which, amongst other changes, causes the clay to be converted into a ceramic material. It is because of these properties that clay is used for making pottery items, both practical and decorative. Different types of clay, when used with different minerals and firing conditions, are used to produce earthenware, stoneware and porcelain. An oven specifically designed for firing clay is called a kiln. Early humans discovered the useful properties of clay in prehistoric times, and one of the earliest artifacts ever uncovered is a drinking vessel made of sun-dried clay. Depending on the content of the soil, clay can appear in various colors, from a dull gray to a deep orange-red.
Clay was also used as the very first writing medium. Thousands of years BCE the cuneiform script was written in clay tablets.
Clays sintered in fire were the first ceramic, and remain one of the cheapest to produce and most widely used materials even in the present day. Bricks, cooking pots, art objects, dishware and even musical instruments such as the ocarina can all be shaped from clay before being fired. Clay is also used in many industrial processes, such as paper making, cement production and chemical filtering.
Some varieties of clay
Montmorillonite, with a chemical formula of (Na,Ca)0.33(Al,Mg)2Si4O10(OH)2·nH2O, is typically formed as a weathering product of low silica rocks. Montmorillonite is a member of the smectite group and a major component of bentonite.
Quick clay is a unique type of marine clay, indigenous to the glaciated terrains of Norway, Canada, and Sweden. It is a highly sensitive clay, prone to liquefaction which has been involved in several deadly landslides.
- Clay (industrial plasticine)
- Clay court
- Clay minerals
- Clay pit
- Grain size
- List of minerals
- London clay
- Modelling clay
- Graham Cairns-Smith, proposer of the "clay theory" for abiogenesis
- Clay mineral nomenclature American Mineralogist.
- WHO (2005), Bentonite, kaolin, and selected clay minerals, number 231 in ‘Environmental Health Criteria’, WHO. Available from: http://www.who.int/entity/ipcs/publications/ehc/ehc231.pdf
- Historical information about the clays of North Staffordshire, UK
- Information about clays used in the UK pottery industry