Читать книгу Monument Future - Siegfried Siegesmund - Страница 156

For the restoration work locally available material is used. The raw material for the plaster production comes from a quarry some 17 km WNW off the Takht-e Soleyman. Two slightly different qualities of gypsum with respect to the contents of non sulfate accessory minerals are mined. The gypsum rock contains dolomite (ankerite), calcite, quartz, feldspar, clay minerals and celestite as accessory minerals which can sum up to 8–10 mass%.

The firing of the raw material is done in the traditional way as it was done many centuries ago (Soleymani, Pirak 2012, Sobott 2018). A shaft furnace built of bricks with a diametre of 1.90 m was sunk 2.90 m deep in the ground. The quarried gypsum lumps are piled up in the furnace in such a way that something like a corbeled vault is formed. Large pieces of gypsum rock are on the inside of the construction facing the firing chamber and small pieces are used to fill the space between the furnace wall and the rock pile. The apex of this artful cone-shaped construction surmounts the upper end of the furnace (Figure 4). A mix of combustible material, mostly wood, is piled up inside the vault.

Once the fire is lit the uncontrolled firing process lasts about 8 hours. Due to the construction of the furnace the temperature distribution in the gypsum filling is extremely variable. Thermocouples installed at different positions in the gypsum pile showed that the temperature difference between the central part and the margin may be as great as 800 °C so that the gypsum lumps are exposed to temperatures ranging from 200 and 1,000 °C (Jäger 2017, Jafarpanah 2017). The cooling period after the extinction of the fire lasts about 24 hours. Then the fired material is removed from the furnace and crushed with large hammers by hand at which high fired gypsum lumps disintegrate easily into powder while low fired lumps break into smaller pieces. The crushed material is sieved and filled into plastic bags. For use at the construction site the material was sieved to discard grains larger than 2 mm. The phase composition of the fired product is variable and depends mainly on the grain size and composition of the raw material, the firing temperature and time, and the resulting partial water vapour pressure on the surface of the lumps. For the assessment of the firing results, 116gypsum pieces adjacent to thermocouples were sampled and studied in situ by colouring tests and in the laboratory by polarized light microscopy of thin sections, X-ray diffraction and thermoanalysis (DTA, DTG). In order to improve the data interpretation of the field samples, experiments with raw material from the gypsum quarry were carried out under controlled temperature and time conditions in the laboratory and the resulting samples analyzed in the same manner.