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Introduction

Glass has long been admired for its functionalityas well as longevity and distinctive aesthetic quali-ties. Day-to-day experience is replete with evi-dence that glass is undeniably a stable, enduringmaterial which can be relied upon for generations,if not centuries, of consistent service. This point isillustrated by the fact that original glass installa-tions can still be found in medieval cathedrals aswell as municipal and residential structures datingfrom the colonial era. However, there are situa-tions where both the aesthic features and function-ality of glass can be severely challenged by factorsthat exist in many ambient environments. Forexample, airborne particulates, runoff from con-crete facades and spotting from hard water sourcesrepresent three common forms of contaminationthat can affect glass. Each of these elements canact either individually or in combination with theothers to severely degrade glass surface qualityand compromise performance characteristics.What is perhaps less well known is the fact thatparticulates contain components that are harderthan glass and thus maintain the potential for caus-ing scratches in specific circumstances. In addi-tion, both concrete runoff and certain types ofwater spots can chemically bond to glass surfaces.As a consequence, the lack of systematic washingand maintenance often leads to situations in whicheventual glass cleaning procedures become morelabor intensive and commensurately more expen-sive than normal. In severe cases, even higherpriced restorative techniques may be required inorder to refurbish soiled glass to its original bril-liance and lustre.

The following paragraphs present a number ofreal-life examples where glass installations havebeen significantly affected by one or more of theseadverse factors. The intent in so doing is toemphasize that such situations need not beendured by neither commercial property managersnor homeowners. Furthermore, the examples citedserve to develop a rationale for the regular clean-ing and maintenance of glazing installations byexperienced professionals in order to effectivelyprevent such conditions from occurring in the firstplace.

It is worth pointing out before proceeding that theimportance of establishing systematic cleaningpreventive maintenance programs for glass has notgone unnoticed among building owners and man-agers. For example, in a recent article entitled“Pay Now or Pay Later” which appears in TheBOMA Magazine issue of May/June 2012, it ishighlighted that a preventive maintenance pro-gram for windows is an integral part of overallexterior building maintenance.1

Dust and Particulates

Glass that appears dirty and soiled is often affectedby an accumulation of particulates that are charac-teristic of the everyday environment and not nec-essarily composed of exotic airborne pollutants.For example, the soiled door shown in Figure 1exhibits an accumulation of ordinary dust andgrime that originated from a relatively common-place environment.

The Systematic Cleaning and Restoration of Architectural Glass

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.

A close-up view of the door in question is dis-played in Figure 2 and provides a more detailedview of what appears to be a significant amount ofdirt and soiling. While this particular view of dirtyglass does not appear to be inherently unusual, adifferent perspective is unveiled by examining thissame grit and grime with the aid of a scanning

electron microscope. Such a view is presented bythe photograph presented in Figure 3.The left hand frame of Figure 3 was taken at amagnification of 250X and reveals the dirty glassof interest to be littered by a variety of irregularlyshaped particulates. Many of these measure lessthan 100 microns in either length or diameter (0.1millimeters or 4/1000 of an inch). However, moreimportant than size is the chemical composition ofthe particulates. For example, the right hand framein Figure 3 focuses on a sharp-edged entity magni-fied 1200 times for which the compositional anal-ysis reveals the presence of silicon oxide (SiO2).In other words, a particle that can be more com-monly identified as a grain of sand.

Although small in size, the presence of sand parti-cles is not a trivial matter since SiO2 is harder thanglass and thus has the potential to initiate surfacescratches. However, surface scratching is not nec-essarily a forgone conclusion if particulateremoval is undertaken by a skilled window clean-ing and maintenance specialist.

Figure 1: Entrance door on a municipal building located within 30 feet of a busy roadway.

Figure 2: Close-up view of municipal building entry door reveals a significant accumulation of dirt and soil.

Figure 3: Microscopic view of particulates sampled from municipal building entry door which include entities that are composed of silica (sand). Left hand frame shows particles magnified 250 times (250X) while frame on right is shown at 1200X.

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A second example of the accumulation of soils anddust on a window glass installation warrants con-sideration in order to demonstrate that the resultscited in the first example are not necessarilyunique. In this instance, attention is focused on thevacation home pictured in Figure 4 which islocated in the mountains of northwest Virginia.Here again, as was the case with the municipalbuilding door, some significantly soiled glass canbe identified as illustrated in Figure 5.

When examined with the aid of a scanning elec-tron microscope, the dust and dirt in this examplereveal the interesting physical features that areshown in Figure 6. As in example 1, the observedwindow dirt is an assembly of numerous irregu-larly shaped particles that are on the order of 100microns or less in size.Compositional analysis in this example alsoreveals the presence of silicon oxide (sand grains)as well as alumino silicate material which are bothharder than commercial flat glass. As a conse-quence, each of these materials has the potentialfor initiating surface scratching when removedfrom a glass surface without the appropriate pre-cautions that are employed by window cleaning-professionals.

The photographs presented to this point indicatequite convincingly that the glass examples ofinterest were not participants in any regularlyscheduled cleaning and maintenance regimens.While the decision not to engage in such programsremains the prerogative of the building owners ormanagers, such choices do not come without thepossibility for adverse consequences occurring at

Fig. 4: Vacation home in the mountains of Virginia.

Fig. 5: Close-up view of dirt and soil on window at Virginia vacation home.

