Mechano-Chemical Scratching by Asperities

The IWCA and the Glass Committee have learned from the studies done by Penn State that the surface of window glass is very different from other glass. This matters to us as Window Cleaners because as it turns out window glass surfaces are much more prone to mechano-chemical wear. Especially when the atmospheric humidity reaches 90 percent. At this level the sodium atoms will exchange places with the hydrogen atoms in water molecules leaving the silicon based builder matrix of glass exposed. Then in time this begins to break down and dissolve. Of course we are only talking about an extremely thin layer of glass. All of this happens naturally without our intervention. Simply put we do nothing and it happens. However when it does windows become much easier to scratch. This is when the glass surface is under mechanical attack in a 90 percent or above humid environment. Which is always the case when we wet the window for cleaning. Further the higher the temperature the worse it gets. It is because of this that all glass surfaces should be sealed from the natural elements. Which would be another source of income for us. Some products are better than others. Of course every manufacturer says they have the best product. The burden then is on us to determine which one we want to use.

Cerium oxide works very well because it acts both chemically and by abrasion. Those who use it know too that it works best at very high RPMs. Which raises the temperature of the glass surface. As the cerium reacts with glass so does water. Which adds to the effectiveness of the abrasive properties. It doesn’t take much cerium in water to get results. Those surface technicians within our ranks such as Marc Tanner will use cerium as a final step in the stain or scratch removal process. It is true that they always use the highest quality. But the finger test always demonstrates that the new surface is in fact much less smooth then that of a quality brand new glass surface. This is because of what are called asperities in tribology. Which is the study of friction, wear, and lubrication, of interacting surfaces in relative motion. Asperities cause an unevenness of surface roughness. They are patterns of sharp, projections, which usually cover the entire surface. It is also very important to note that these also show on different scales of measure. Marc and I have had numerous discussions about the technology he uses and I have learned that it is very easy to make serious mistakes. I have also been called out many mes over the years on buildings that have been damaged by the wrong polishing technology. But not necessarily the wrong products! We all know you shouldn’t put a squeegee in the wrong hands. Yet streaks can be fixed. However abrasion haze (that can only be seen in the direct sun) can’t be so easily remedied. I always say do a single window and stand back in the direct sun;...and really look at it. If you are working on two floors do just one window on the second level and look real careful at it around three o clock. This isn’t a bad practice either when just cleaning windows. Especially if you are using a razor blade or anything else to remove anything such as paint or silicone caulk.

What is so easy ‘not to get’ is that smooth glass is never perfectly smooth. And we have all been on what I call ‘ice glass’. But all glass surfaces have asperities. When two seemingly smooth surfaces come into contact, they first make contact at just a few of the asperity points. Such ‘points’ might be so small that we could virtually count the atoms in each using an atomic force micro- scope. They also cover only a very small part of the total surface area. But. This is where friction and wear originate. Especially when two surfaces come in contact. The more pressure used the greater the friction and wear. Or in other words the greater the potential damage. By damage I mean scratches. So asperity points can cause scratches. Also the more humid it is (especially over 90% relative humidity), and the higher the temperature, the more likely it is that asperities will scratch glass. Especially if the pressure exerted by either a metal or even a plastic blade is high.

We have learned from this some very basic truths. First the natural chemical weathering of window glass gives rise to asperities and makes the glass much more prone to scratches. Next every glass surface has asperities which are the greatest reason for scratches on glass. Also humidity and high temperature increases the ability of asperities to create scratches. We also need a quick and easy way to determine the exact size and distribution of window glass asperities. Then we can demonstrate to the builder and home owner just how diffcult it would be to remove paint, wood stains, or silicone caulk from the windows without causing scratches. We would also be able to show the difference between the size and pattern of the asperities of different glass surfaces.