SCIAU, P., RELAIX, S., GOUDEAU, P., BELL, Anthony M. T., JONES, R. L. and PANTOS, E. (2006). X-Ray synchronotron study of phase transforms in illite clays to extract information on sigillata manufacturing processes. In: Synchrotron Radiation in Art and Archaeology 2006, Berlin. (Submitted)
Documents
12780:41279
PDF (conference paper)
__staffhome.hallam.shu.ac.uk_staffhome2_b_acesamb_MyWork_Tony Personal_AMTB papers_Pantos_Sciau_SRABerlin.pdf - Submitted Version
__staffhome.hallam.shu.ac.uk_staffhome2_b_acesamb_MyWork_Tony Personal_AMTB papers_Pantos_Sciau_SRABerlin.pdf - Submitted Version
Download (142kB) | Preview
Abstract
The technique of sigillata really began in central Italy during the first century B. C. with the
development of red vitrified slips obtained through vitrification of a clay preparation. These
ceramics, usually decorated with raised motifs and standardised shapes, quickly took over as
semi luxury crockery. Given this success, this technique quickly extended to the entire Italian
peninsula and then to the Mediterranean coast. From the very start of our era, great centres of
production were set up in the south of Gaul.
The aspect of sigillata comes from the nature and the texture of its slip. Studies have shown that
sigillata slips of quality were obtained from a non calcareous clay while the local calcareous clay
was used for the bodies. During firing the slips are vitrified and get a specific microstructure
containing hematite and nanometric corundum crystals [1]. An investigation of the clays
surrounding La Graufesenque site started and it seems that only the Trias levels are chemically
compatible with the composition of antique slips. Apart from the in depth study of the
mineralogical nature of these clays realized at a geological Laboratory, we have studied the
structural transformations as a function of temperature of two of these clays, chosen for the
quality of vitrification in the firing temperature range of sigillata [1030-1080°C]. The main
difference between the chemical composition of these two clays is the amount of Mg (2.4 % and
4.5 % in oxide weight).
Time-resolved measurements were made at
Daresbury (station 2.3) up to 1100oC in
oxidizing conditions. An abrupt increase of the
hematite cell was observed around 850°C.
Above 1000°C, the hematite peaks get sharper
which indicate an increase of coherence length
(Fig. 1). A spinel phase with cell parameter
close to MgAl2O3 was detected from this
temperature. As for the hematite, its coherence
length increases with the temperature but also
during the beginning of the cooling. For the
clay sample with the smaller amount of Mg, a
corundum phase with very small coherence
length was detected above 1000°C. Slips were
prepared from the last clay by modern potters and firing at 1050°C in oxidizing atmosphere. A
mineral quantitative analysis performed using the Rietveld method revealed that the amount of
spinel phase is very high while the corundum contributes to a small part of crystal phases. It is
the inverse in the antique slip where the amount of Mg in oxide weight is around 1%. It is clear
that the amount of Mg plays a key role in the corundum/spinel competition and that the present
slips contain too much Mg. Two questions arise: (i) As the Trias levels are quite heterogeneous
is it possible to find clay with less Mg? and (ii) Did the gallo-roman potters eliminated a great part
of Mg during the slip preparation process? We discuss the merits of these two alternative
hypotheses.
More Information
Statistics
Downloads
Downloads per month over past year
Share
Actions (login required)
![]() |
View Item |