Geology Reference
In-Depth Information
logical material). Frequency, size, shape and the orien-
tation of macro- and micropores assist in understand-
ing production technologies. Round pores may suggest
high firing temperatures, irregular and elongated pores
relatively low firing temperatures (Middlestone et al.
1995). The identical porosity values of typologically
uniform ceramic sherds can indicate that comparable
techniques were used. Pore structure analysis is im-
portant in grouping ancient potteries with regard to fir-
ing temperatures and techniques (Gosselain 1992).
occur, depending on the local geological situation and
the genetic history of the clay. Artificial admixtures,
added by the potters to improve the tempering process,
include quartz, various minerals, marble and rock frag-
ments and crushed shells as well as ash, slacks, glass,
ore grains, crushed pottery and organic material, e.g.
straw, wheat and plant stems. Thin section studies of
argillaceous inclusions require specific descriptive
methods (Whitbread 1986).
• Shape:
Differences in the angularity and roundness
of temper grains especially quartz grains, may provide
a means of discriminating natural and additional tem-
per material.
• Size and sorting:
The maximum size of grains pro-
vide a first approximation in differentiating ceramic
types (Riederer 1985; Schubert 1986). Distinct grain
size hiati between matrix and temper components can
point to an addition of temper grains, whereas a gradual
transition of temper grain sizes into matrix grain sizes
should indicate the lack of additions. However, cau-
tion is necessary, because of the possibility of a natural
mixture of clay matrix and coarse temper grain frac-
tions (e.g. in lake sediments).
•
Orientation of temper grains:
Orientation patterns
seen in ceramic thin sections provide clues to the tech-
niques used in fabrication (Philpotts 1994). Pronounced
orientation patterns reflect the mode of the manufac-
ture of the pottery.
•
Frequency:
Important data describing variations in
temper composition are the amount of temper grains.
The mineralogical composition of the matrix can not
be sufficiently resolved by petrographical means. Ma-
trix and temper grains are studied by means of X-ray
fluorescence analysis and X-ray diffraction as well as
neutron activation analysis and Mössbauer spectros-
copy (Riederer 1987; Wagner et al. 1986). The temper
grains are investigated in thin sections of ceramic sherds
or in artificial grain fractions derived by crushing and
sieving of the samples (Hagn 1985). The mineralogi-
cal composition of temper grains and matrix reflects
burning techniques. Temper grain analysis directed to
the understanding of the provenance and technology
of ceramics must consider the following points.
•
Mineralogical composition:
Common natural ad-
mixtures of pottery clays are quartz, feldspars, micas,
calcite and cherts. In addition, other minerals and rock
fragments (e.g. amphiboles, pyroxenes, epidote, opaque
ore minerals; quartzite, granite, crystalline rocks) can
Fig. 19.4.
Roman ceramics with
white marble tempering.
'Auerberg
pots'. These Roman pots from the
first century A.D. were imported
from the border region between
Austria and Italy and found in a
Roman settlement on the Auerberg
near Bernbeuren in southern
Bavaria. White dots visible on the
pots are marble grains which had
been mixed into the clay. The larg-
est pot is 19 cm high.
