Clay and vitrification: pick the incorrect statement about fluxing, composition, sand addition, and why fireclay is vitrified during manufacture.

Introduction / Context:
Vitrification is the process of forming a glassy phase during firing that bonds grains, reduces open porosity, and improves strength and impermeability. Understanding how fluxes and fillers modify firing behaviour is central to ceramic engineering and refractory design.

Given Data / Assumptions:

• Lime and other fluxes lower the softening/vitrification temperature of clays.
• Clay is primarily an alumino-silicate material.
• Sand (silica) addition reduces plasticity and can raise fusion temperature while limiting shrinkage.

Concept / Approach:
The goal of controlled vitrification in fireclay refractories is to decrease open porosity and develop strong ceramic bonding between grains at service temperatures. Statements that claim vitrification increases porosity are therefore incorrect, because the opposite occurs when a glassy phase infills pores and necks particles together.

Step-by-Step Solution:
Evaluate (a): True — fluxes lower vitrification temperature.Evaluate (b): True — alumina and silica dominate clay chemistry.Evaluate (c): True — sand lowers plasticity, raises fusion point, reduces shrinkage.Evaluate (d): False — vitrification reduces open porosity and improves strength.

Verification / Alternative check:
Processing texts show porosity decreasing with well-controlled vitrification, improving modulus of rupture and permeability resistance.

Why Other Options Are Wrong/Right:
Only (d) contradicts fundamental ceramic processing principles.

Common Pitfalls:
Overfiring can cause bloating or excessive glass which may trap pores; however, the intent of vitrification is to reduce porosity, not increase it.

Final Answer:
Vitrification of fireclay material is done to increase its porosity.