3. Juni 2012 - New Ceramics

Forum
Terra sigillata bowl with its mould.
rheinzabern, 3rd century.
tible to faults during drying and firing
and dries faster, which is important for
mass production. These experiences of
the Roman potters are an example of
unknowing knowledge in ceramics. The
causes have only been discovered in our
times: the finest clay minerals form a
relatively large group named after the
US state of Illinois, where this mineral
was first described. Illites have so small
a particle size that even now their structure
has not been precisely determined.
They contain the flux potassium oxide in
a quantity that is just adequate to form
the silky sheen of a terra sigillata vitreous
slip that makes the body impervious
in the same way as a glaze. Experience
raced ahead of knowledge. This is typical
of the need of knowledge to play catch
up in ceramics. Potters had acquired this
experience, even if not in such detail, in
the 4th millennium B.C.E. Knowledge of
this technique was then lost, and it had
to be reinvented by the Greeks and Romans.
This led to the techniques of Greek
vase painting and Roman terra sigillata.
The managers of terra sigillata factories
would not have dreamt of exchanging
it for the risk of using a glaze. The Roman
Empire had to fall before glaze was
accepted as a normal part of ceramics,
thus heralding the advent of a new era
in ceramics.
Around 7,000 B.C.E. in the Near East,
Horseshoe kiln in northern China from 10th – 14th century.
From medley, m. “The Chinese Potter“. oxford: Phaidon 1976.
pots were fired in pits. The earliest pottery,
Jômon ceramics in Japan were
fired in open flames, similar to Native
American pottery. By 6,000 B.C.E. in
the Near East, pots were fired in proper
kilns, which means to say the ware and
the fuel were separated, which only happened
in China a thousand years later, in
the 5th – 4th millenniun B.C.E. during
the Yang-shao culture. The kilns were of
updraught design. Around 6,000 B.C.E in
Mesopotamia, 850-1,050°C were reached,
500 years later 1,150°C. With the Greeks
and Romans and up to the present, potters
usually fired to 850-1,000°C. Cross
draught kilns first appeared in China
around 100 C.E. In northern China, it
was the “horseshoe" kiln with a chimney,
in southern China the “dragon kiln,
a long, climbing kiln. The cross-draught
kiln was necessary for natural ash glazes
to be invented. The cross-draught kiln
was introduced in Western Europe by the
Slavs during the period of mass migration
in the Middle Ages. It was the precondition
for the invention of saltglaze, which
arrived earlier in Germany then in England
because of the culture transfer from
the east. In Mecklenburg, cross-draught
kilns have been excavated in two locations.
It was built on a rock wall and had
a firewall to stop the fire "escaping". It
was made of interlocking pots. From the
late Middle Ages, cross-draught kilns in
the Rhineland have reached 1,250°C. The
Chinese reached such temperatures 600
years earlier in the Tang period (618-906)
and later fired their porcelain to these
temperatures, which was soft paste, not
hard paste porcelain.
By means of draught in woodfired
kilns and pressure in gas kilns, the gases
flowing through the kiln carry away
the gaseous products of the firing. That
is the difference to firing in an electric
kiln. Something always evaporates from
the glaze during firing, which is then
carried away by the kiln gases, as every
component part has its own partial pressure.
With copper it is so great that with
oxblood glazes, copper oxide evaporates
away from the edges. During the firing,
the edges become hotter than the surfaces.
That is to say that one and the same
object is heated to differing temperatures
during the firing – it is the same with
people. The nose burns sooner than the
cheeks when skiing, and in the sauna,
the knees and elbows become hotter than
the trunk. In ceramics, these differences
are taken into account by soaking, i.e.
holding the maximum temperature, and
in enamelling, an object has an edge
enamel and a surface enamel.
www.gustav-weiss.de
PART II FOLLOWS
42 NEW CERAMICS maY / june 2012