We have discussed the Water-Clay relationship and the plastic and drying behaviour of clay. Now we will discuss the behaviour of clay when subject to firing.
When subject to high temperature, clay undergoes various changes in its properites. These varying properties of clay on firing is a very important aspect of clay in the process of ceramic manufacturing. Be it tableware, wall tiles, electrical porcelain, sanitary ware, floor tiles, fired clay products, advanced ceramics, bricks, roofing tiles, stone ware ceramics or any ceramic article - All these heavily rely on the behaviour of clay when fired.
When plastic clay is dried in room temperature, it becomes rigid however it is fragile. When it is fired it becomes hard and durable.
Upto 150oC, the residual water (also called mechanically combined water) gets expelled.
Between 450oC and 600oC, chemical decomposition takes place and the chemically combined water gets expelled. For pure (true) clay, this will be around 14% of the weight of the dry clay. However this varies a lot from clay to clay and depends on the mineralogical composition of clay.
Between 350oC and 850oC, contaminants like organic matter, carbonates, sulphates, chlorides etc get decomposed and the organic matter is burned away expelling carbon di-oxide, sulphur etc.
Between 900oC to 1000oC, vitrification starts. For some clay, this can extend upto 1500oC. Clay looses its porosity, shrinks fully, attains its full mechanical strength and enters its next stage as a result of the different pyro chemical reactions it went through when fired.
