Atmospheric Surface Damage Explained

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Glass surfaces are seriously vulnerable to the chemistry of atmospheric conditions.  There are molecular contamanents in the air that can form various monolayers on the glass.  Some of these will  cause the surface to become more easily damaged by wear when for example two plates are slid past each other during storage.   Or certain other monolayers can actually “lubricate” the surface making the surface more resistant to wear or scratching.   The difference between whether such molecules cause a lubricating effect or what is called “interfacial wear”, depends on whether they are adsorbed to the surface or actually react with the surface.   Sometimes these monolayers will cover virtually the entire area of the glass plate, and sometimes only localized “patches” are covered. 
It is also true that water molecules are quite capable of changing the “mechanochemical” properties of glass surfaces. This can happen with liquid or vapor.  There is a chemical reaction involving ion exchange.  This involves the swapping of sodium atoms in the glass surface and the hydrogen atoms in water molecules.  This process is called “weathering” also “glass corrosion”.  It is greatly enhanced by temperature and time.  This condition can reveal subsurface damage or a change in the molecular matrix of the glass at that level.  It can also cause such a change in the molecular matrix including oxygen and silicon atoms.  As window cleaners in the great north east know, old storm windows are much more rough to the touch of a dry fingertip outside rather than inside.  This is the weathering effect.  What most do not know is this particular effect begins immediately when the glass comes off the bath of molten tin.  It also can become rather severe under the right atmospheric conditions.  When glass is stored for six months before installed on the building it should be kept in a very dry, cool environment.  The glass sheets should be kept separated from one another by what is called an interleaving system. 
The point that I am attempting to make here is that glass surfaces are certainly not chemically and physically inert and unchanging.  They are in fact quite alive with activity.  The chemistry of the surface can be so altered by atmospheric conditions that certain previously damaged “subsurfaces” are preferentially exposed to physical attack by whatever means. Also various monolayers of atmospheric pollutants form monolayers that will either enhance the likelihood of mechanical damage to the surface or actually lubricate it and actually protect it from such damage.  
I know from my own experience that when glass is polished with an optical silica based compound or a cerium oxide, it becomes rough to the touch.  It also becomes much more water loving.  This same exact condition can be caused by a very dilute solution of hydrofluoric acid.  Once it is created either physically , chemically, or both;...the glass surface takes on an alternate “microtexture”.  If this condition is measured on a level less than a micron it is called a “nanotexture”. Microtextured surfaces are much more prone to surface damage.  Whether that be interfacial shear,  stress cracks, or scratches.  It is also possible to apply certain “lubricating” monolayers to microtextured surfaces using silane chemistry. Once applied correctly the monolayer will not just adsorb but react with the nonbridging oxygen atoms and resist mechanical wear, and scratches.  Many years ago I discovered a product created by DuPont in Wilmington Deleware that did precisely this.  It was based on a highly concentrated water based alkyltrialkoxysilane and tetraalkylammonium chloride.  This product was so concentrated that I could actually reduce it by 100 times with pure water and get the same effect on glass.  If applied correctly that is, by first microtexturing the surface, and then rubbing the silane solution into the micropores so as to effect the maximum number of reactions with the nonbridging oxygens.  I also discovered by testing other glass sealants that they did not have the same effect.  So chemistry is very important and necessary to properly protect glass surfaces from mechanical wear or scratches. Protection from the weathering effect of humidity/water can be accomplished with a variety of glass sealants.
I have also learned by talking with “true” glass restoration professionals that window glass surfaces which are prone to scratches can be “fixed”.  This is done by means of a grinding and polishing process.  It follows then from what we have learned thus far that such surfaces can now be both “repaired” and protected from further scratches.  The surface and subsurface can be precisely re-engineered and then chemically sealed by a reactive silane chemistry.  You will find some of the gurus of this science/technology on the Glass Smart Facebook Group.

Written by Henry Grover Jr.
henrygroverjr@gmail.com
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