Porcelain and ceramics
Porcelain originates from China and first appeared during the Tang Dynasty
(618 – 906 A.D.) Porcelain was developed from stoneware. Unlike stoneware,
porcelain requires very high combustion temperatures (starting at 1,200 °C).
Hence, porcelain was invented so late, as, in order to produce high
combustion temperatures, the fire had to be brought under control, which was
very difficult to accomplish with primitive means. Chinese master craftsmen of
pottery were the first who succeeded in doing this.
Components
The components of porcelain are the mineral kaolin
(a fire-resistant decomposition product of feldspar), feldspar and
quartz.
Processing
The components of porcelain (kaolin approx. 40 –
60 %, feldspar approx. 20 – 30 % and quartz approx. 20 – 40%) must be carefully
processed in order to achieve optimum results.
Thus, the raw materials must
be carefully freed of dirt. Iron, copper, nickel and
manganese must be
removed in order to obtain the pure white colour.
The components are
processed to form a mass by adding water. This then has to rest for a while (up
to 2 years).Only then the mass has the right consistency to be shaped by hand,
turned on a wheel or poured into a mould. The drying process follows
this.
The first firing (biscuit firing in up to 900 °C) removes the water
from the porcelain and gives it the stability required for further
processing.
After the first firing, the clay is still porous and
absorbent. Now it is dipped into the glaze which has a similar chemical
composition to that of the porcelain mass. The porcelain treated in this way is
now exposed to the glost firing at a
temperature of approx. 1,400 °C. The
glaze then becomes permanently attached to the clay; a beautiful, smooth surface
is created that is not only
very hard, but also immune to attacks by acids,
with the exception of fluoric acid. In order to give the porcelain elaborate
decorations, the merchandise can now be painted or provided with a transfer
picture.
The now following third firing, the so-called over glaze firing,
seals the decoration onto the porcelain.
Bone China
A further type of porcelain is the
so-called bone china, which has its origin in England. Its mass is made of up to
50 % by bone ash as well as parts of kaolin, feldspar and quartz. Thanks to
adding bone ash, the porcelain receives a soft, warm glow. It is very
translucent and has a pleasant touch to it. It is beige in colour.