Martina Kohoutková

Identification of causes of degradation of Gothic ceramic tiles by thermal analyses

Abstract Thermal analyses are suitable methods when studying not only nowadays, but also archaeological ceramic materials. In this case, they were used to study the mineralogical composition and the ageing process of a collection of fragments of Gothic mosaic tiles which were found during archaeological research of Břevnov Benedictine Monastery in Prague. The ceramic bodies of the tiles were differently coloured and showed different degrees of degradation. The ageing process of low-fired ceramics is accompanied by irreversible moisture expansion of a ceramic body which can lead to its degradation. The expansion of ceramic bodies of selected tiles was evaluated by the modified dilatation method. Thermal analyses with the combination of other methods (XRF, XRD and IR) and induced hydrothermal treatment proved the presence of clays in raw materials and helped to identify the causes of different degrees of degradation of the studied tiles.

Kinetic studies on leucite precursors

Kinetic studies were performed on two types of leucite precursors. These precursors were prepared using a hydrothermal method at 150°C; the reaction time was 1.5 hours. In order to obtain precursors having different amounts of seed the molarity of KOH was changed. These intermediate products were subsequently calcinated from 5 minutes to 72 hours at temperatures of 850°C, 900°C, 950°C, 1000°C and 1050°C. The crystallinities of the powders were calculated by X-ray diffraction analysis. The crystallization curves for the synthesis of leucite exhibited a typical sigmoidal characteristic. Using different kinetic equations it was found that the Avrami-Eroféev model is the most appropriate to describe the experimental data. Using the Avrami-Eroféev model n reaches an average value of 2.9 which is connected with the three-dimensional growth of nuclei. The calculated activation energy of crystallization of leucite was 385 kJ mol-1 for non-seeded precursors and 246 kJ mol-1 for seeded precursors, respectively.

Preparation of Leucite Powder with Controlled c-Phase Fraction for a Variety of Dental Applications

Leucite (KAlSi2O6) is the main crystalline phase in feldspathic dental materials used for ceramic-fused-to-metal restorations. It occurs in two modifications, low temperature – tetragonal and at temperatures above 600 °C high temperature – cubic modification. The aim of this work was to develop a low-temperature preparation technology of submicron leucite powders with varying ratio of tetragonal and cubic modification. The results show that analcime synthesized in hydrothermal conditions is the suitable precursor for the preparation of leucite with controlled fraction of the c-modification. Homogenous t-leucite having particle size from 2 - 4 μm was prepared by 4h ion-exchange in 4M KCl. Partially stabilized c-leucite was obtained by dual ionexchange; 4h in 4M CsCl led to 48 % of c-modification in the final product.