The nature of marbled Terra Sigillata slips: a combined μXRF and μXRD investigation

Abstract

In addition to the red terra sigillata production, the largest Gallic workshop (La Graufesenque) made a special type of terra sigillata, called "marbled" by the archaeologists. Produced exclusively at this site, this pottery is characterized by a surface finish made of a mixture of yellow and red slips. Because the two slips are intimately mixed, it is difficult to obtain the precise composition of one of the two constituents without contamination from the other. In order to obtain very precise correlation at the appropriate scale between the color aspect and the elemental and mineralogical phase distributions in the slip, combined electron microprobe, X-ray micro spectroscopies and micro diffraction on cross-sectional samples were performed. The aim of this study is to discover how potters were able to produce this unique type of terra sigillata and especially this particular slip of an intense yellow color. Results show that the yellow component of marbled sigillata was made from a titanium-rich clay preparation. The color is due to the formation of a pseudobrookite (TiFe 2 O 5 ) phase in the yellow part of the slip, the main characteristics of that structure being considered nowadays as essential for the fabrication of stable yellow ceramic pigments. Its physical properties such as high refractive indices and a melting point higher than that of most silicates widely used as ceramic colorants are indeed determinant for this kind of application. Finally, the red parts have a similar composition (elementary and mineralogical) to the one of standard red slip.

Thin coatings of ceramics: some new developments in France and in Europe

“…TEM investigations have also strongly renewed the archaeometric studies of antic ceramic slips [60,67,68]. The identification of yellow pigment of marbled terra sigillata could open outlooks in material science [49]. On the other hand, significant advances have been made in the field of non-abrasive elementary analysis of thin coatings [15,54].…”

“…The color is usually black or red following the nature (reducing or oxidizing) of the firing atmosphere during the vitrification phase. However, a specific type of terra sigillata, called "marbled" by the archaeologists, is characterized by a yellow and red coating [49]. Coatings obtained from a clay material were also used on Neolithic ceramics and some of them were fired at temperatures high enough to lead to a partial vitrification of the clay matrix with a rather good control of the color (red, black or white).…”

“…In order to obtain very precise information concerning the yellow component of slips, synchrotron techniques and transmission electron microscopy were combined. From the correlation between elemental and mineral compositions, the crystallographic nature of the yellow component was determined [49]. It contains small crystals of pseudobrookite, a Ti-rich phase, which is certainly responsible for the yellow color.…”

Raman spectroscopy analysis of terra sigillata: the yellow pigment of marbled sigillata

“…Micron‐X‐ray powder diffraction analysis demonstrated that the composition of red part was similar with the slip of the standard terra sigillata while the yellow component was obtained from a titanium‐rich clay preparation. The yellow color is attributed to the formation of pseudobrookite (Fe 2 TiO 5 ) phase during firing process . Indeed, pseudobrookite was proposed as yellow pigment for modern ceramics, and studies were devoted to its syntheses .…”

A superbend X-ray microdiffraction beamline at the advanced light source

“…Xray microfluorescence and monochromatic X-ray microdiffraction have been combined to identify the composition of the slip which consists of two layers, a 10 micron thick layer of red slip and a 30 microns thick layer of yellow slip sequentially dip-deposited at the surface of the pottery. [12] The red slip turns out to be the standard hematite-based slip used in the conventional red Terra Sigillata, while the yellow color of the second slip was found to be due to a titanium-rich mineral called pseudobrookite (Fe 2 TiO 5 ). This yellow color can only be obtained through specific firing conditions indicative of the high level of mastery achieved by the ancient potters.…”