How / what scratches windows? All substances can be rated according to their "scratch ability," that is, if a specimen can be scratched by a known mineral, it is softer than that mineral. If it in turn will scratch another known mineral, it is harder than that mineral. One formalized rating system of scratch hardness is the famous Mohs scale, which establishes a comparative ranking of common minerals with diamonds rated at the top as a 10 and talc rated as the softest at number 1. Other common household items can be ranked in this system as well such as:
2.5–3 Gold, Silver
3 Copper penny
5.5 Knife blade
6.5 Iron pyrite
For purposes of the question at hand, substances with a higher Mohs rating higher than 6.5 will tend to scratch glass; substances with a lower rating won't. While the brass, plastics and rubbers used by window cleaners are never suspected of scratching glass, many people automatically assume that the razors on every window cleaner's tool belt are suspect. But a close examination of the Mohs scale clearly shows that steel (razor blade) and glass are very close to one another in hardness. It's a fact that razor blades simply can't scratch glass. Many window cleaning conventions will feature a booth where participants are given a razor and a sample piece of glass which they are asked to intentionally scratch. Even when the glass is subjected to punishment that would never be seen in the course of any window cleaning, scratches can't be produced.
So what does? Using the relationships demonstrated in the Mohs scale, anything with a higher number than glass will scratch it. That includes aluminum oxide (sand paper), ceramics and corundum. Many common building materials such as mortar and stucco contain substances that will quickly scratch glass as well. The window cleaner is the last in a long line of trades people who have come in contact with glass in new construction environments (carpenters, painters, masons, floorers, and drywallers), they are often assumed to be the culprit when in fact, they only uncover what has been hidden by the dust, mud, dirt and grime that are prevalent on every construction job site.
There is one other source of scratched glass that is relatively new and is becoming alarmingly more pervasive: an increasing problem with defects in poorly tempered glass known in the industry as Fabrication Debris. "Fab Debris" consists of microscopic particles left on glass as a result of tempering--a re-heating process used to make shatter resistant glass for use in shower doors, entryways, doors and windows. As building codes have been revised to increasingly require tempered glass, the demand for tempered glass has also increased, putting pressure on tempered glass fabricating processes. If tempering ovens are not meticulously maintained, they can cause microscopic particles (known as "fines") to be left on the roller side of tempered glass as it moves through the oven. Undetectable by the unaided human eye, these fines become ticking time bombs.
When dislodged during normal cleaning processes, this fabrication debris is dragged across the glass, leaving hundreds of tiny scratches, which are many times not noticeable except under specific lighting conditions.
Because of the increasing pervasiveness of this problem, many window cleaners are now requiring signed waivers before doing any work on tempered glass.
Click here to learn more about tempered glass