Clays exhibit plasticity when mixed with water in certain proportions. When dry, clay becomes firm and when fired in a kiln, permanent physical and chemical changes occur. These reactions, among other changes, cause the clay to be converted into a ceramic material.

800px-Gay head cliffs MVBecause of these properties, clay is used for making pottery items, both utilitarian and decorative, and construction products, such as bricks, wall and floor tiles. Different types of clay, when used with different minerals and firing conditions, are used to produce earthenware, stoneware, and porcelain. Prehistoric humans discovered the useful properties of clay. Some of the earliest pottery shards recovered are from central Honshu, Japan. They are associated with the Jomon culture and the deposits from which they were recovered have been radiocarbon dated to around 14000 BC. Depending on the content of the soil, clay can appear in various colors, from a dull gray to a deep orange-red.

Clay minerals are typically formed over long periods of time by the gradual chemical weathering of rocks, usually silicate-bearing, by low concentrations of carbonic acid and other diluted solvents. These solvents, usually acidic, migartigiano lavora argillarate through the weathering rock after leaching through upper weathered layers. In addition to the weathering process, some clay minerals are formed by hydrothermal activity. Clay deposits may be formed in place as residual deposits in soil, but thick deposits usually are formed as the result of a secondary sedimentary deposition process after they have been eroded and transported from their original location of formation. Clays sintered in fire were the first form of ceramic. 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. Clay is also often used in the manufacture of pipes for smoking tobacco. Until the late 20th century bentonite clay was widely used as a mold binder in the manufacture of sand castings. Clay, being relatively impermeable to water, is also used where natural seals are needed, such as in the cores of dams, or as a barrier in landfills against toxic seepage.argilla

Pottery is our oldest handicraft. In prehistoric times, most likely water was carried in woven baskets lined with river clay. After the water was poured out of the container the layer of clay dried. The loss of moisture caused the shape to shrink and separate from the sides of the basket. When the clay, now shaped like a pot, was removed, and dried in the sun on hot sand, it retained the basket pattern. Early men and women then discovered that they could harden the molded pottery in hot ashes and make sturdy containers to transport and store foods tuffs. From these would have been extended the pots formed by hand and decorated with crude tools. From a very early date in history, some say at least 400 B. C., earthenware pottery was produced on a mass scale by a potter's wheel in many parts of the world.

The Egyptians made kilns to place their clay pots in for firing. The kiln was lined with a kind of insulation brick that was made from a mixture of straw and clay which had been dried in the sun. Later, the ancient Egyptians used a finer clay with a high quartz content for their delicate pottery. They rubbed the pieces with a smooth stone to give the a dull sheen or coated them with a fine layer of another color of clay.

135Further experimentation lead the Egyptians to coat their clay objects with a bluish-green substance to make them non-porous. This was a glaze composed of quartz, soda, and a mineral containing copper which when fired covered the clay bowls and vases with a glass-like surface. Ancient Greek vases are highly valued for form and decoration. The graceful lines an perfect balance speak to our desire for beauty. The pottery was decorated with pictures of the daily lives of the people and stories of their gods, goddesses and heroes. On the red figure vases the background was painted black and the figures were left the natural red color of the clay. The color was reversed on the black-figured vases.

In medieval times sand was mixed with clay to make cooking pots strong enough to be placed over an open fire. Today, for the same reason, casseroles used for baking are made from clay mixed with grog which is a ground-up fired pottery. The openness of grog clay allows water to evaporahomete more evenly as it dries and prevents cracking and warping during the firing. Grog clay eases the problem of heat expansion which can cause large thick pieces of pottery or sculpture to blow up in the kiln. Around the middle of the thirteenth century German potters started to produce stoneware. This pottery was made form finer clays and fired at a higher temperature than earthenware. Stoneware was tan or gray in color, strong and naturally non-porous. Light, transparent porcelain was first produced in China. Porcelain was made from a very plastic and pure clay called kaolin mixed with felspar. The colorful decoration of the porcelain was accomplished by firing each color individually after it was applied. These delicate china dishes and figurines were in demand all over Europe. In their efforts to unravel the secret of the composition of the Chinese porcelain, European and other Asian potters developed many variations in their glazing techniques. Rakuware is another type of pottery of special interest. The crackled glaze of raku originated in Japan where tea bowls were modeled by hand from a very coarse clay.

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