How did kiln firing make clay bricks stronger and more weather resistant?

Kiln firing transforms clay into durable building materials through a process of partial vitrification. As clay is heated in a kiln, impurities within the raw materials melt to form a glassy liquid phase. This liquid facilitates the bonding of particles and fills the voids between them, a process that reduces overall porosity and increases the material's density.12

This consolidation is critical for both strength and weather resistance:

• Strength: The glassy phase acts as a binder, while the growth of crystalline mullite needles within the matrix provides structural reinforcement, allowing the finished product to bear heavy loads.12
• Weather Resistance: By reducing porosity through liquid-phase sintering, the firing process makes the brick less permeable to fluids. This is essential for preventing damage from moisture and frost, as underfired bricks with high porosity are susceptible to failure in freezing conditions.12

Achieving the correct balance is vital; while higher glass content improves chemical resistance and water impermeability, overfiring can result in a product that is too brittle to withstand mechanical or thermal stress.2