Common glass is made from Silica Sand, Lime Stone and Soda Ash. It erodes when blasted with sand and this property is used to produce blind glass and to mark glass with figures etc. Sand blasting roughens the surface and scatters the light in different directions so it looses its see-through property.
The glass does not decompose in the atmosphere naturally unless amosphere is severely polluted but can be etched chemically using hydro-fluoric acid. Etched glass is produced by this method.
Glass is made from molten sand. It does not decompose and they only erode with wind, water and other forces. If you've ever found a smooth piece of sea glass at the beach, you have seen the erosion of glass. Rubbing glass against all the sand dulls the sharp edges of the glass and scratches the surface, making it less transparent. If left for a long time, it will completely disintegrate into the sand, but it won't decompose. It will still be glass, just broken into tiny grains, blending in with the rest of the sand.