Clay
:
Clay is a term used to describe a group of hydrous
aluminium phyllosilicates 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 high
plasticity index.
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.
Montmorillonite, with a chemical formula of (
Na,
Ca)
0.33(
Al,
Mg)
2Si4O10(
OH)
2·n
H2O, 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.
Varve (or
varved clay) is clay with visible annual layers, formed by seasonal differences in erosion and organic content. This type of deposit is common in former glacial lakes from the
ice age.
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 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 hardened. A fireplace or oven specifically designed for hardening clay is called a
kiln. These properties make clay an ideal substance for making durable
pottery items, both practical and decorative, with different types of clay and firing conditions giving
earthenware,
stoneware and
porcelain. 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.
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,
spark plug bodies, and even musical instruments such as the
ocarina are all made with clay. Clay is also used in many industrial processes, such as
paper making,
cement production,
pottery, and chemical
filtering.
*
Ceramic*
Clay pit*
Clay minerals*
Grain size*
List of minerals*
Plasticine*
Clay (industrial plasticine)*
Pottery*
Clay court*
London clay*
Modelling clay*
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
* Clay used as building material.
Building with clay
