Clay and Ceramic Materials

Three main properties render clay suitable for making ceramic materials its plasticity when wet, its hardness when dry, and the toughness, increased hardness, and stability that it acquires when fired. The addition of water to dry clay produces a clay-water mixture that, within a narrow range of water content, has plastic properties it is deformed, without breaking or cracking, by the application of an external stress, and it retains the acquired shape when the deforming stress is removed. Wet clay mixtures can, therefore, be modeled, molded, or otherwise made to acquire a shape that will be retained after the forming operations. Water-poor mixtures are not plastic, however, and excess water results in mixtures, known as slips, that are too fluid to retain a shape, as shown in Table 56. 

As for the drying properties of clay-water mixtures when wet clay dries at ambient temperature, the water of plasticity that surrounds the particles 

FIGURE 51 Drying clay. The illustration shows a schematic, highly magnified cross section of a mass of wet ciay at three different leveis of water contents. Initiaiiy, aii the space around the ciay particies is filled with water (a). As the wet mass dries up, water is lost from between the particles, which gradually come nearer to each other (b), and the clay mass shrinks in size. When all the water has evaporated, the particles are in contact with each other and the mass cannot shrink any further (c). From this point onward the bulk volume of the clay mass does not change. 

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The types of substance that are thermoluminescent, either in their natural state or after radiation bombardment, include (112) the alkali metal halides, calcite, dolomite, fluorite, aluminum oxide, magnesium oxide, gypsum, quartz, glass, feldspars, feldspathoids, certain dried clays, and ceramic materials. Of over 3000 rock samples examined for thermoluminescence, some 75% showed visible fight emission (112). Nearly all limestones and acid igneous rocks are naturally thermoluminescent, due mainly to the presence of trace elements of uranium, thorium, and so on. Calcium and magnesium... 

Clays and Ceramics from Sites near Hesi. Analyses of the materials from sites neighboring Hesi have been disappointingly uniform. The... 

Porcelain is a special kind of ceramic. Porcelain is made from pure white clay and other materials. It is translucent, waterproof and is used for making chinaware. 

Worrall, W.E. Clays and Ceramic Raw Materials, Applied Science Publishers London,... 

Glass, clay, and ceramics are also natural materials, but they are not typically used in the fabrication of prototypes. 

For clay and ceramics extruders or pug mills clay, materials for bricks, tiles and ceramics. 

In British Standards, definitions usually relate to the content of a particular standard. BS 5416 defines ceramics as materials generally made from a mixture of clays and other materials, distinguished from glass and glass ceramics by the fact that they are first shaped and then rendered permanent by firing at a temperature generally well in excess of 1000°C. ... 

Assessment of acid resistance is important for chimney linings, incinerators, and heavy clay and ceramic kiln linings. Methods such as BS EN 993-16 1995 (2001) and ISO 8890 1988 (1998) assess a material s resistance to attack by a boiling/ condensing method. 

After drying, the bricks ate put into a kiln where the temperature is raised slowly to between 870 and 1316°C or higher depending on the temperature needed to fuse the clay. With the clay particles pattiaHy melted and fused together, the brick is a ceramic material with exceHent strength and fire resistance. 

Composition. Among the most commonly used support materials are aluminas, siUcas, and aluminosihcates with a wide range of alumina to sihca ratios, as well as activated carbon, siUcon carbide, selected clays, various ceramics, artificial and natural 2eohtes, kieselguhr, and pumice. Polymeric... 

Ceramics and minerals present many common problems, but ceramics warrant special treatment because elements of low atomic number predominate in them and they consequently offer x-ray emission spectrog-raphy of the light elements an excellent opportunity to prove its usefulness. Scott,8 in making this clear, emphasized the absorption and enhancement effects to be expected, and pointed out the need for careful sample preparation. By use of a General Electric XRD-5 spectrograph and associated equipment, he set up working curves for alumina, silica, potash, lime, phosphate, titania, and iron oxide in clays, refractories, and other ceramic materials. 

With the following example, we illustrate how in a sequence of activities the students intuitive notions about the influence of particle size and the sintering temperature of the clay on the properties of ceramic materials have productively been used (Klaassen Lijnse, 1996 Mortimer Scott, 2003 Duit Treagust, 2003 ... 

Catalysts can be metals, oxides, sulfides, carbides, nitrides, acids, salts, virtually any type of material. Solid catalysts also come in a multitude of forms and can be loose particles, or small particles on a support. The support can be a porous powder, such as aluminium oxide particles, or a large monolithic structure, such as the ceramics used in the exhaust systems of cars. Clays and zeolites can also be solid catalysts. 

Swollen clay materials are soft and easy to mould. They serve to produce ceramic materials. High quality fire-clay has a high kaolinite content. Upon firing, the intercalated water is removed first at approximately 100 °C. Then, beginning at 450 °C, the OH groups are converted to oxidic O atoms by liberation of water, and after some more intermediate steps, mullite is formed at approximately 950 °C. Mullite is an aluminum aluminosilicate, Al(4 )/3[Al2 Si,05] with x 0.6 to 0.8. 

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