The Cause of Scratched Glass & How To Protect It - A Quest For Answers

Did the IWCA test disprove that glass fines/particles can fuse to glass?

For decades now, window cleaners have reported encountering glass with a “rough surface” that has a “drag sensation” and also many times exhibits a “tinkling sound” when a razor blade is passed over the surface.  Both the roughness of the glass and the tinkling sound does vary from window to window.  This type of surface has historically been described as being impregnated with fabricating debris in varying amounts. Historically, “fabricating debris” has been described as particles that were not adequately washed off prior to entering the tempering oven, thus getting fused or “baked” onto the surface during the tempering process.  Such particles could be glass fines resulting from the scoring and cutting of glass sheets.  Or sand/silicate particles from atmospheric conditions inside the glass factory.   However, research by the IWCA in recent years has shed light on some inconsistencies in that theory.  First, lab tests that attempted to fuse glass and sand particles failed. After heating and cooling glass at the industry standard temperature of tempered glass, particles with the same coefficient of expansion as glass, and silicate particles did indeed fail to fuse to the float glass surface.  Such particles simply "popped out" after they were pressed in.  The simple answer then to the above question is an absolute yes.  It is not only plausible but also quite possible and very probable.

Microscopic Analysis;... What of It?

When this information was presented to me my first reaction was to simply get out a microscope and look at the animal.  Isn't that what we do with everything? Richard Feynman used to say, "look at the animal". So this is what the IWCA Glass Committee attempted to do.  Very powerful microscopes were used to attempt to see fused particles on actual samples of offensively scratched glass "from the field".  However;...no such particles could be identified!  So it was not possible to actually "see" what our ears were telling us was there.  Several years passed as the IWCA went through this phase of exploration.  This was the most irritating exercise in futility.  But it did serve as an educational experience for everyone involved.

So, after more than five years of searching, no actual physical specimens of glass tainted with fused fabricating debris have been delivered to the IWCA. However, the Glass Committee was able to gather scratched glass samples from actual construction sites in all parts of the country.  Sure enough, these scratched glass samples exhibited a “rough surface” whereas many times similar glass windows on the same job site exhibited a smooth surface and did not scratch even though the same cleaning methods were used.

Nano-Scale Roughness Exposed

At last years IWCA Convention we introduced the idea that the rough surface of glass that appears more likely to scratch could be due to a change at the microscopic or even the nanoscopic level. From this the IWCA introduced the term nanoscale roughness.  In fact there is an entire field of science that discusses how surfaces with a nanoscale roughness may be more prone to physical damage.  3D surface metrology is a very real science.  The question remained however; how do the “rough surfaces” and that "tinkling sound" which are encountered in the field originate? Do they develop during the tempering process? Or are there other considerations? Could the development of "nanoscale roughness" go back to the actual float glass process where glass is made on a bed of molten tin - producing what is known as an “air side” and a “tin side”?  All float glass has two sides.  The underneath where it rides the molten tin down the glass ribbon.  Also the side that faces up and is only in contact with air.  It is in fact interesting to note that in the IWCA's collection of scratched glass with “rough surfaces",  the tin side was the rough side.  Also, the tin side was more easily scratched.  Should this surprise us? Is it possible that the tin side of float glass can develop a random roughness?  But exactly how?

A Closer Look at the Tin Side

To begin to understand this, I direct your attention to the paper “Statistical Analysis of the Metrological Properties of Float Glass” by Brian W. Yates and Alan M. Duffy.  This paper speaks of a “surface roughness” that develops on the tin side of float glass and reports “It can be seen that the overall tin side surface roughness average is indeed rougher on average than the air side”. The conclusion reads: “Statistically significant differences were found between the tin and air side surface roughness values for both untreated and acid treated, with the tin side being significantly rougher than the air side.”  Is there a definite  link between the rough tin side of some glass surfaces and surface scratching? Does post production heat treatment of glass exacerbate the roughness of some glass surfaces?  These questions will be closely examined as you are about to see.

