Bentonite is highly objectionable in the raw material mix for white ware
because due to its high water absorption and high swelling power, it increases the filter
pressing time for the raw material mix during mould preparation, and only a very small
quantity (less than 1%) is sometimes added to the raw material mix for white ware, mainly
with a view to improving plasticity. But, sodium bentonite is used in the glazing mixture. The
semi-finished ceramic bodies are dipped in the glaze and fired at 100-13000C. The glaze
should be deposited on the surface of the body uniformly, and for this purpose it is necessary
that the ingredients of the mixture do not settle down but remain in suspension for a long
time. The role of bentonite is to help in this by virtue of its high dispersion.
94 Kaulir Kisor Chatterjee
In 1988, the Bureau of Indian Standards (BIS) has prescribed a set of specifications the
critical parameters of which are: size: (-) 45 micron; Fe2O3: 4% (max); TiO2: 3%(max);
Fe2O3+ TiO2: 6% (max); CaO: 3% (max); MgO: 3% (max); CaO+MgO: 5% (max).
Fe2O3 has a colouring effect. TiO2 (over a period of time) makes the product coloured and
so they are objectionable. Besides, TiO2 has a high melting point and it will unnecessarily
increase the firing temperature.
Lime (CaO) is highly hygroscopic. So, if it is present in clay, the product will absorb
water over the course of time on exposure, and ultimately, crumble. Also, at 11000 C (i.e.
below the firing temperature), CaO reacts with alumina and silica and forms new compounds,
mostly silicates. Some of these silicates lower the fusion point of clay. Also, if lime is present
in the form of CaCO3 or CaSO4, then CO2 or SO3 is expelled on heating, and the ware is left
more porous. Finally, lime makes the melt more fluid and it reduces the range between
softening and flowing temperature. Sometimes this range may be as short as 400C only. The
result is that it becomes difficult to control the temperature of the furnace to remain within
this range. For these reasons, lime is very objectionable in bentonite. MgO is also highly
refractory in nature and so is undesirable in white ware, because white wares, by definition,
are non-refractory in nature. Besides, it is hygroscopic, absorbing 120% of its volume of
water slowly over a period of time.