کاربرد تحلیل‌های بلورشناسی در مطالعات فنی آثار تاریخی گچی (مطالعه‌ی موردی گچ بری کوه خواجه‌ی سیستان، ملات گچ شادیاخ نیشابور و ملات گچ قلعه الموت قزوین)

Investigation of gypsum-based historical materials has two main problems: philological problems and technical issues. Technical issues, the main topic of this article, are normally consequences of physicochemical complexity of gypsum crystals in different conditions and variety of additives or mixed materials. Neglecting this technical problem, leads researchers to misunderstand or misinterpret these materials. Moreover, exclusively use of one analytical method, such as scanning electron microscope or X-ray fluorescence, provides an incomplete picture of samples that regularly produces further misunderstanding of specimen or its history. The methodology of this study is a multianalytical approach via XRD, SEM and using image analysis software on electron photomicrographs to estimate amount of free spaces in microstructure of samples. A group of three experimental samples were made from traditional recipes of craftsmen in Iran, in order to make a connection between philological problems came from tradition, ad technical problems came from mineralogical property of gypsum. Gach-e Tiz, or quick setting mortar shows a very dense microstructure in SEM micrographs. Variety sizes of monoclinic crystals of gypsum are present in it, and moreover some shapeless and bulky regions are also visible. Gach-e Kam-Mayeh, or low setting mortar reveals a much more open microstructure than the former one, filled with needle like crystals and a lot of micro porosity between crystals. There is no evidence of bulky forms area in Gach-e Kam-Mayeh. Gach-eKoshteh, or mechanical low-setting mortar which is a traditional Iranian recipe, shows a parallel layers of laminar gypsum crystals setting upon each other. This form also demonstrates no evidence ofbulky regions. Historical samples were collected from a stucco decoration belong to Kuh-e Khwaja ofSistan; a gypsum mortar fragment from Shadyakh, Neyshabur and a piece of gypsum mortar from Alamut castle of Qazvin. The results of XRD show that both layers of Kuh-e Khwaja stucco containanhy r te as the major phase and the sample of Shadiyakh mostly composed of Gypsum. Organic materials extractions represent no evidence of organic additive to this historical materials. Thereforethe images of their microstructure are comparable with experimental samples that made without anyadditive. Lower layer of Kuh-e Khwaja’s stucco shows a lot of tiny crystals that are not very look likegypsum, but precipitated on the surface of bigger and deformed crystals. The microstructure of it isvery open and include big amount of micro porosity in compare with other samples. Upper layer ofthis stucco shows not any form of gypsum crystals but very disturbed flaked shapes, supposedlygypsum crystals transformed to anhydrate. The micrographs of Shadiyakh’s mortar also reveal not anyclear form of gypsum crystals but base on XRD result it still as gypsum materials. This samplecontains mostly from that bulky and dense structure which shows the usage of low amount of waterfor its paste. The results of image analysis on electron micrographs at magnification of ×250 and×500 provide a good and comparable estimation for free space evaluation in the micro structure ofgypsum based materials. The combination of these tniques and methods lead to betterunderstanding of gypsum based historical material