Low Iron Opacity Glass Stair Treads. The walking surface of glass stair treads should be safe and durable. For this application, Nathan Allan developed “Glass Sandpaper”, an exclusive slip resistant safety feature, designed specifically for glass stair tread surfaces. Real glass particles, which do not easily erode, are fused onto the walking surface of the top glass layer, to create an extra grip for shoe treads. Tested in laboratories for slip co-efficient friction ratings, our Glass Sandpaper produced the highest test results in both wet and dry conditions (product data is available), which allows for indoor and outdoor installations.