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The Ancient World

Tile-making evolved from primitive pottery manufacture, and the earliest architectural sites give evidence of the use of tiles. As soon as the art of glazing was discovered, it became possible to use the thin slabs of hard-burned clay, decorated in colors, as a decorative adjunct to architecture. This aesthetic use of tiles as a facing for walls distinguishes them from other ceramic products, such as brick, terra-cotta, and roofing units, which are essentially structural. Colored glazed tiles dated from 4700 B.C. have been found in Egypt .

Ancient ceramics were perfected in Mesopotamia . Large wall surfaces were faced with bas-relief decorations executed in enameled tiles resembling modern bricks in shape, most notably at the palace at Khorsabad (722–705 B.C. ) in Assyria, near ancient Nineveh, and the Ishtar Gate (c.7th cent. B.C. ) in Babylon . From these regions ancient Persia acquired ceramic techniques for the fine bas-reliefs of animals and archers in the palaces of Susa and Persepolis (5th cent. B.C. ).

The earliest tile sewer pipes are those excavated at Crete (c.1800 B.C. ). The Greeks also employed tile drains and conduits as well as tiles for roofing. Their architectural ceramics were mostly confined to cornices and cornice adornments and are customarily classed as terra-cotta. The Romans made wide use of floor tiles of various shapes and of floor mosaics, as well as a variety of wall tiles, including a type similar to modern hollow tiles, which were used in bathing establishments for the passage of warm air and smoke and as insulation. Roman tiles received no colored or glazed decoration.

Clay and Pottery, an ancient art

Story by Phyllis McKee

Since the first piece of clay fell into a fire and was transformed into a glasslike material, people have used clay for domestic wares, ritual tokens, and decorative items. The oldest known pottery fragments stem from the Hittite civilization, 1400-1200 B.C.

Where Clay Comes From

Clay comes from the ground, usually in areas where streams or rivers once flowed. It is made from minerals, plant life, and animals—all the ingredients of soil. Over time, water pressure breaks up the remains of flora, fauna, and minerals, pulverizing them into fine particles. Larger particles are filtered out through rocks and sand, leaving silt to settle into beds of clay. How far silt travels from its source and how pure the silt is determines the type of clay it becomes.

Kinds of Clay

The three most common types of clay are earthenware , stoneware , and kaolin . Earthenware , or common clay, contains many minerals, such as iron oxide (rust), and in its raw state may contain some sand or small bits of rock. Earthenware is a secondary clay that has been transported by moving water some distance, picking up minerals and other materials before settling in a river bed. Because of its many impurities, earthenware melts at a cooler temperature than other clays. Called a low-fire clay, earthenware fires (or bakes) in a temperature range of 1700 to 2100?F (926–1150?C). After firing, it is still porous and—unless glazed—is often white or gray. Earthenware is commonly used in the making of terra cotta pots, roofing tiles, and other low-fire ware.

Stoneware is a hard and durable clay that is fired to temperatures between 2100 and 2300?F (1205–1260?C). Its natural colors vary from light gray or tan to dark gray or chocolaty brown. Historically stoneware was used for crocks and jugs and is now typically used to make dinnerware.

The purest clay is kaolin , or china clay . Called a primary clay because it is found very near its source, kaolin has few impurities and is the main ingredient used in making porcelain. Because its particle size is larger than other clays, it is not very plastic . This means that in a moist unfired state, kaolin tears when it is bent. Kaolin is a high-fire clay, needing heat from 2335 to 2550?F (1280-1400?C), to vitrify . Fired porcelain can become very hard and translucent, its melted surface becoming so smooth and shiny that a glaze is not needed.

Techniques for Making Pottery

The earliest method for making bowls and jugs is handbuilding —using only hands and clay. This approach is still used today. A ball of clay is pinched or pressed to form a bowl. Or the clay is rolled into ropes or coils that are then wrapped in upward circles until the desired height is reached. The coils are then often smoothed so that they are no longer distinct.

Another handbuilding method is slab-building. A large ball of clay is flattened into a pancake-like slab. The slab is then cut into rectangles, which are attached together by moist clay, making the sides of a clay box. To make a bowl, the entire slab is placed over a round mold.

Around 5000 B.C. the potter's wheel was invented, probably by the Sumerians of the Tigris-Euphrates basin or by the Chinese . The potter's wheel allows the potter to throw even, symmetrical shapes in much less time and with far less effort. They are thought to have been in operation even before wheels were used for transportation.

There are many different types of potter's wheels in use. Some are powered by hands or feet, which spin the platter on which the clay sits. Others use treadles, like the foot pumps seen on old sewing machines. Many wheels are turned by electricity.

