Unappreciated Materials (4): Clay

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clay-carSome materials catch headlines, are held in awe, but not all.  Some get little respect, despite having changed the world.  They have become commonplace, anonymous, ignored and (particularly if they are cheap) cast aside when no longer wanted.  If they had feelings, they would be hurt. This brief series of blogs is to draw attention to their plight.

Stone, wood and clay are the oldest building materials on Earth.  Of these, clay is the most widely available. Two-thirds of the world’s population live in buildings made from clay in one guise or another: as brick, as adobe, as rammed earth or as wattle and daub.  Entire industries (brick-making, pottery, china, and porcelain) have grown up around it.  Few other materials are as versatile.

Clays are deposits of fine (<2 micron) particles of hydrated silicates typified by Kaolinite (Al2Si2O5(OH)4).   They form by the chemical weathering of siliceous rock, often carried down-stream and deposited on lake beds.  Wet clay is plastic and can be shaped by hand or by machine.  When dried it becomes rigid, with enough strength for gentle handling.  If it is fired at temperatures between 800 to 950 °C (low fire clays), 950 to 1,050 °C (earthenware), 1,100 to 1,200 °C (stoneware), and 1,200 to 1,400 °C (porcelain), the lower-melting silicates melt and bond those with higher melting points into a dense, strong ceramic body that can be decorated and made waterproof by subsequent glazing.  Versatile though it is, clay demands equipment and maintenance.  Kilns are expensive.  Clay must be kept wet and it must be “thrown”—pummeled before use”—it cracks if dried or fired too quickly and mess is involved.  Clearly, this is not easy to manage with children.

Modeling clay (Plasticine, Plastilin, Newplast) enables the creativity without the mess.  Replacing the water in clay by oils and waxes creates a material that is almost as moldable as clay but does not dry out, so it can be kept or modified or reused.  It softens when warmed, stiffens a bit when cooled, allowing control, and it is inert, non-toxic, and benign.  Like many inventions, modeling clay seems to have surfaced in several countries at about the same time (the “ripeness of time” effect).   One such was the brain-child of William Harbutt, an art teacher in Bath, England, who wanted a less troublesome material for his sculpture classes.  In 1897 he devised the still secret formulation of Plasticine.  Mr. Harbutt had business sense as well as artistic talents”—the company flourished until 1983, when it was sold to a foreign investor.

The creative potential of modeling clay continues to inspire engineers and artists. Plasticine is used in mold making for metal casting.  Industrial-designers use industrial-design grade modeling clay to mock up bodies for concept cars, and film-makers like Aardman use it for the characters of stop-motion animations.

Clay has a rich cultural history.  One of the earliest (4000 BCE) forms of written language, cuneiform, was incised in soft clay tablets, then fired to give “pages” so durable that they survive and can be read today.  And like most of our unappreciated materials, the word “clay” has acquired symbolic as well as literal meaning.  The Christian bible describes man as shaped from a lump of clay, making “clay” a synonym for human flesh and its weaknesses.  “Feet of clay” describes weakness of character, particularly in people of prominence such as politicians.  Fired clays have less derogatory associations: to describe someone as a “brick” used to mean that they were reliable, trustworthy; and a “porcelain complexion” is one of flawless, transparent beauty.

Clay is a material that in every way”—practical, economic and artistic”—deserves our respect.

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