81 research outputs found
The nature of marbled Terra Sigillata slips: a combined μXRF and μXRD investigation
X-Ray synchronotron study of phase transforms in illite clays to extract information on sigillata manufacturing processes.
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
Commensurate-Incommensurate Magnetic Phase Transition in Magnetoelectric Single Crystal LiNiPO
Neutron scattering studies of single-crystal LiNiPO reveal a spontaneous
first-order commensurate-incommensurate magnetic phase transition. Short- and
long-range incommensurate phases are intermediate between the high temperature
paramagnetic and the low temperature antiferromagnetic phases. The modulated
structure has a predominant antiferromagnetic component, giving rise to
satellite peaks in the vicinity of the fundamental antiferromagnetic Bragg
reflection, and a ferromagnetic component giving rise to peaks at small
momentum-transfers around the origin at . The wavelength of the
modulated magnetic structure varies continuously with temperature. It is argued
that the incommensurate short- and long-range phases are due to
spin-dimensionality crossover from a continuous to the discrete Ising state.
These observations explain the anomalous first-order transition seen in the
magnetoelectric effect of this system
EXAFS study of lead-free relaxor ferroelectric BaTi(1-x)Zr(x)O3 at the Zr K-edge
Extended X-ray absorption fine structure (EXAFS) experiments at the Zr K-edge
were carried out on perovskite relaxor ferroelectrics BaTi(1-x)Zr(x)O3 (BTZ) (x
= 0.25, 0.30, 0.35), and on BaZrO3 for comparison. Structural information up to
4.5 A around the Zr atoms is obtained, revealing that the local structure
differs notably from the average Pm-3m cubic structure deduced from X-ray
diffraction. In particular, our results show that the distance between Zr atoms
and their first oxygen neighbors is independent of the Zr substitution rate x
and equal to that measured in BaZrO3, while the X-ray cubic cell parameter
increases linearly with x. Furthermore, we show that the Zr atoms tend to
segregate in Zr-rich regions. We propose that the relaxor behavior in BTZ is
linked to random elastic fields generated by this particular chemical
arrangement, rather than to random electric fields as is the case in most
relaxors.Comment: 13 pages, 12 figures, 4 tables. Submitted to Phys. Rev.
Ceramic technology: how to characterize terra sigillata ware
As part of the Topical Collection on the archaeometric study of ceramics, this paper focuses on terra sigillata ware. The main aims are to provide a review on the state of the art of the studies and to provide a guide to the most suitable analytical techniques. The text is divided into four main parts: (1) a brief archaeological introduction on the ancient production of terra sigillata; (2) a summary of the archaeometric studies carried out to date; (3) a reasoned list of the most suitable techniques for the investigation of the ceramic body and (4) an in-depth discussion on the most effective techniques for the study of the coating. The application of both destructive and non-destructive techniques is critically evaluated as well as the advantages and disadvantages provided by the different instrumentation, in terms of sample preparations and expected results
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