And Now, the Fire

After a piece of pottery has been formed and dried completely, it must be fired to achieve permanency. Without the chemical transformation that occurs through firing, an uncooked bowl dissolves back into mud once it comes in contact with water.

Giant bottle kilns fueled by coal were commonly used to fire clay in the 18th and 19th centuries, and the first half of the 20th century, and have been replaced by cleaner fuels like natural or propane gas, wood, and electricity.

Some people fire their work in pits in the ground. They layer the pots with hay, seaweed, or other combustibles. The fire burns from top to bottom over the course of a day or two. The chemicals in the hay, or other chemicals that are added to the fire, are absorbed into the pot and create colors in the clay.

Wood-fired Pottery

The Japanese have created clay art for thousands of years. They are famous for their wood-fired pottery, fired in an anagama (a single-chamber, tunnel-shaped kiln) or a noborigama (a multi-chamber kiln). The use of these wood-fired kilns has spread worldwide.

These specialized wood firings can take up to a week to complete. The fire is started with tiny pieces of wood and the kiln is stoked every five minutes. When the kiln becomes hot, large pieces of pottery are added at regular intervals. The fire is kept burning 24 hours a day for several days until the clay has matured. The kiln is left to cool for several more days—if it is opened too soon, the pots will crack and break. Because it is so labor-intensive, potters who use these kilns often fire only once a year. They save up an entire year's work, perhaps hundreds of pots, for one firing.

Most wood-fired pottery doesn't have a glaze. As the fire gets hotter, drafts pull wood ash through the kiln where it is deposited on the pots. The pots are so hot from the flames (they glow red like charcoals in a barbecue) that the ash melts on the clay and creates its own glaze. The patterns produced are unpredictable.

Porcelain

porcelain [Ital. porcellana ], white, hard, permanent, nonporous pottery having translucence which is resonant when struck. Porcelain was first made by the Chinese to withstand the great heat generated in certain parts of their kilns. The two natural substances used were kaolin, also known as china clay, a white clay free of impurities that melts only at very high temperature, and a feldspar mineral called petuntse that forms a glassy cement, binding the vessel permanently. Although proto-porcelain wares exist dating from the Shang, by the Eastern Han high firing glazed ceramic wares had developed into porcelain, and porcelain manufactured during the T'ang period (618–906) was exported to the Islamic world where it was highly prized. The ware was refined during the Sung period (960–1279). During the Yuan period (1280–1368), blue and white ware was produced by utilizing cobalt blue from the Middle East . The Ming period (1368–1644) developed this blue and white ware but used other colors as well. The Ch'ing period (1644–1912) designed porcelain especially for export often utilizing Western designs. In Europe porcelain was first commercially produced (1710) in Meissen , Germany . Most of the European porcelain is soft paste (made from clay and an artificial compound such as ground glass) and is not as strong as the Chinese hard-paste porcelain. Important European centers for porcelain are Bow, Chelsea , Worcester , Staffordshire, Vienna , Meissen , S?vres, Limoges , and Rouen .

There are three types of porcelain

  • Bone China
  • Hard-paste
  • Soft-paste

Bone China

bone china, variety of porcelain developed by English potters in the last half of the 18th and early 19th cent. The clay is tempered with phosphate of lime or bone ash. This innovation greatly increased the strength of the porcelain during and after firing.

  • Stronger than hard-paste porcelain and easier to manufacture.
  • Its ivory white appearance is created by adding bone ash to the ingredients for hard-paste porcelain.
  • Bone China is a hybrid hard-paste porcelain containing bone ash.

The initial development of bone china is attributed to Josiah Spode , who introduced it around 1800. The original basic formula of six parts bone ash, four parts china stone, and three and a half parts china clay remains the standard English body.

Hard porcelain is strong but chips fairly easily and, unless specially treated, is usually tinged with blue or gray. Bone china is easier to manufacture, is strong, does not chip easily, and has an ivory-white appearance.

Very soon the bone china was copies by Minton , Coalport , Davenport , Derby , Worcester , and the Herculaneum factory at Liverpool .

Later on it was used by New Hall in 1810, Wedgwood in 1812, and Rockingham in 1820.

The quality, as much as form and decoration, varied from factory to factory; some tended, after about 1820, toward brilliant colour, lavish gilding, and overcrowded design; others produced tasteful, simply ornamented tableware. Since much early bone china was issued unmarked, it is often difficult to attribute the pieces.

