Issue No. 2

Abstract

Four clay samples from Mayouom deposit were studied to evaluate their potential uses in the ceramic industry. Characterization of crude clays involved chemical and mineralogical composition, plasticity and particle size distribution. Fired properties of clays were measured in terms of densification parameters viz linear firing shrinkage, bulk density, open porosity and water absorption. The samples denoted as MY38, MY41g and MY45g were found to be poor in sand and highly plastic, whereas MY45s is sandy and has low plasticity. The alkali and alkaline earth oxides compositions are low (< 0.9 %) while the colouring oxides (TiO₂ + Fe₂O₃) are 0.4- 6.1%. In all the samples, kaolinite is the dominant mineral associated with quartz, illite, goethite, apatite, rutile or anatase. Test-discs from sand-poor samples show important linear firing shrinkage vitrifying above 1300°C, whereas the shrinkage in MY45s was small, its ceramics also vitrified at high temperatures. These properties show that these clays can be successfully used as refractories. In addition, due to its white firing colour, MY45s can be very suitable for use in the production of porcelain and vitreous china.

Abstract

The possibility to obtain a cheap ceramic material based on 50 wt % basaltic tuffs and 50 wt % industrial clays was studied. The tuffs were characterized by optical microscopy, X-ray diffraction and differential thermal analysis. It was shown that this material is appropriate for the ceramic industry: it is easy for milling and is characterized by low liquidus temperature. The sintering behaviour of the ceramic was evaluated by hot-stage microscopy and dilatometry. The structure of samples, heat-treated at different temperatures, were observed by scanning electron microscopy while the densification degree was evaluated by dry flow and gas pycnometery. Samples with 13% total porosity and 4% water absorption were obtained at 1150 °C for 30 min soaking.

Abstract

In this study, conventional ball mill and hybrid (roll crusher + ball mill) grinding techniques were compared in the preparation of porcelainized stoneware tile body mix in terms of energy consumption. In the conventional ball mill grinding technique, raw materials for porcelainized stoneware body mix were ground in the fast mill alone. In the case of hybrid grinding technique, the same material was first crushed using roll crusher and then subsequently ground in the fast mill. Specific energy consumptions were determined for both techniques. Comparing the results it was found that hybrid grinding appears to offer considerable energy saving over conventional ball milling alone. In this example energy saving was about 40%. Some of the important technological properties of products obtained from conventional and hybrid grinding techniques were
found to be similar indicating that hybrid grinding offers energy saving without altering technological properties of the products.

Abstract

Feldspar porcelains are ceramic materials that allow full or partial dental ceramic restorations similar to natural teeth, combining aesthetics (color, shape and texture) to mechanical strength. The ceramic restorations are made of a ceramic support (alumina, zirconia, spinel or glass-ceramics) on which layers of porcelains with pigments are applied and fired to provide color, translucency and fluorescence. In the present work, the ceramic support was a densely sintered alumina plate for an easier colorimetric evaluation using CIELab method. A previous manufacture of finely milled frits for each different batch of porcelain was essential, in order to obtain a nice layering by firing at relatively low temperatures (900 to 980 °C). Among four different formulations (pigments and frits) for porcelain layers using Brazilian raw materials, the so named B1 and B2 were those which presented shades and textures near to dentine-3 reference.

Abstract

The paper presents results of investigations on the application of glass-ceramic to bonding the CBN grains into abrasive composite. The application of the glass with specific chemical composition makes it possible to generate the designed crystalline phase in it. A type of crystalline phase has a significant influence on the elastic modulus.

Abstract

A l₂O₃/TiC ceramic composites were produced by hot pressing for the use of wire-drawing dies. Performance of these ceramic drawing dies was tested with a universal tensile testing machine on 65Mn steel wire. The wear surface features of the ceramic drawing dies were examined by scanning electron microscopy. Finite element method was used as a means of numerically evaluating stress and its distribution inside the ceramic dies during wire-drawing processes. The wear mechanisms of the ceramic drawing dies were investigated. Results showed that drawing speeds had no profound effect on the drawing forces; while the lubrication conditions had great influence on the drawing forces. The ceramic dies exhibit brittle fracture with evidence of cracks and scale chipping on both sides of the corners at the approach zone. Greater compressive stresses were found to be the main reason that caused the failure at these areas. While at the centre bore area of the approach section, the ceramic dies showed plowing features, and the primary wear mechanism at this area was abrasive wear. Examination of the center surface at the bearing zone demonstrated that the wear occurred by small abrasive. Brittle fracture was found to be the primary wear mechanism at the exit corner of bearing section owing to the greater stress concentration.

Abstract

The 11^th century Greek monasteries of Dafni and Hosios Lukas exhibit excellent mosaic decorations covering ample surfaces and evaluated as the most important of the mid-Byzantine period. They are the examples of the iconographic and stylistic conceptions of their time, generated at the end of iconoclastic crisis of the Church of Constantinople. The notable artistic preciousness of the mosaics is created by the lights and shades effects obtained by the great variety of the colours of the glass tesserae. A selection of samples of different colours has been examined through chemical analysis techniques in order to investigate the raw materials and the technology of production. The glasses are of the plant-ash type, belonging to a time when the use of natron was abandoned in the Mediterranean area. The comparison between the compositions of the original glasses from the two sites has shown that the glasses, although of the same type, were produced with different raw materials; their compositions have been also compared to those of the first Byzantine period from S.Vitale in Ravenna (Italy).