Did you know that Chinese ceramics were once considered
high-tech? For many years, China was the only civilization
that knew how to make porcelain. Through years of
research and experimentation, the Chinese developed
secret techniques that no one else knew about. People
from all over the world wanted these wonderful treasures,
and the Chinese were very eager to sell them.
How Are Ceramics Made?
Ceramics are generally made from three basic elements:
Clay + Glaze + Heat
They are formed into shapes by hand, using molds, or by turning on a potter's wheel.
Clay is basically rock that has been
ground down over thousands of years. It is different
from sand or ordinary mud because it holds its shape
and keeps shape when it is heated (or fired) in
a kiln. Kilns are ovens that are specially made
for firing ceramics.
Clay, although common, is not found everywhere,
and ceramics industries often grow 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 turns.
The clay used to make fine porcelain china is
called kaolin, from the Gaoling Mountains
of southeastern China, where it was first mined.
Chinese potters mixed the kaolin clay with a powder
ground from a stone called baidunzi, a rock
that contains feldspar, a glassy mineral. In the
very hot kilns, the stone melts and makes the clay
hard and shiny like glass.
Glaze acts as the skin of the pot,
making it waterproof and providing decoration. 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,
that melt to make the body hard,
- a flux, a mineral such as feldspar or
calcium that allows the glaze to melt at a lower
- 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
kept their formulas very secret.
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.