  • Bone china is extremely hard, intensely white and will allow light to pass through it.
  • Strength is provided by the fusion of body ingredients during firing. This unique English pottery body is made from the following: 50% animal bone , 25% china clay , 25% china stone . First or biscuit firing 1200 C - 1300 C. Second or glost firing 1050 C - 1100 C.
  • Porcelaneous ware was first made in China , hence its common name china. Chinese porcelain is less vitrified (and therefore softer) than its modern European counterpart, which was developed in Germany in the early 18th century.
  • Josiah Spode II (1754-1827) introduced in his new bone china pottery in c1797. This was to prove the English solution to the quest for porcelain. Technically bone china is a form of hard paste porcelain because it is a mixture of clay and another non-glassy material. The standard formula is 25% china clay, 25% Cornish stone, 50% bone ash. Bone china became the English porcelain because - It is less liable to loss in firing than soft paste porcelains which contain glass. The firing temperature is much lower (1250? C) than for hard paste porcelain (1400? C). The potters could use their existing methods and ovens. The brilliance of enamel colours and gold was greater than on other porcelains.
  • It very quickly became a popular body for several reasons - The diminishing trade with China caused by very heavy import duties on porcelain (108% in 1799). Less merchant shipping available because of the need to sustain naval and military forces overseas. The patronage of the Prince of Wales, leader of taste at the beginning of the nineteenth century. The growth of the professional and merchant classes provided a market. It was easy for existing factories to convert to making bone china because the sequence of processes as well as the glost and enamel firing temperatures were the same as earthenware. Bone china is more difficult to use in a plastic state that earthenwares, but quicker to cast with than earthenwares.

Calcined bone ash:

Calcined bone ash is used in the production of bone china and makes up about 50% by weight of the final body recipe. It is produced from animal bone, which is first processed to remove any adhering meat which is generally sold as pet food. The bone is then treated to remove glue, which is processed and upgraded for use in normal applications where glue is used, and also for the sizing of expensive paper. The raw bone which is left after the meat and glue have been extracted is then heated to about 1000 C at which temperature any residual organic material is burned off and the structure of the bone is changed to form suitable for the manufacture of bone china. The high temperature used also sterilises the bone. Prior to use the bone is finely ground with water before inclusion in the bone china body and it is calcined bone which gives traditional English bone china its translucency and whiteness.

Hard-paste Porcelain

Hard-paste porcelain is made from a mixture of china clay (kaolin) and china stone (petuntse). The use of china stone dispenses with the need for the ' frit ' used in soft-paste porcelain.

The strength and whiteness of the porcelain was improved by ageing the paste in store

  • This type of porcelain often has a grey appearance and is extremely hard, it is fired at a much higher temperature than soft-paste porcelain. The ingredients melt and fuse into a dense strong body . It will allow bright light to pass through it. Colours lie on top of the glaze .
  • Hard-paste porcelain recipe: 50% china clay , 30% china stone , 20% flint . Firing: Biscuit temperature 900 C - 1000 C. Glost firing 1350 C - 1400 C.
  • Porcelaneous ware was first made in China , hence its common name china. Chinese porcelain is less vitrified (and therefore softer) than its modern European counterpart, which was developed in Germany in the early 18th century.

Soft-paste Porcelain

First produced in Europe in 1738.

Soft-paste porcelain is produced by mixing white clay with ' frit ' - a glassy substance that was a mixture of white sand, gypsum, soda, salt, alum and nitre.

Lime and chalk were used to fuse the white clay and the frit, the mixture is then fired at a lower temperature than hard-paste porcelain.

  • Soft-paste porcelain is soft and the body is granular since the ingredients do not melt together.
  • The glaze , translucent layer that coats pottery to give the surface a finish or afford a ground for decorative painting. Glazes—transparent, white, or colored—are fired on the clay. Of the various artificial mixtures used for glazes, that for whiteware contains borax and lead, whereas a salt glaze is used for stoneware. No lead is used for porcelain. The coloring agents are oxides of different metals. In the 16th and 17th cent. glazes were also used in painting to enhance the luminosity of oil or tempera colors. Titian and Rembrandt were especially adept at glazing techniques. It is clear and thick and sometimes gathers into pools. The enamel colors sink into the glaze.
  • Glassy porcelain has no standard recipe but the body is made from - Glass , China stone , other ingredients. Firing temperatures: biscuit 1200 C - 1300 C, glost 1050 C - 1150 C
  • Porcelaneous ware was first made in China , hence its common name china. Chinese porcelain is less vitrified (and therefore softer) than its modern European counterpart, which was developed in Germany in the early 18th century.
 

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