Real Science

Armed with this knowledge, questions and the collection of scratched glass samples, the IWCA has embarked on the first of its kind scientific research during the summer of 2016. Dr. Paul Duffer has greatly expended his time and energy leading the way for the IWCA to participate in a special research program at the world class Department of Materials Science and Engineering at Pennsylvania State University. Professor Seong Kim, phd  is quite dedicated to materials science and has much experience with glass surface studies. He has been and will continue to oversee the IWCA research of scratched glass at Penn State.  Already we have learned so much about the real science of float glass surfaces.  But this is only the beginning.

  

Where Will This Take Us?

The IWCA is dedicated to helping window cleaners better understand the glass surfaces that all of us work on every day of the week! Armed with knowledge as an industry we can develop much more effective, efficient, and safe cleaning methods.  Such advanced knowledge will result in the very best products, and practices that will assist us in maintaining the integrity and beauty of glass, reduce possible damage, and adequately protect ourselves legally should aggressive cleaning techniques and restoration become necessary on a job site.

The GANA/IWCA Joint Bulletin

GANA TD 03­1003 (Reapproved 2010)

Construction Site Protection and Maintenance of Architectural Glass

Steps Must Be Taken to Avoid Permanent Damage to Glass

This bulletin focuses on the fact that attention and blame should be put on general contractors for not protecting the glass during the construction process.  It states that, "The complex nature of construction projects and site management requires well‐planned and executed material delivery and storage.".  Regarding storage the bulletin states that the duration of site storage must be minimized.  Also storage should be under roof to avoid direct rain and water runoff.  Further adequate ventilation must be maintained to prevent damage from condensation.  Deposits from concrete or masonry runoff can cause hard water spots during storage.  It discusses glass surface or edge damage as being the result of inadequate on‐site protection and ill‐advised or vulnerable storage locations, also exposure to other trades.  Stating also that, "Deep  surface  scratches,  contact  by  hot  weld‐splatter  and  edge  damage  threaten  the  structural integrity of glass and may require glass replacement.  Surface contact with harmful materials will require  prompt  cleaning by  professional window  cleaners, such  as members  of  the  International Window Cleaners Association (IWCA).".  It is further stated that it is the responsibility of the general contractor to educate all of the other trades, regarding glass protection during storage and installation.  For this reason the glass in storage should be inspected daily.  There is much information in this bulletin which expertly identifies the chemical and physical damage that can result during storage and how to determine the cause of this damage. 

Protection Options During Construction

The very first thing that must be addressed is the fact that glass with nano-scale roughness is exposed to scratching during storage and every other phase of the construction process.  Glass with this type of surface must first be identified.  We are the very best choice for the general contractors to do this.  Then once it has been identified adequate steps must be taken to protect it from scratching and other surface damage.  There are temporary self clinging films that can be applied prior to storage which can be quickly and easily removed after construction.  There are also paint on protective plastic coatings that can be pealed off too.  When these are contaminated by construction debris such as concrete splatter, hard water spots, silicone caulk, stucco runoff, paint, wood stain, or other deposits;... such will come off with a simple pull.  There are other wipe on completely transparent products/sealants that are intended to be much more permanent and everlasting.  They are not intended to be removed.  Certain ones even offer a measure of scratch protection.  However just use caution, as the brush on temporary peal off films will actually stick to these permanent sealants.  So you will want to apply the permanent sealants after first pealing off any temporary brush on films.  Protection during construction, (from glass storage to the finished construction project), is an aditional service that window cleaners should offer.  Especially if we are doing post construction work.  It will be necessary to first become familiar with this new technology of glass protection, the various products available, and then educate the general contractors we work for.

The IWCA and the Glass Committee is your best defense against the ignorance surrounding the scratched glass issue.  It is also your best source of information that will help you to earn greater respect from your customers and a much higher profit margin.  Join the cause today.

Written By Paul West and Henry Grover Jr.

Members of the Glass Committee of the IWCA

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