Porcelain
Its origins
Porcelain
has its origins in China where it first appeared during the Tung
Dynasty (618-906 A.D.). Porcelain is a further development of
stoneware. As opposed to stoneware, porcelain requires very high firing
temperatures (from 1,200°C). Hence, the discovery of porcelain occured
quite late. In order to create firing temperatures this high, the fire
had to be brought under control which was very difficult to be achieved
with the primitive methods used during that time period. The Chinese
pottery masters were the first to do this.
The path
The term porcelain has its origins in the Latin language, it means
piglet and was a slang term for a special type of escargot. Indeed, the
smooth, beaming surface of this sea creature is a very refined entity
of finest porcelain. The word porcelain (porcellana) was first used by
Marco Polo, the famous traveller. He undertook a mercantile expedition
to China in the year 1271 and lived there for approximately 20 years as
a servant to Kublai Khan (one of the grandsons of Dschingis Khan) and
could tell truly legendary stories of the Middle Kingdom. Rarely did it
happend that porcelain wares came to Europe from China during this time
period. The delicate material was often marvelled at and people tried
to copy it, however, quite some time had to pass until the secret of
its processing was discovered. The Portugese Vasco da Gama was the
first who developed the sea way to India around Africa in 1498.
Innumberable possibilities of trade were the result. Trading companies,
such as the East Indian Company, were founded and more and more
rarities from the Far East conquered Europe. Hence, it came into
fashion to take a liking for the new drinks such as tea, coffee and
cocoa. Of course, also porcelain as a rare treasure was amongst these
new fashions. Porcelain trade flourished. Especially the European
princes were delighted by the material. First and foremost August the
Strong, electoral prince of Saxony and King of Poland. He employed the
physicist and mathematician Ehrenfried Walther von Tschirnhaus of
Dresden and, later, the Alchemist (which was, during these times, a
serious profession) and pharmacist Johann Friedrich Böttger. Von
Tschirnhaus got to know the French man Velette who was the inventor of
the burning mirror. He immediately realised the possibility to create
very high firing temperatures with this device. However, Vellette did
not want to disclose the function of his invention. Von Tschirnau saw
himself forced to experiment and was successful with lenses made of
glass. He succeeded in producing a porcelain-like material which he
called wax porcelain. In the light of these events, August the Strong
ordered von Tschirhaus and Böttger to work together. After many
experiments and failures a first success could be announced in November
1707. The red stoneware or "Jaspis-porcelain" consisted of clay from
Zwickau and easily melting clay from Aue. It was very durable and the
colour ranged from beige to dark brown. The colour red-brown was very
popular, because it reminded of the Yi-hsien stoneware from close to
Shanghai which was mainly used in teapots. In the year 1708, Böttger
succeeded in producing the first white hard porcelain with Colditzer
clay and feldspar. On March 28th in 1709 he officially declared the
invention of hard procelain which is entirely imporous and retains its
form even under very high temperatures.
Its components
The components of porcelain are the mineral kaolin (a fire-resistant
decomposite of feldspar), feldspar and quartz. The name kaolin comes
from the name of the Kao ling mountains in the North East of the
Chinese Province Ching-te-chen. The characteristic, crystalline
component of porcelain is mellit which is a decomposite of kaolin. The
higher the amount of mellit which is produced, the better the technical
characteristics such as durability and chemical resistance of the
porcelain. The transparency, however, declines. Kaolin can be deformed
plastically. The plasticity is originally in the form of the kaolin
minerals. When chemical bonds are introduced, the plasticity is
increased. One possibility to do this is the urea. The very
thinly-walled Chinese and Japanese Egg-shell porcelain can only be
produced if the kaolin is stored in and processed with urea.
Its processing
The
components of procelain (kaolin ca. 40 – 60%, feldspar ca. 20 – 30% and
quartz ca. 20 – 40%) need to be processed carefully in order to receive
an optimal result. The raw materials need to be thoroughly freed from
impurities. Iron, copper, nickel and mangan need to be eliminated in
order to receive the desired white coulour. The components (the
specific proportions are carefully kept secrets of the manufacturers)
are processed with water to a mass. This then has to rest for a while
(up to 2 years) until it has reached the right consistency to be formed
by hand, on a disc or to be filled into a form. Afterwards, the drying
process starts.
The European porcelain contains two firings, as opposed to the Chinese porcelain.
The
first firing (ca. 900° C) withdraws the water from the porcelain and
gives it its necessary stability for the following processing steps.
The shard is still porous and absorbent after the first firing. Now it
is dipped into the glaze which has a similar chemical composition like
the porcelain. The so-treated porcelain is now fired with 1,400°C.
During this firing process, the glaze is irreversibly combined with the
porcelain and a very nice, smooth, white surface is created which is
not only very durable, but is also not harmed by acids, except by
hydrofluoric acid. In order to add sumptuous decorations, the product
can now be painted or decorated with a transfer picture. Now the third
firing takes place, which binds the decoration to the porcelain. As
opposed to European porcelain, Chinese porcelain only receives one
firing because the glaze and the decoration are already applied after
the drying and before the first firing. Glazes consist of kaolin,
quartz, chamotte and chalk. In order to get coloured glazes,
metal-oxides are added, such as chrome, nickel, cobalt, iron-oxide.
Only few oxides sustain the high temperatures and are suitable. Another
type of porcelain is the so-called Bone China which has its origin in
England. Its material consists of up to 50% of kinebone ash and also
kaolin, feldspar and quartz. When kinebone ash is added, this porcelain
receives a soft, warm shimmer. It is very transparent, soft in the hand
and has a pleasantly warm, beige colour.