The technology of polychrome glazed ceramics in Ifriqiya: new data from the site of Chimtou

Abstract

Ifriqiya (roughly Tunisia and eastern Algeria) is believed to have played a significant role in the diffusion of ceramic glazed technologies into other regions of the Western Mediterranean. However, due to limited analysis on North African glazed ceramics, its role in technology transfer remains poorly understood. This paper uses SEM-EDS and petrographic analyses to understand the technology employed in the production of Tunisian ceramics through the study of 30 polychrome glazed ceramics from a medieval settlement at the site of Chimtou (ancient Simitthus), Tunisia, dated to the late ninth-twelfth century. The results show that these are lead-rich glazes with varying contents of alkalis, coloured with copper, iron and manganese oxide and applied over a calcareous body. Opaque glazes were obtained using cassiterite crystals as opacifier or by adding crushed quartz. The use of lead stannate as a colourant and opacifier in one light yellow glaze raises questions about the mechanisms of introduction of tin opacification technology in North Africa. Scrap metal seems to have been used as a source of lead for the glazes; while iron slag was probably used as a source of iron to colour the glaze in one sample, pointing to a cross-craft interaction between glazemaking and metallurgy.

Supplementary information: The online version contains supplementary material available at 10.1007/s12520-024-01974-x.

Keywords: Cross-craft interaction; Glaze technology; Medieval Tunisia; Polychrome glazed ceramics; Recycling; Tin opacification.

Fig. 1

Plan of Chimtou showing location of medieval settlement and key places named in the text

Fig. 2

General types of the glazed wares from Chimtou

Fig. 3

BSE images showing details of the microstructure of the ceramic bodies. a) Image showing characteristic elongated porosity, round quartz inclusions and microfossils inclusions (Chi 13). b) Image showing decomposed lime with white halos (Chi 21). c) Image showing detail of foraminifera microfossil (Chi 11)

Fig. 4

Photomicrographs showing the fabrics of the different petrographic groups identified and the three petrographic outliers  (Chi 8, Chi 12, Chi 27). All photomicrographs were taken in cross polarisation

Fig. 5

Plots showing the composition of the glazes analysed. a) Ternary diagram showing the major chemical compositions for the high lead low alkali, lead alkali and one outlier. Samples Chi 2, 7 and 3 present a weathered glaze. Chi 27 is opaque yellow. b) Biplot of 15b Owt% vs Na₂O + K₂O wt% for the glazes analysed showing that the cassiterite opacified samples are enriched in alkalis. Two areas (and thus two points) of samples Chi 6 and Chi 30 are shown in graph b, to take into account the inhomogeneity due to the presence of tin-rich areas (encircled in graph b)

Fig. 6

BSE micrographs of the polychrome transparent glazes analysed showing: a) detail of a cross section of a yellow and brown glaze showing kentrolite particles (marked with an arrow), unmelted quartz and K-Pb feldspars at the ceramic interface; b) the rare occurrence of tin oxide (or tin and antimony) particles in the transparent glazes; c) the presence of a large agglomerate of tin oxide particles in sample Chi 26; d) an iron slag inclusion in sample Chi 3

Fig. 7

BSE micrographs of the opaque glazes analysed with picture of the respective sherd showing: a) the abundant presence of cassiterite crystals as opacifiers (Chi 30); b) the use of tin opacifier in a frit-like glaze showing numerous unmelted quartz grains in the glaze (Chi 6); c) the presence of large bubbles, unmelted quartz and overglaze brown decoration with associated kentrolite crystal near the surface of the glaze in sample Chi 28. This sample was likely opacified using quartz rather than tin oxide

Fig. 8

BSE image showing lead stannate particles in sample Chi 27 (right) and picture of the sample analysed showing the surface decoration and the light-yellow background (left)

Fig. 9

Body-glaze comparison plots for SiO₂ and Al₂O₃ contents after subtraction of PbO and normalising the composition

Fig. 10

Comparison between Na₂0 vs CaO and Na₂O vs K₂O contents of the bodies (a, b) and glazes (c, d) from Tunisia (data from: Ben Amara et al. ; Ben Amara et al. ; Salinas et al. 2022b, 2020), Algeria (data from: Djellid et al. 2023) and Chimtou (analysed in this paper)