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What clay is

Clay minerals are hydrous aluminium phyllosilicates, built of aluminium and silicon ions bonded into tiny, thin plates by oxygen and hydroxide ions. Clays develop plasticity when wet but can be hardened through firing. 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 iron oxide.

Clay minerals

The main groups of clays include kaolinite, montmorillonite-smectite and illite. Kaolinite is a soft, earthy, usually white mineral produced by the chemical weathering of aluminium silicate minerals like feldspar. In many parts of the world, kaolin is coloured pink-orange-red by iron oxide, giving it a distinct rust hue. Bentonite is an absorbent swelling clay consisting mostly of montmorillonite. The montmorillonite in bentonite takes the form of microscopic platy grains, which give the clay a very large total surface area and make bentonite a valuable adsorbent. Illite is a group of closely related non-expanding clay minerals.

Terracotta

Terracotta is a clay-based non-vitreous ceramic fired at relatively low temperatures. The typical firing temperature is around 1,000 °C. During firing, the iron oxides in the body react with oxygen, often giving the reddish colour known as terracotta.

The smell of earth

Geosmin is produced by bacteria, especially cyanobacteria, often present in soil and water supplies. The main genus in the Actinomyces that produces geosmin is Streptomyces. Geosmin contributes to petrichor, the strong scent that occurs when rain falls after a spell of dry weather or when soil is disturbed. Petrichor is the earthy scent often associated with rain falling on dry soil or rock.

History

Clay is the longest-known ceramic raw material. Prehistoric humans discovered the useful properties of clay and used it for making pottery. Some of the earliest pottery shards have been dated to around 14,000 BCE.

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