Biomedical Engineering Reference
In-Depth Information
Chapter 9
A 7,000 Year Old Story: Ceramics
Some six to seven thousand years ago, Chinese craftsmen managed to manu-
facture pottery by firing clay. Definitely not more than thousand years later, the
Egyptians did the same (independently?). Throughout the successive millennia, both
regions refined this art to dazzling heights. Faience was already invented in predy-
nastic Egypt around 3000 B.C. and the potters wheel was introduced there around
2500 B.C. The start of the
ceramic industry
in the Neolithicum figures among
the major stepping stones in the great adventure of human kind. Evermore since
then and throughout the developing world ceramists manufactured art, objects for
kitchen, religious (
ex votos
!) or medical purposes and today they still do the same.
9.1
Greek Pottery, a Useful Intermezzo?
The summit of Greek craftsmanship in ceramics was the invention around the sev-
enth century of a technique to produce red pots with a black glaze decoration. We
had to wait till the eighteenth century P.C.(8) to uncover the potter's secret.
Man
does not discover, he uncovers
as Larry Hench used to say. For decoration the pot-
ters used a paste, obtained by concentrating a colloidal suspension of illitic clay
low in calcium but rich in iron oxides-hydroxides. The suspension formed spon-
taneously in rain water. By this suspension technique, the right particle size was
sequestered. Potash (K
2
CO
3
) was added as a flux responsible for what is called
making the
black vitreous slip
. After decoration, the pots were fired in a single cycle
beginning by heating the kiln to around
950
ı
C with all kiln vents open. In this
oxidizing atmosphere both pot and glaze (from the paint) become reddish brown
(formation of hematite Fe
2
O
3
). The red color of fired clay is due to the presence of
iron which is rather the rule than the exception: iron is after all the fourth most abun-
dant element in earth's crust. Subsequently, the vents were closed and green wood
was introduced in the kiln. In this reducing atmosphere (CO) hematite was reduced
to a mixture of black wuestite (FeO) and magnetite (Fe
3
O
4
) while the temperature
went down to
850
ı
C. At this stage, the vents were re-opened and the pots turn
again to orange-red except the black glaze because this is, say
oxygen tight
due to
the vitrification of the illite, whereas the vase is porous.
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