California glass expert performed a failure analysis on several broken glass tiles. These tiles were above the water line in a swimming pool. The failed tiles were removed and submitted for failure analysis. It was impossible to open the tiles to examine the fracture surface. The thin set mortar had “squeezed” into the crack and was firmly holding the tile pieces together. The tiles were soaked overnight in muriatic acid to allow separation. They could then be pulled apart to expose the fracture surface for failure analysis. The figure above is an 8X photomicrograph of a representative fracture surface. All the failures initiated on the backside of the tile. It appears that the glass tile backside was forced against a small hard particle during mounting in the thin set mortar. The failure initiated at a point on the glass tile backside and spread into the tile (indicated by the arrows). The root cause of the glass fracture was impact against a hard particle, The fact that the mortar had squeezed into the crack indicated that the glass tile failure occurred during mounting. Perhaps the tile setter was using a mallet to align the glass tiles.