Original Article
On-site pXRF analysis of body, glaze and colouring agents of the tiles at the excavation site of Iznik kilns

https://doi.org/10.1016/j.jeurceramsoc.2019.01.050Get rights and content

Highlights

  • First on-site, non-invasive analysis of tiles excavated at Iznik kilns are performed with portable XRF instrument.

  • A new type of paste, which is rich in calcium is evidenced for Iznik tiles.

  • A technological similarity is found between the monochrome, turquoise tiles of Iznik kilns and Edirne mosques.

  • This study documented the potential use of bornite mineral in red decors.

Abstract

The excavation at Iznik tiles kilns continues as the third period of the field mission. We present here the first on-site, non-invasive analyses performed with portable XRF instrument on twenty-five excavated tiles and two residue materials of the kiln. The shards studied were attributed to the productions from 14th- to 17th-centuries. The comparison was made by the discussion of characteristic elemental ratios selected from the ceramic technology criteria and PCA/Euclidean distances analysis. Three groups of body and glaze technologies were evidenced. We encountered that the amount of tin oxide in the glaze decreased over the centuries. Besides, two different types of fluxes were used in the glaze, some containing only potassium, and the others having potassium and calcium. The decors, which were investigated in this study were blue, turquoise, green, red colours, and black lines. A copper-iron mixture in the red areas was documented, which reflects the use of bornite.

Introduction

The excavations in Iznik, which aim to present the history and cultural heritage of the city, started in 1969 in the sites of Orhan Imaret and its Bath (Turkish Hamam) and continued for kilns [1]. Since 4th-century BC, Iznik, known as Nicaea under Byzantium rule, has been located in Anatolia at the intersection of the roads linking the Middle-East region to the Balkan states [2]. Iznik came into prominence when the ceramic production had developed during the Ottoman Empire. The excavation work of Iznik tiles kilns continues as the third period of the field mission [3]. The origin of Iznik technology remains debated because the links with the know-how of Seljuk, Timurid, and Byzantine productions of Nicaea are not well defined due to the limited studies carried out at the excavation site and on the ancient Ottoman buildings historically well documented. Most of the holy places, mosques, and similar architecture built in Istanbul, Edirne, and Bursa were ornamented with the “Iznik” style tiles [[4], [5], [6], [7], [8], [9], [10]]. Indeed, Ottoman sources state that the Iznik production was used more as a trade name than a provenance [11]. This study aims to identify the technology of tiles produced in the kilns of Iznik city and define the technological links between Timurid and Seljuk productions.

Since the 1980s, mostly laboratory studies with large-scale, destructive/invasive instruments and few on-site measurements with some portable, non-invasive techniques were conducted about the characterization of Ottoman period ceramics, where the research on Iznik and Kütahya productions lead, by international [[12], [13], [14], [15], [16], [17], [18], [19], [20]] and national researchers [[21], [22], [23], [24], [25]]. Mainly, the material studied on-site concerns the collections of the museums [19,20], and rarely on buildings [25]. The place or places of the production of “Iznik” style ceramics are still debated, the number of shards found in the excavation context is reduced and the measurements have been focused more on the analysis of paste and mostly published in an inaccessible form for international researchers due to the national language used [1,11,27,28]. The analyses concerning the colouring agents are old and limited to a few fragments whose origin is poorly or not documented. The most comprehensive work comprises the analyses of the shards preserved in the vaults of the Topkapi Palace Museum with scanning electron microscopy-energy dispersive spectrometer (SEM-EDS) and Raman instruments [11,26,27]. The aim was to characterize non-inventoried shards and attribute the productions of Iznik, Kütahya, Tekfur Palace (Istanbul), and some other Ottoman ceramics of unknown origin by comparing the microstructure of the cross sections, glaze signatures and chemical composition of the bodies, glazes, and pigments found in the coloured areas.

On the contrary to the analyses of movable and immovable cultural assets of Iznik production, very few works were conducted on the excavation materials of the Iznik tile kilns and the studies were performed only with destructive techniques [2,[28], [29], [30]]. During her Ph.D. work, Füsun Okyar [28] performed petrographic and chemical analyses of the bodies, slips, glazes, and coloured decors of the shards of redware and stone-paste type, on twenty-four samples in total. Chemical compositions were defined with SEM-EDS method. The research on the excavation materials continued with the studies of T. Tulun and her colleagues by using similar techniques (e.g. XRD, SEM-EDS) to identify the technological properties [2]. In this study, the results obtained by F. Okyar [28] and T. Tulun [2] are used as references to compare the data obtained with pXRF.

