174 Chapter 7
damage for certain sensitive glasses, such as medieval glass types or water-
logged glasses. In order to avoid iridescence or micro-cracking, the relative
humidity during storage and display has to take into account the degree of
degradation of the objects, their composition and provenance, and possible
damage due to previous treatments.
3 Deterioration and Conservation of Ceramics
3.1 History and Technology
Manufacturing of ceramic objects, both for daily use and to express artistic or
religious merit, represents an ancient craft, dating back to at least 10000 BC,
but still influencing modern technology. Ceramic artefacts are characteristic
for the artistic expression of a certain culture and thus have been used by
archaeologists as indicators for a social unit and for trade between different
units. Ceramics testify cultural activities by providing traces of mankind back
to before the Neolithic age and thus being of special importance for civilisations
from which no written documents are available.
Although the basic technology for the production of ceramics is rather sim-
ple, it leads to a large variety of shapes and objects with characteristic fea-
tures: by adding the right amount of water, clay is obtained as a soft plastic
mix that can be formed easily by hand, e.g. to produce simple pots. The walls
of a vessel may also be built up by a series of coils, winding round and round
a flattened pad. On the other hand, flat sheets of rolled clay can be joined to
obtain more rectangular pieces. Figurines, and other complex hollow forms,
are produced by pouring liquid clay into an absorbent mould, made from
low-fired, unglazed clay or wood. As the water is absorbed by the mould, a
thick layer of clay precipitates on its inner surface. After the remaining liquid
slip has been poured away, the mould can be opened to remove the object.
Different components can be cast separately before joining together with a
slurry of liquid clay. The invention of a rotating turntable or wheel has facil-
itated the mass production of vessels. “Throwing” a pot involves the center-
ing of a ball of clay on the rotating table and forming of the vessel walls with
both hands.
The characteristic variety of ceramics is not only created by the different
possibilities of shaping clay, but also through variation of raw materials, dec-
oration and firing technology.
3.2 Raw Materials
The Earth’s crust is composed of clay minerals and accessory minerals such
as quartz, feldspar and calcite. The clay mineral portion consists of parti-
cles essentially smaller than 2m, whereas the accessory minerals may range