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Monday, October 14, 2019

Glass Surface Chemistry

In pure water chemistry one out of every 10,000,000 water molecules is dissociated into a hydroxide 'and' a hydronium ion.  Hence the water maintains a neutral pH.  It is neither acidic or alkaline.  If there are more hydronium ions (H3O) than hydroxide ions the pH is below 7 and the "pure water" is acid.  If there are more hydroxide ions then hydronium ions the pH will be above 7 and the water will be alkaline or a base.  Cleaning solutions use alkaline chemistries because most films and dirt on glass are hydrocarbon in nature.  Which are acidic.  Hence the two chemistries neutralize and cleaning happens.  However it is the hydronium ions that swap places with the sodium atoms in the near surface of soda lime/window glass.  The hydronium ions and sodium atoms inter-diffuse.   Which is the reason why Dr Paul Duffer says that glass is "alive".  It is very easy to change the chemistry of a glass surface with pure water if it is acidic.  And especially if the ambient temperature is raised.  An other way of saying this is that glass surfaces are very reactive.  This is demonstrated in our work every day.

Any window cleaner that has been working for ten years or more knows that not all surfaces are the same.  If you doubt this just start checking clean dry surfaces with a penny.  Then feel and listen.  They can pop and fizz as in the video here.  Or just be very rough.  They can also be perfectly smooth.  Not rough with no sound.

Everything I have been describing is what happens naturally.  We also change glass surfaces by polishing with microcrystalline super-abrasives such as cerium oxide.  Certain acids such as hydrofluoric, sulfuric, and ammoniumbifluoride do the same thing in a matter of thirty seconds.  Whenever hard water spots are removed with polishing slurries the glass becomes rough, very easy to scratch, and a stain sponge.  For all intents and purposes the surface is now DEFECTIVE!  This is an issue that no one in the window cleaning industry has recognized.  

I remember a building that Marc Tanner told me about that he restored.  It was four stories.  The first three stories released the stain with ease.  But the fourth floor was a total bear.  Come to find out an etchant/acid had been used before to remove the stains from just the fourth floor.  Then the stain reformed.  

It is also true that stains come off reflective metallic surfaces much easier than clear transparent glass.  Which releases stains much easier than dark tinted glass.  Dark glass can be an absolute bear.  Especially if it has been polished or "restored" with an acid such as HF.  Usually etchants will destroy dark glass.  They will leave an orange peal effect, or discolor.

The days of simply cleaning windows with a buck, mop, and a squeegee are long gone.  When we take on a store front we take on the maintenance and protection of the windows we clean.  This is true for window cleaners and auto detailers who work on vehicular glass. 

Vision glass that has been restored by having water stains removed should absolutely be sealed with a hydrophobic sealant.  This protects the window surface from scratches and more stains.  Here is a demonstration by Marc Tanner showing off  Nanovations NG1010.

Written by Henry Grover Jr.

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