Study of Colorants Materials on the Decoration Patterns on the North and Southeast Porches of Soltanieh Dome, Zanjan, Iran

Colorful decorations and wall paintings are one of the most important and widely used decorating methods applied in the construction of Gonbad-e Soltanieh. Understanding the type of colors and how these paintings and decorations were applied can be one of the most important discussions in knowing more and better about this building and the techniques and paints common in ancient times. Therefore, the present study aims to study how the ground was prepared for the implementation of colorful decorations and paintings, as well as what type of colors was used to reach a comprehensive knowledge of the techniques applied by Ilkhanid painters. To this end, the Scanning Electron Microscopy (SEM) analysis is used to investigate the order of layers and plastering and also, the X-ray diffraction (XRD) analysis and more chemical methods are used to study the type of green, blue, yellow, red, white and black colors. SEM analysis showed the type of plastering and the order of the painting layers well. Also, XRD and more chemical analyses identified the colors of malachite green, colored green, azurite blue, red ochre, yellow ochre, anhydride white and carbon black. 
Keywords: Colored Ornaments, Mural, Ancient Colors, Soltanieh Dome, XRD, SEM.
 
Introduction
The Soltanieh (or Soltaniyeh) Dome is located in the city of Soltanieh, one of the capitals of Ilkhanate, a Mongol dynasty built by Jaljaitü (a.k.a. Muhammad Khodabandeh). It was inscribed on the list of the UNESCO World Heritage Sites as the seventh building in Iran in 2005. In terms of outstanding architecture, colored ornaments, and murals (frescos) on the various beds (substrates, subgrades) of the Soltanieh Dome, they are the most important and widely used buildings in the Ilkhanate era. This study focuses on various important features of the Soltanieh Dome building, including interior decoration. In the interior of the building, a series of elevated porches have been formed, including decorations implemented in various ways. The Soltanieh Dome was decorated in two stages during the Ilkhanate era, during the reigns of Jaljaitü and Abu Sa’id, according to politico-religious considerations or special personalized tendencies. The decorations of the first period include the main decorations of the dome, bricks, and tiles, and the decorations of the second period include plaster molding, painting, and Pateh (needlework folk art) on the decorations of the first layer. In this regard, the identification of colors and the implementation of these decorations can be considered as one of the most important issues for better identification of this building and the techniques and colors common in the Islamic Middle Ages. Therefore, this study seeks to answer the following questions:
1) According to studies, what kind of dyes (dyeing materials) were used in the Ilkhanate era?
2) Considering a prominent building such as the Soltanieh Dome, what are the layers identified in the background of the decorations used in the paintings belonging to the Ilkhanate era? And how was the background prepared?
3) What kind of chemical compounds were used to make dyes in Soltanieh Dome?
By scrutinizing similar studies in the context of comparative background, this study sought to gather comprehensive information on the dyeing materials and techniques used by Ilkhanate era painters. This study employs an empirical-descriptive methodology. Initially, a series of field observations were made on the dyes (colors) used in the interior decoration of the north and southeast sides of the Soltanieh Dome, followed by sampling as per the sampling principles. The scanning electron microscope (SEM) method, including secondary electrons (SE) and backscattered electrons (BSE), was used to examine the arrangement of the layers in the painting motifs. Moreover, X-ray diffraction (XRD) and wet chemistry methods were used to identify the types of pigments used: green, blue, yellow, red, white, and black. According to the results of macroscopic and microscopic investigations into the arrangement of the layers in the Soltanieh Dome decorations, it has been determined that the layers are located in the following order: support, scratch coat (arriccio), fine coat (intonaco), primary layer, and paint layer, respectively. In the Soltanieh Dome, brick and tile decorations from previous periods are used as a support for paintings. Scratch coat was implemented using coarse-grained meal plaster.
Furthermore, fine-grained plaster of Paris was used to prepare fine coat plaster. Finally, the primary layer was made using fine-grained dead burnt plaster, canvassed, and prepared by a medium. Additionally, studies performed using the wet chemistry method showed the composition of gypsum, lime, and ash. XRD analysis results also indicated an 84% gypsum phase and a 16% quartz phase. As mentioned earlier, failure to detect calcite phases may be attributed to its small amounts in the composition. According to the results of a qualitative wet chemistry test, a mixture of gypsum and small amounts of ash and lime were identified, applied to the decorations of the first period, to the gypsum layer. However, XRD analysis showed only gypsum and some silica, and no calcite was detected, which can be attributed to its small amount in the composition. Moreover, ash is an organic material and cannot be detected in XRD analysis.
The colors green, red, yellow, white, black, and blue, are applied in decorations and paintings in the porch and hallway of the Soltanieh Dome. Examination of the colors used in the decorations applied to the paintings in this building indicates the existence of two types of green colors with different tonalities. One is the dark green color that is widely used in a wide range of buildings, and the other is the much lighter green color used to paint the leaves and flowers among the brandeburgs and the floral patterns of the porches, visible with a little care. Nevertheless, this color is not visible at first glance, which is because it is severely damaged and is currently seen in gray and dark colors. Most of this color is gone, and only small parts of it can be seen in the more solid parts of the color, left in light green-blue at the margins of the leaf or flora patterns as a thicker layer of color. Malachite green is used in the plaster molding of the first-floor corridor and light green-blue color in the painting decorations of the southeast porch. Analysis of light green in porch paintings by XRD indicated the presence of arsenic and copper acetate (CuOAc) (a.k.a. cupric acetate) as pigment compounds. In addition to light green, a darker green color can be seen in porch decorations, specifically due to the change in color of azurite blue to malachite or copper trihydroxychlorides.
Nonetheless, this section emphasizes light green, which is not the result of azurite blue discoloration (changes in azurite blue). Dye traces and degradation can be seen in several areas. Given the compounds identified in the XRD analysis for light green, it is likely that verdigris green was also used. The presence of copper arsenic may be attributed to the presence of arsenic in the primary copper composition used to make verdigris green. Blue was detected in painting decorations, Pateh (needlework folk art) margins, and azurite building plaster molding.
Furthermore, the results of wet chemistry and the XRD test indicated the use of azurite blue. Most of the raised parts of the first-floor corridor (hallway) are decorated in white. Besides, encircling was done with white paint to create brick patterns on the walls of the corridor after painting the surface yellow. Heat test results showed the presence of sulfate ions (SO42-) in this dye, indicating the use of anhydride as a dye. Yellow is used in the decoration of the first floor of the Soltanieh Dome to make decorative brick patterns on plaster. The results of wet chemistry experiments (tests) indicated the presence of iron in the composition of this pigment. In addition, the pigment turned reddish-brown due to heat. Therefore, it was found that yellow ocher was used in these parts.
 
Conclusion
Moreover, red is used on the plaster molding in the first-floor hallway. The results of laboratory studies using wet chemistry and heat also indicated that red ocher was used in these areas. This red color is also used in many buildings of other Ilkhanate periods. Carbon black has also been used in Soltanieh Dome decorations to encircle decorations and other colors. To identify the type of black pigment used, a sample of the desired color was placed on direct heat. After a while, it disappeared due to high heat (high temperature), which can be attributed to burning carbon, indicating the presence of carbon black. Thus, following the determination of the type of carbon black dye utilized, instrumental methods such as XRD were not used to identify its nature (to characterize it).
In summary, the results of this study using instrumental analysis suggested that the following dyeing materials were used:
Malachite and verdigris green for green, azurite for blue, and red ocher in the form of iron oxide for red, ocher for yellow, and anhydride and carbon black for white and black, respectively.