Contractors can increase their business by learning the special techniques glass block requires. Here are some of them: Store glass block carefully. Isolate the sill, head, and jambs of the surrounding construction from the glass block panel. Use Type S mortar, as specified in ASTM C 270, Specification for Mortar for Unit Masonry. Because glass bonds weakly, it's important to use a high-bonding mortar, even if the blocks have an edge coating that improves bond. Lay glass block in a stack bond pattern. Install reinforcing in horizontal joints of panels larger than 25 square feet. Attach glass block panels to heads and jamb of surrounding construction with panel anchors. Seal perimeter expansion joints with a nonstaining sealant, such as silicone or urethane. Clean the faces of the glass block with a wet sponge or a stiff-bristle brush.