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Archaeometrical studies on plasters of some historical buildings

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2007
Çiftçi, Burcu Devrim
The present study aims to investigate the composition of historical plasters to get information about their material characteristics and their technology. Plaster samples were obtained from four Ankara Citadel houses built in late Ottoman period. In order to determine the raw material characteristics and mineralogical properties of plasters; chemical analyses, optical observation of cross sections, petrographic analyses of thin sections, elemental analyses by ICP-OES, X-ray powder diffraction analyses for the determination of mineral phases, thermogravimetric analyses and FTIR analyses were carried out. Interpretation of all the analytical examination was used to understand the composition and unique character of plaster samples studied. Observation of thin sections revealed more plaster layers than those observed in cross sections. Up to twelve layers could be observed with different colours, such as blue, red, yellow, green, white and brown. Generally, thicknesses of white plaster layers were found to be thicker than the others. In two samples, two black boundaries between plaster layers were identified which could be an indication of the use of asphalt for isolation purposes, like dampness proofing or heat insulation. Soluble salt contents of the plaster samples were in the range 3.04%-9.22%, with an average being 6.62%. The anions identified were Cl-, SO42-. In few samples, PO43-, NO2- and NO3- were found. Binder was found to be lime and gypsum. The amount of binder in terms of total calcium oxide, CaO, was found to be in the range of 33.5-43.6%, with an average being 37.9%. Amount of aggregate was about 62.1% as average. The main minerals identified in plaster samples were calcite and gypsum. Gypsum might be added to increase the strength of the plaster. Beside calcite and gypsum, quartz and pozzolanic activity related mineral, Opal-A, were found in some of the samples. In red plaster layers hematite mineral was also identified. Other colour effective elements were found to be Fe, Sb, Mn, Cu, Cr and Ni. Presence of organic additives was observed but clear identification was not established.