Ceramics...What are They? 

Although technology has somewhat obscured the boundaries of the word "ceramics," it is still reasonable to adopt the more classical definition that encompasses the three types of kiln fired clay bodies: earthenware (pottery); stoneware; and kaolin (porcelain). It is the transformation that occurs within the kiln that provides the beauty and mystery and emotional draw of such items. Clay has been used in bricks and pottery for millennia. When I hold a piece of porcelain or pottery in my hand, I stand at the portal of an old and practiced craft. It weaves its spell through the senses. It is tactile. This is stuff of earth and of generations. It has history and family and tradition. -- Is it the warmth and even the smell of the clay in a pottery form? the richness of its glaze? Is it the bell like ring of the porcelain? or is it the warm translucence that entrances? In truth, it is all of this and more. Let me take you on a short walk through Ceramics 101 if you will.

Porcelain is a hard, white, translucent ware made by firing a compound containing white clays, especially kaolin clay, with fluxing agents at high kiln temperatures 2335 to 2550°F (1280-1400°C and then glazing it (or not) with variously colored fusible materials. There are three types of porcelain: hard-paste, soft-paste and bone china.

Pottery, by contrast, is formulated with less pure clays and is fired at lower temperatures 1700 to 2100°F (926–1150°C) resulting in a less durable opaque ware that is porous. It can only be made waterproof by application of a glaze coat.

Stoneware is intermediate in character to porcelain and pottery. Although it is fired at high kiln temperatures 2100 and 2300°F (1205–1260°C) making it a hard and durable clay, its natural colors vary from light gray or tan to dark gray or chocolaty brown. It can be translucent if thin enough, but often has more utilitarian uses than porcelain.

As an aside, please note that cold cast resin is an opaque material that has not been fired in a kiln. This material is a strictly contemporary formulation that will not be further considered here. 

Clay is a natural, fine-grained earthlike material, the product of the geological weathering or aging of the surface of the earth. The root of the word "clay" is from Latin and Old English words meaning "to stick," and clay is a sticky soil that is plastic when moist but hard when fired. Chemically, clays are hydrous aluminum silicates, ordinarily containing impurities like iron, in small amounts. Clays are classified as either primary or secondary. Primary clays weather pretty much in place, making them less likely to pick up impurities. Secondary clays are transported some distance (e.g., by water) and pick up many minerals and other substances before finally becoming sedimentary

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, and its many impurities result in it melting 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. Another word for earthenware that is perhaps more commonly used in describing the finished product is "pottery."

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 (vitrified) surface becoming so smooth and shiny that a glaze is not needed.

But the kaolin or china clays have poor plasticity, so they are often used in conjunction with additives--usually ball clays (for stoneware) and petuntse (china stone, a granite derived feldspar) for porcelain. Keeping in mind that these represent only typical "recipes," note the proportions that differentiate stoneware from porcelain, from bone china (a type of porcelain).

So it is the ingredients or "recipe" and the temperature of the kiln that determine the properties of the finished ceramic item. Kilns used to be wood fired, so even with a good recipe, it used to be difficult to control the final outcome of the firing. Of the three ceramic products--porcelain, stoneware, and pottery, one must consider the results of the recipe, much like a favorite cookie or a fine wine.

Pottery may have originated in Japan from a site known as Odai Yamomoto, in the north of Honshu, the main island of Japan, where shards have been found that date back to 13,000 years ago. Using the latest radiocarbon calibration gives a date of 16,000 years ago. (or 14,000 BC), but it is considered to be in the Heian period (794-1191 BC) that potters first began to glaze pottery in today's sense and to fire pottery at considerably high temperatures. Stoneware in contrast dates to the Shang dynasty in China around 3400 years ago. It is believed that porcelain was first made by Chinese potters toward the end of the Han period (206 BC-AD 220) when pottery generally became more refined in body, form, and decoration. The Chinese made early vitreous wares (protoporcelain) before they developed their white vitreous ware (true porcelain) that was later so much admired by Europeans. Europeans began importing Chinese porcelain in the 14th century, but it was not until the early 18th century that they were able to reproduce its much-prized hardness, whiteness, and translucency for themselves.

The clay content of the three types of porcelain--soft-paste, hard-paste, and bone china--is kaolin, but the fluxes [melting agents] differ.  Recall the "recipes" mentioned earlier.

The first European soft-paste or frit porcelain [a mixture of fine clay and glass-like flux substance fired at about 2,200 F, or 1,200 C] was produced in Florence, Italy, about 1576. Soft-paste porcelain is fired at lower temperatures and does not completely vitrify--that is, it remains somewhat porous. Breaking a piece of soft-paste porcelain reveals a grainy body covered with a glassy layer of glaze. By the 1700s, porcelain manufactured in many parts of Europe was starting to compete with Chinese porcelain. France, Germany, Italy, and England became the major centers for European porcelain production.

The secret of true hard-paste porcelain, similar to the porcelain of China, was discovered in Europe by a German chemist named Johann Friedrich Bottger. This is no simple story and is steeped in intrigue. You should consider reading the details of this astonishing story from any good reference. The discovery of porcelain led to the establishment of a porcelain factory in Meissen in 1710. For nearly a century Meissen porcelain surpassed in quality all other hard-paste porcelain made in Europe.

The ingredients for the English bone china body were discovered around 1750. The addition of calcined bones to the hard-paste porcelain formula provided England with an extremely hard, intensely white and translucent porcelain. Although not as hard as true porcelain, it was more durable than soft-paste porcelain. The bone ash greatly increases the translucence of the porcelain. The firing temperature is much lower (1250º C) than for hard-paste porcelain (1400º C). England still produces nearly all the world's bone china and 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.

References:

http://www.he-and-she.com/english-china/china-clay.html
http://crownminerals.med.govt.nz/minerals/docs/comreports/report20_clays.pdf
http://www.thepotteries.org/types/porcelain.htm
http://www.wordiq.com/definition/Porcelain
http://www.potters.org/subject67803.htm
http://www.artlex.com/ArtLex/p/porcelain.html
http://en.wikipedia.org/wiki/Porcelain
http://62.1911encyclopedia.org/K/KA/KAOLIN.htm
http://www.alumnifriends.mines.edu/fun_stuff/kaolin_mining/
http://www.americanscientist.org/template/AssetDetail/assetid/34141/page/1
http://www.compulink.co.uk/~archaeology/world/stories/fareast/jomon/jomon.htm

Gladstone Pottery Museum stoke.gov.uk web page under rev; factmonster has the same clay and pottery article (try manual Google search).