Wallhuggers comments are right on! On a few of the artsy modern homes I've done, they have a steel railing on a staircase next to massive windows. Needless to say, those areas near the welds have spatter on the glass. The same goes for concrete patios with sliding glass doors. If you try to scrape either of them, IT WILL SCRATCH! I can feel the pain of the homeowner when I tell them that glass is ruined because either one will pit the glass so badly that scratch removal is usually not feasable.