How does firing bond the particles in structural clay products?

Firing bonds particles in structural clay products through a process known as partial vitrification. When these products are heated in a kiln, the clay or shale ingredients—which contain various impurities—melt to form a glass phase.12

This glassy liquid acts as a matrix that bonds the remaining solid particles together. During this process, crystalline mullite needles grow into the glassy phase, creating a microstructure of large filler particles (typically quartz) embedded within this fine-grained matrix.12

The bonding is further facilitated by liquid-phase sintering, where the liquid promotes a denser rearrangement of particles. Additionally, a process of solution-precipitation occurs, where small particles and the surfaces of larger particles dissolve into the liquid and then reprecipitate at the contact points, or "necks," between larger particles. This creates a permanent, strong bond that provides the finished product with its characteristic load-bearing strength and durability.12