When we fire the clay at high temperature the foreign oxides in the clay along with free Silica form a glass which can be detected by its transformation point in thermal expansion curves. This glass is responsible for the loss of in mechanical strength of fired clay at the softening point and the body deforms under load.
The glass formed acts as a bond at low temperature and increases the strength and is the primary responsible factor for all the properties of ceramic products. On rapid heating small amounts of not very viscous glass is formed where as on slow heating large quantities of highly viscous glass are formed.
Glass formation in the body during heating can be traced out by the addition of CoO which yields a blue colour only when dissolved in a glass. Alternatively this can also be traced out by the addition of Uranium Oxide which form a yellowish green fluorescent glass.
.. to be contd.
