How does feldspar affect the firing of bricks and tiles?

In the production of bricks and tiles, feldspars serve as essential fluxing agents. These aluminosilicate minerals—which contain sodium, potassium, or calcium—function by reducing the melting temperatures of the aluminosilicate phases present in the clay mixture.12

By lowering the temperature at which melting begins, feldspars facilitate the process of vitrification, which is the formation of a glassy liquid phase during firing. This liquid phase is critical for the consolidation of the ceramic material, as it promotes:2

• Particle Rearrangement: The liquid allows for a denser packing of particles through viscous flow.2
• Bonding and Densification: Through a process of solution-precipitation, the liquid phase helps dissolve smaller particles and surfaces, which then reprecipitate to form "necks" between larger particles, effectively bonding them together.2
• Porosity Reduction: This densification process reduces the overall porosity of the fired object, contributing to its final strength and impermeability.2

While impure clays used for common structural bricks may contain natural fluxing agents, the addition of feldspar is a key method for controlling the firing behavior and final properties of ceramic products.13