“Surfaces that Scratch”

Have you ever cleaned a window and then lightly ran a dry fingertip across the dry clean glass? If you make a practice of doing this in public you will definitely get attention. But you will also learn something. You will discover how the surface of even brand new glass can be quite different from one window to the next. Even the same sheet of glass can be more rough in one area yet quite smooth in another. Now let me ask you;...what type of surface do you think would be more likely to take a scratch? A really smooth surface or a rough one? Fingertips drag more across rough surfaces. So do metal objects and other various solid particles. Whatever kind of particle that might be. What I have been calling “drag” is known in the scientific community as the “coefficient of friction”. This can be very accurately measured and given a number so that one surface can be compared to another. The more rough the glass surface the more easy it is to scratch. Also even very small scratches begin with what are called “micro-cracks”. Further large cracks begin with micro cracks. So from this we learn that rough glass is easier to crack. To prove this Penn State used very small microscopic metal points or tips. These are called microindentors or nanoindentors depending on their size.

Now what makes this really interesting is exactly “how” glass surfaces can be made more rough. So that they are more likely to scratch. This is simple to do with a good abrasive powder. Such as a water based mix of cerium oxide. Or even an op cal grade microcrystalline silica powder. It is also possible to make glass more rough by using the right chemical solution. Hydrofluoric acid of only 1% or higher will do. Sulfuric acid and certain other chemicals will also do it. But what is totally fascinating is how Penn State did it. They wanted to selecvely remove the sodium atoms from the glass surface. To do this they used plain water. No abrasives, acids, or alkalis. They did adjust the percentage of water in the air and the temperature of the air. The percentage of water in the air is called the Relative Humidity or RH. It was learned from this test that very humid conditions at high temperatures did the greatest damage to window glass. It cracked much easier, scratched much easier, and wore much easier. Overall the general strength and physical integrity of a plate of window glass is greatly reduced by the chemical reaction of plain water with the surface. In particular the exchange of sodium atoms with the hydrogen atoms of water molecules. If you look up this chemical process on the internet you will find it is called weathering. As mentioned this effect is greatly enhanced only when the temperature is rather high and the relafive humidity is 90%. Otherwise the effect is not very noticeable.

Penn State also learned something else which is of special note to us as Window Cleaners. Simply that when new glass is covered with a lm only one molecule thick it either becomes easier to scratch or more diffcult to scratch. Which one depends on the molecule. If that molecule adsorbs (spelled correctly) to the surface it promotes scratching. If however the molecule does not adsorb it will act as a lubricant. Therefore various sealant products do much more than help to keep stains of glass. Even at only one molecule they help to prevent scratches by acting as a lubricant. They also help to prevent the removal of sodium atoms and the weathering of windows which helps to prevent scratches and even cracks. Remember every large crack starts with a small one. It should be mentioned at this point that all sealants are not created equal. Some are very effective at serving as a barrier to water. Some are more effective at guarding against scratches. Also some will stand up to the shorter more powerful ultraviolet rays of the sun. Whereas others are burnt of in a manner of days. Nonetheless for the reasons discovered by IWCA funded research all brand new pristine glass surfaces should be sealed and maintained. That is big. Very big!

Penn State used water to selectively remove sodium atoms from window surfaces because this process quite effectively reveals any abnormalities or defects in the subsurface. The chemical reaction of water with glass surfaces can even cause abnormalities in the subsurface. Water molecules move into the surface of glass reacting with the silicon, sodium, and nonbridging oxygen atoms there creating different molecular species. This “new network” can create abnormalities in the subsurface. Which is nothing more than the glass that is directly under the surface. The surface is literally what you touch. Also known as the glass/air interface.

The next step for IWCA research will be to look at the coefficient of fric on of the glass that was recovered from the eld which randomly scratched. How does the scratched surface compare to the smooth unscratched surface? What caused the difference in the coefficient of friction? Does the humidity level, ion exchange, coeffcient of fric on and construction debris all come together to make the "perfect storm" of scratched glass? The Glass Committee must finish their eight year project to find out.