178 Chapter 7
40 SiO 2 , 1 K 2 O, 3 Al 2 O 3 , 1 Fe 2 O 3 , 3 CaO and 2 CuO (two others). Thus, the
main components and even the colouring agents used are again very similar
to glass. Various oxides of Fe, Cu, Co, Mn and Sb are considered traditional
colourants for antique ceramic glazes, whereas a larger variety of coloured
glazes have been available since the 18th century. Fine decorations can be
achieved by painting the colourants directly on the ware before glazing, as
“in-glaze colour” or “overglaze”, when applied on a glazed ground.
For the stability of the composite material, it is important that the body and
the glaze have similar expansion coefficients, to avoid cracking and crazing.
The bond between the glaze and the underlying body is often weak, for low-
fired glaze. In contrast, high-fired glazes can stabilise a friable ceramic sup-
port. Firing of glazes onto ceramic bodies implies a sequence of chemical and
physical reactions, leading to the formation of an interface, which represents
a transition between the dense surface and the porous substrate (Figure 15).
The adhesion is ideal if the surface tension of the applied glaze is low enough
to enter the pores to create the connection. On the other hand, components
from the ceramic substrate may diffuse into the glaze during firing. The thick-
ness of this transition layer may reach up to 50m and depends on the time
and temperature during firing.
3.5 Dating and Provenance
The typology of pottery styles has been the most reliable dating method for
archaeological sites for centuries. Through comparison of characteristic pottery
Figure 14Glazed terracotta, an architectural element from a 16th century building in
Northern Germany