Glass block has expanded its market thanks to extra fire resistance, colored block, and a new installation system. EXTRA FIRE RESISTANCE Tests by Underwriters Laboratories (UL) show that 6x6-inch clear glass block can withstand the ASTM E 163 fire test for window assemblies for 45 minutes. NO-TROWEL INSTALLATION SYSTEM Using a patented polyvinyl chloride (PVC) formwork system, masons can lay up glass block walls and floors fast and with uniform joints. COLORED GLASS BLOCK Most colored glass block are not made of colored glass. The inside of each hollow block is sprayed with a colored coating that is baked (annealed) onto the inner surface. The block itself is clear glass, giving the color a soft glow. Hundreds of colors are available.