Clay, compositions firing behavior

In the fourth century B.C., the influential Greek philosopher Aristotle (384—322 B.c.) described the composition and behavior of matter in terms of the four qualities shown in Figure 3.1 hot, cold, moist, and dry. Although we know today that Aristodes model is wrong, it was nonetheless a remarkable achievement for its day, and people using it in Aristotle s time found it made sense. When pottery was made, for example, wet clay was converted to ceramic because the heat of the fire drove out the moist quality of the wet clay and replaced it with the dry quality of the ceramic. Likewise, warm air caused ice to melt by replacing the dry quality characteristic of ice with the moist quality characteristic of water. 

Forming techniques used for clay-based ceramics require control of water content in the batch. Water content, in turn, affects the response of the clay during forming [27], As the water content of the batch increases, the yield point of the clay-water mixture, and thus the force required to form the desired shape, generally decreases [26], However, the relationship is complex and depends on the composition of the clay, its structure, additives to the batch, and other factors [14], One method for quantifying the behavior of clay-water pastes is to measure the plastic yield point as a function of water content [14], The water contents and maximum yield points in torsion are compared for several clays in Table 9. Kaolins and plastic fire clays require the least amount of water to develop their maximum plasticity, ball clays require an intermediate amount, and bentonite requires the most. 

The performance of clay-based construction products, e.g., bricks and roofing tiles, can be monitored using thermal methods. The types of raw materials, viz., clay and accessory minerals, and their reactions that occur during the firing process and the durability of clay products can be examined through the application of DTA, TG, TMA, and dilatometric methods. This is particularly important for quality control as physical and chemical behaviors are dependent on the raw material characteristics, e.g., composition, particle size, and morphology. 

Ceramics - Whitewares use a combination of china clay (usually airfloat) and ball clay selected for consistent chemical composition so that firing and vitrifying characteristics of the body are controlled and color and translucency of the fired ware are satisfactory. The ratios used are chosen to optimize green strength, plasticity, and casting behavior. 

A field of growing attention is the development of polymer-clay nanocomposites due to the significant technological applications of these materials. Therefore, composite systems produced by organic polymers and clay minerals prepared at the nanoscale level, which typically present a unique layered structure, rich intercalation chemistry and availability at low cost, have been used to develop plastic materials with superior mechanical properties, molecular barrier behavior, fire retardant abilities, enhanced thermal stability, among other properties, compared to the individual polymeric materials [3-5]. 

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