Investigation of Glaze and Coating Structure of Historic Arg-e Bam Pottery Using Micro Raman Spectroscopy and Laser-Induced Breakdown Spectroscopy (LIBS)

In addition to reading historical books and texts, an examination of the natural and artificial artifacts left over from the past can also provide a comprehensive and appropriate view of human life style. Pottery is a work of art, due to their nature as well as their compositional structure, has been able to retain its originality for many years and present itself as a viable option for study purposes. Despite the widespread use of both Micro-Raman Spectroscopy, and Laser-Induced Breakdown Spectroscopy (LIBS) techniques in the world of archeology, these study methods are still unknown within the country and have not been applied to the study of artwork in accordance with their global status. Careful study of historic Arg-e Bam pottery is done for the first time in the country in a scientific and practical way, in order to reveal some of the historical and artistic aspects of these works. Studies also show that in the Middle Ages and during the Seljuk, Khwarezmshahi, Ilkhanid and Timurid rule to beautify and extend the life of pottery from seven types of matte turquoise blue glaze, matte milky glaze, clear turquoise blue glaze, milky gray glaze, monochrome glaze, embossed glaze and also golden glaze were used. The current paper applies both above mentioned study methods, following a comprehensive review of the nature of glazes used in the pottery industry and their historical antiquity. The results of the study are then described on seven samples of historic Arg-e Bam pottery to illustrate the source of cobalt oxide for blue, chromium and copper glazes for green and turquoise glazes as well as tin for white glazes.
Keywords: Arg-e Bam, Pottery, Glaze, Micro Raman Spectroscopy, Laser-Induced Breakdown Spectroscopy (LIBS).
 
 
Introduction
One of the most important and main ways to be aware of the style and context of human life throughout history is to study the objects made by them, especially works of art. Considering that clay objects have the ability to maintain their originality for many years and remain intact in the face of various dangers; Archaeologists and researchers have always been interested in various studies. On the other hand, by combining archeology with other sciences such as physics, chemistry, etc., more accurate and reliable analyzes can be achieved, and as a result, a clearer picture of the past lifestyle of human beings can be obtained.
In this research, using Raman spectroscopy and Laser-Induced Breakdown Spectroscopy (LIBS), studies have been conducted on the origin and structure of glazes on a number of pottery of the historic citadel of Bam. Using this study method on glazed pottery, in addition to requiring a short time, can provide us with complete information about the structure, contents and history of these samples. In this research, it is shown that these two spectroscopic methods are able to determine and introduce the materials used in the production of glaze on Pottery samples with a high reliability coefficient.
It can be said that the first generation of glazed pottery found in Bam belongs to the Sassanid period, at the end of the historical period, which archaeologists consider to be rough blue glazed pottery related to this period. Researchers believe that the Bam region in the Islamic era was much richer in terms of pottery and its species diversity than the two prehistoric and historical periods; Because most of the pottery discovered in this area belongs to the Islamic era.
In the leading research, the word glaze and the necessity of using it in the pottery industry are first introduced. In the next step, the history of pottery discovered from Bam citadel and also the use of glaze in these samples were reviewed and then, by introducing materials and study methods, the common materials used to produce different types of glaze in different colors were introduced. Finally, by examining seven samples of pottery in the historic citadel of Bam, we will talk about the characteristics of the glazes used to cover these samples, as well as the results obtained from this research.
 
Discussion
Ceramic glaze is an impervious layer or coating of a vitreous substance which has been fused to a ceramic body through firing. Glaze can serve to color, decorate or waterproof an item.[1] Glazing renders earthenware vessels suitable for holding liquids, sealing the inherent porosity of unglazed biscuit earthenware. It also gives a tougher surface.
 
Green and Turquoise Glazes:
Copper oxide: It is one of the most widely used colored oxides that has been used since ancient times to produce green glazes by potters and artists. Zinc oxide: By using this material and copper oxide at the same time, a turquoise color can be created in different glazes. Chromium oxide: The use of this material together with one of the alkaline metals in the structure of the glaze makes its color green. The green color created in this way has a very good stability.
 
White Glazes
Tin oxide: This material is generally used for the purpose of opaque and opaque different glazes, but if we use only tin oxide and lead in the production of glaze, then after heating it, a white and milky glaze is obtained.
For experiments in this research, a green laser Raman microscope with a wavelength of 532 nm was used. The intensity and amount of Raman rays scattered from the surface of each sample is a function of the location and three-dimensional arrangement of its constituent atoms in space relative to the other components. With this description, the Raman signal received from the sample surface can be introduced as a fingerprint identifier that can play an important role in identifying materials.
Also for LIBS spectroscopy method, (In this spectroscopic method, a small part of the material is converted to plasma using a laser as a light source).  Nd: YAG Q-Switch laser with a wavelength of 532 nm, a pulse width of 10 nanoseconds and a repetition rate of 3 Hz is used.
 
Conclusion
in the study of pottery discovered from the historic citadel of Bam using the Raman spectroscopy method, it was found that cobalt oxide was generally used to create blue glazes in the coating of these materials. But in one of the samples, the presence of azure or Lapis lazuli was also visible. In addition, while copper oxide can be introduced as the most important factor in the formation of green glazes in the above samples, the study of the range obtained from a number of samples showed that chromium ions are the main structure of their green glazes. In addition, it was confirmed that in white glazes, a sudden increase in the band in the 500 region could be evidence of the presence of tin oxide in the glaze structure.
The use of laser induction sales spectroscopy also showed that copper-dependent compounds and most likely copper oxide were generally used to prepare the blue glazes on these pottery. The identification of copper radiation lines in the spectra obtained from samples 2, 4, 5 and 6 is a good confirmation and expression of this issue.
On the other hand, in the study of samples 1, 4 and 7, it was found that the glazes of these materials, while containing copper-dependent compounds, were prepared using compounds prepared from chromium (most likely chromium oxide) and the application of different percentages of this Two materials during the preparation and baking of the glaze have caused us to see a range of different colors from green to turquoise on these samples. Although it was expected that cobalt radiation lines could be found in some of these samples, the presence of this element and its related compounds, such as cobalt oxide, was not found in any of the obtained spectra.
In the sample review section, it was also shown that in white glazes, it is possible to identify elements such as tin, barium and titanium, and these elements are responsible for creating this color in the glazes of samples 6 and 7. Examination of the glaze structure on these 7 samples also showed that only one of them has a glaze of lead nature and lead lines are visible only in sample 3.