Question

Once a piece of clay has been fixed to above $500^{\cir}C$ it will be completely dehydrated and it will also have lost -

• A. its colour
• B. its shapes
• C. its shrinkage
• D. its plasticity

Hint:

Logic Tip: "Ceramic change" occurs roughly around $500^{\circ}C$ - $600^{\circ}C$. The clay becomes a permanent rock and can never become plastic mud again.

Answer 1

The Correct Option is D

Concept:
Firing clay transforms it from a workable, water-soluble mud into a permanent, hardened ceramic material. This transition is tied to chemical changes at specific temperatures.

Step 1:
When clay dries in the air, physical water evaporates, but the clay can still be slaked down (dissolved) back into workable mud. This means it retains its plasticity.

Step 2:
As the kiln temperature passes $350^{\circ}C$ and reaches between $500^{\circ}C$ and $600^{\circ}C$, a process called "ceramic change" or "dehydroxylation" occurs. The chemically bound water (the $2H_{2}O$ in the kaolin formula) is forcefully driven out of the crystal lattice.

Step 3:
Once the chemical water is gone, the molecular structure of the clay is permanently altered. The flat, slippery platelets that allowed the clay to bend and stretch without breaking are fused.

Step 4:

• The clay does not lose its shape; it locks into it.
• It may change color, but it does not "lose" color entirely.
• It will actually undergo more shrinkage later during vitrification.

Step 5:
Because the molecular change is irreversible, the clay permanently loses its ability to be molded and shaped with water. This property is known as plasticity. \[ \boxed{\text{(4) its plasticity}} \]