Porcelain phase composition

The experimentally established phase composition of porcelain is in good agreement with the values obtained by analyses of the phase diagram. For the various types of porcelain, the contents of the individual phases lie in the following ranges ... 

The facts mentioned above indicate that one of the ways to increase the mechanical strength of porcelain is to change the glassy phase composition in order to increase its thermal expansion coefficient, and thus reduce the microstress at the boundary with quartz. Practical experience shows that the same effect is obtained just by reducing the quartz grain size the small quartz grain size suppresses the adverse effects of microstresses at the boundaries. 

Ceramic materials are divided according to various aspects, for example, according to chemical and phase composition, porosity (dense and porous ceramics) and structure (coarse and fine ceramics). The group of fine ceramics includes, for instance, porcelain and technical ceramics, whilst the coarse ceramic materials are represented by brickmaking products and by refractories for the construction of industrial furnaces. According to the purpose and fields of application, ceramics are also divided into building ceramics, technical ceramics, domestic ware, refractories, etc. 

Qz = quartz Tr = tridymite. The ternary eutectic is at the triple point Kfs-Mu-Tr with the melting temperature of 985°C. This is also the equilibrium phase composition of porcelain. 

Because the mix used to form the bodies that are fired to produce the products are made in this manner from naturally occurring mixed or quarried materials, the chemical analyses of the bodies varies considerably, not only from manufacturer to manufacturer, but from year to year, as the deposits of the basic materials will vary, and compositions can only be indicated in general or relative terms. What they do determine is the crystalline phases of the mineral content, and all analytical data is based on the identity of these phases. The only characteristic that is an absolute with porcelain brick (or tile) in comparison with other forms of fired chemical-resistant blocks is zero porosity. 

The composition of raw materials mixes used to obtain the mentioned products and stoneware, composed of still more glassy phase than porcelain, is shown in Fig. 2.6. 

A typical porcelain body has the composition 50 clay-25 feldspar-25 quartz. Sketch an expected microstructure of such a body, indicating scale when (a) fired to achieve phase equilibrium (1450°C for 6 h) and (b) fired to 1300°C for 1 h. 

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