Secret techniques of Chinese porcelain

Source:Global Times Published: 2010-7-20 15:02:00

Did you know that Chinese ceramics were once considered high-tech? For several ages, China was the only civilization that knew how to make porcelain. Ceramics are generally made based on three basic elements: clay, glaze and heat. Generally speaking, they are formed into shapes by hand, using molds, or by turning on a potter's wheel.

The whole process of making Chinese porcelain:

1. Forming: The relatively low plasticity of the material used for making porcelain make shaping the clay difficult. In the case of throwing on a potter’s wheel it can be seen as pulling clay upwards and outwards into a required shape and potters often speak of pulling when forming a piece on a wheel, but the term is misleading;

Clay in a plastic condition cannot be pulled without breaking. The process of throwing is in fact one of remarkable complexity. To the casual observer, throwing carried out by an expert potter appears to be a graceful and almost effortlessly, but this masks the fact that a rotating mass of clay possesses energy and momentum in an abundance that will, given the slightest mishandling, rapidly cause the workpiece to become uncontrollable.

2. Glazing: Unlike their lower-fired counterparts, porcelain wares do not need glazing to render them impermeable to liquids and for the majority of the time are glazed for decorative purposes and to make them resistant to dirt and staining. Many types of glaze, such as the iron-containing glaze used on the celadon wares of Longquan produced in Zhejiang Province, were designed specifically for their striking effects on porcelain.

3. Decoration: Porcelain wares may be decorated under the glaze using pigments that include cobalt and copper or over the glaze using colored enamels. Like many earlier wares, modern porcelains are often bisque-fired at around 1,000℃, coated with glaze and then sent for a second glaze-firing at a temperature of about 1,300℃ or higher. Another early method is once-fired where the glaze is applied to the unfired body and the two fired together in a single operation.

4. Firing: In this process, green (unfired) ceramic wares are heated to high temperatures in a kiln to permanently set their shapes. Porcelain is fired at a higher temperature than earthenware so that the body can vitrify and become non-porous.


Key elements involved in the making process:

1. Clay


Clay is basically rock that has been underground for thousands of years. The clay used to make fine porcelain is called kaolin, from the Gaoling Mountains of southeastern China, where it was first mined. Chinese potters mixed the kaolin clay with a powder from a stone called baidunzi, a rock that contains feldspar, a glassy mineral.

Kilns are ovens that are specially made for firing ceramics. In the very hot kilns, the stone melts and makes the clay hard and shiny like glass. It is different from sand or ordinary mud because it holds its shape it is heated in a kiln.

Clay, although common, is not found everywhere, and ceramics industries are often set up where this key raw material is available. Because heat is also needed, there is another crucial raw material: fuel for firing the kiln.

After cleaning and preparing the clay, potters mix it with water to make it easier to handle. Clay vessels (pots, dishes, vases, and other containers) and sculptures are formed in several ways: Building by hand (pinching, coiling, and slab-building); Shaping on a potter's wheel; Forming in molds. Chinese tomb sculptures were formed in molds and then painted or glazed. The other ceramics were formed on potter's wheels. A lump of clay is thrown on a turning wheel and shaped with the hands as the wheel spins.


2. Glaze

Glaze acts as the skin of the pot, making it waterproof. It often feels like glass when you touch it.

Glaze is made up of four main ingredients: Clay, often the same clay as the object; Glassy minerals, often silica found in sand, melt to make the body hard; A flux, a mineral such as feldspar or calcium that allows the glaze to melt at a lower temperature; Minerals to add color such as cobalt (blue) or manganese (purple).

The potter mixes the glaze ingredients with water and then applies them to the vessel or sculpture, sometimes with a brush or by spraying, pouring, or dipping the object into the glaze. There are many different glaze recipes, and Chinese potters sometimes keep their formulas a secret.


3. Heat

To become usable ceramics, clay objects have to be fired in kilns, in the same way that bread dough is baked in an oven to become bread.

The water must be removed from the wet clay and the clay particles must melt together for the pot to harden and keep its shape. Over the centuries, pots have been fired in kilns varying from simple bonfires to long kilns that climb up the side of hills.

Kilns usually have three main sections: A firebox (containing the fuel); A firing chamber (containing the pots) and a chimney.

Heat moves from the firebox through the firing chamber and up through the chimney. Traditional Chinese potters used wood and coal as fuel, but today electric kilns are increasingly popular.

Different clays require different firing temperatures and firing times-a clay vessel that has not dried completely before firing can sometimes explode! The colors of the glazes can vary according to the amount of oxygen in the kiln during firing. So, the potter watches the kiln very carefully during firing. After firing, the pots are left to cool before they are taken out.,

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