The advantage of the use of laboratory-type instruments is the possibility of a multi-scale analysis of the sections (including glaze, decor, slip, and body) of the shards. The high-resolution techniques, including petrographic microscopy, SEM-EDS, micro-XRD (X-rays diffraction), micro-XRF (X-rays fluorescence), and micro-Raman, with high detection capabilities, are the best approaches to understand the differences and common points in the ceramic production technologies. Due to the destructive character of the techniques, ethical rules of cultural heritage studies now limit these types of analyses as much as possible. Moreover, the shards from Iznik kilns are rare and the research ethic, especially in Turkey, covers the use of non-destructive techniques to maintain the integrity of the samples. Due to the complexity of the Iznik decors with superposition of several layers of enamels, the analysis by micro-techniques imposes one of the cuts incompatibles with the respect of the artifacts. In addition, most of the documented “Iznik” tiles are in UNESCO World Heritage sites and their analysis can only be performed by non-invasive methods. Therefore, the portable, non-destructive techniques, e.g., pXRF, pRaman, and FORS (Fiber Optics Reflectance (UV-nIR) Spectroscopy) have become essential tools for the analyses of the objects in place (tiles) or exclusive objects preserved in the secure rooms of museums or private collections. Thus, a methodology must be created to make the analytical procedures more reliable when using portable instruments.

Moreover, the main drawback of studying the excavation materials is the interpretation of the results regarding the production technology. The materials found at the excavation site could be trials of the artist to define the best recipe for the artwork he/she prepared or some rejected ceramics due to the inappropriate temperature and atmosphere control of the kiln or wrong choice of the raw materials for the body and glaze. However, the tile shards found inside the kiln are in general statistical representatives of the production and can be compared with the tiles found in the wall revetments of the buildings. The portable instruments, which allow non-invasive and fast measurements, help to analyse a huge number of artefacts leading a statistical approach which compares the tiles of the excavation sites with the tiles mounted on the walls of the buildings of similar periods. In addition, a few tiles can be extracted to be cut and prepared for destructive methods at the laboratory, which will provide individual data rather than a cumulative knowledge. Therefore, portable, non-destructive techniques are gaining in importance. Thus, defining the provenance of the collections will be supported by investigating the excavation materials.

This study presents the characterization of twenty-five shards excavated recently and two residue materials formed on the walls in the Iznik tile kilns. The work has two objectives: i) the comparison of elementary data analysis based on the comparison of the elemental ratio, which is characteristic of the ceramic technology, with the ‘blind’ approach using PCA and Euclidian dendrograms; ii) a better knowledge of the production of Iznik kilns. Moreover, the results of pXRF measurements will correlate the previous measurements carried out on the excavation materials with large-scale laboratory-type instruments in order to assess the capabilities of the portable XRF system.

Section snippets

Tile shards

Twenty-three glazed shards and two shards not glazed but one painted and the other non-painted, were examined with the pXRF instrument in this study in addition to the measurement of kilns residues on-site and in the excavation lab. Fig. 1 shows all the studied shards which were excavated from the tile kilns in Iznik. The shards, which were shown on the first three lines, were attributed to the productions of 14th-16th centuries and shards on the last line are assumed to be later productions of

Body

Before the measurement with the pXRF, a visual examination was performed to identify the colour changes in the paste. When we compared the colours visually, three types of paste were determined, in red, light beige and white colours. In the same atmospheric conditions during the firing process, a high addition of limestone in the body clarifies the colour of the paste from red to yellow, and when it is added in low quantity, the colour of the body turns to red with the same iron content.

Validity of non-destructive elemental analysis with portable instrument

On-site measurements with portable instruments become crucial in terms of the reproducibility and reliability of the results obtained. The accuracy of the data obtained with the pXRF can be provided by using the proper calibration standard (glass standards Corning Brill B, C, D, NIST SRM 611; geological standards red mud, diorite DR-N [46,47]) which allows doing a semi-quantitative analysis for determining the composition of the body, glaze, and pigments used in the decor. In this study, pXRF

Conclusion

The monochrome and polychrome Iznik tiles are the masterpieces of the Ottoman period which were produced from the 14th- to 17th- centuries by the orders of the Sultans for historical buildings. Therefore, most of the tiles were conserved in the wall revetments except the similar tiles excavated in the Iznik tile kilns. They represent mostly the samples of overproduction or waste materials of the workshops. By reason of the need of on-site, non-destructive measurements, a methodology had to be

Acknowledgments

Noyan Özdemir from TROY-MET (Istanbul) is gratefully acknowledged for making Hitachi X-MET 8000 Expert Geo pXRF instrument available in Iznik for on-site measurements. The authors thank Yurdanur Akpınar for providing the coordination between the authors.

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