Figure 6: Particulates from Virginia window magni-fied 1100 times via a scanning electron microscope

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some future date. This contention is supported bythe several observations noted here that dust anddirt normally found on glass are likely to includeparticulate materials that are harder than glass. Ifthe populations of these are allowed to increase tointensities similar to those represented in Figures 2& 5, the probability is enhanced that some degreeof scratching might occur during subsequentefforts at cleaning unless appropriate proceduresare implemented. Therefore, it is very important tosecure the services of a knowledgeable and experi-enced professional in order to successfully addresssuch challenging cleaning projects as those por-trayed by the examples presented to this point.

Figure 7 presents an example of a window unitthat exhibits a variety of small surface scratches.The significant population of these suggests that alarge number of minute, relatively hard particu-lates were moved in a circular motion over theglass surface in question. However, the key pointhere is not necessarily the damage done to this par-ticular unit, but, rather, consideration of the factthat had a skilled professional been engaged in thisproject, there would be little or no discussion ofpost-cleaning damage. A second example forwhich numerous scratches appear on glass is pre-sented in Figure 8. Once again, the observedscratches are indicative of an absence of sufficientcare and diligence having been exercised duringcleaning.

The incidents of glass scratching illustrated inthese examples place renewed emphasis on theinherent wisdom in engaging the regular servicesof a professional window cleaning contractor, suchas a member of the International Window Clean-ing Association (IWCA), as part of a damage miti-gation and preventive maintenance strategy. In scenarios involving initial or post-constructionwindow cleaning projects, the accumulation ofdust and dirt from building materials can be partic-ularly challenging. Relevant information thatadvises both building owners and constructioncontractors in preparing for and coping with these

unique situations can be found in the joint IWCA/GANA Bulletin entitled, “Construction Site Pro-tection and Maintenance of Architectural Glass.”2

Concrete Runoff

Runoff from rainwater cascading over concretecomponents of building facades presents anotherdistinct challenge to maintaining clean and aes-thetically pleasing fenestration (window) surfaces.The consequence of this occurrence has long beena source of apprehension among individuals asso-ciated with the glass manufacturing and window

Figure 7. Close up view of a previously soiled window surface after cleaning. The observed scratches suggest that washing was undertaken by someone lacking the skills of an experienced professional window cleaner.

Figure 8: Another example of a window glass surface where the observed scratches are indicative of cleaning in the absence of an experienced professional.

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cleaning industries. The focal point of concern hasbeen the typical inability to restore the affectedglass to its original condition by means of basiccleaning techniques. For example, the windowshown in Figure 9 provides a graphic illustrationof the typical outcome associated with suchattempts. The close up view of concrete runoffdeposits presented in Figure 10 reveals the charac-teristic pattern associated with this problem.

For many years, conventional wisdom has main-tained that runoff from concrete surfaces damagesglass due to an alkaline etching or erosion of thesurface. However, more recent scientific study hasshown concrete runoff to be characterized bydeposits of amorphous materials that reside on theglass surface.3 Whenever rainwater makes contactwith a concrete surface, small amounts of silicatematerials are dissolved with concentrations of 4 to8 parts per million being quite common. Uponcoming to rest on glass, these dissolved silicatematerials gradually become more and more con-centrated as the runoff droplets evaporate. Finally,a point is reached where formation of water insol-uble polymers begins. This same reaction alsoresults in chemical bonding to the glass substrate.In addition, experiments have shown this processto be significantly more rapid than either glass sur-face corrosion or true alkaline etching.4 In fact,

prior work has shown the reaction to reach com-pletion at the point when the water carrier evapo-rates to dryness. This could take hours or as littleas a few moments to occur depending upon envi-ronmental conditions.

Figure 11 offers a view of concrete runoff obtainedwith the aid of a scanning electron microscope.The magnified image clearly shows the deposit tobe residing on top of the glass. There is no evi-dence of erosion or pitting that would be observ-able had chemical etching occurred. In addition,previous studies (See reference 1.) have shownthese deposits to be glass-like with respect toboth structural and solubility attributes.

Figure 9: Visible gray haze due to deposits on glass from concrete runoff. Implementation of conven-tional cleaning procedures failed to remove deposits.

Figure 10: Close up view of typical concrete runoff deposits on glass. These deposits begin to react with glass upon evaporation of the runoff droplets resting on the glass

Figure 11: Magnified view of concrete runoff deposit on glass taken with scanning electron microscope; 200X.

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It is important to note that concrete runoff can alsohave an adverse effect on glass that bears a firstsurface coating. The same mechanism that pro-duces glass-like deposits on uncoated glass cangenerate similar entities on coated surfaces withequal facility. Figure 12 reveals the outcome ofconcrete runoff accumulating on glass with a solarreflective coating on the No. 1 surface. A magni-fied view is presented in Figure 13. The glass inquestion had been removed from an existing glaz-ing installation due to suspicions that the coatingwas defective as a consequence of compromiseddurability. These reservations arose when it wasobserved that conventional cleaning failed torestore the glass to its pristine condition. However,subsequent investigation identified the deposits tobe silicate residue from concrete runoff

The net result of concrete runoff accumulating onglass is the formation of tenacious, glass-likedeposits that defy removal by means of conven-tional cleaning practices. In essence, a “glass-on-glass” residue is created. If this process is allowedto continue unabated, future cleaning and restora-tion become simultaneously more challenging,more labor intensive and more expensive. In thecase of coated glass surfaces, successful outcomesare even more elusive since such coatings are verythin and often not amenable to polishing or chemi-cal cleaning. However, a preferred approach is topreempt such events by having an experienced

IWCA professional engage in an ongoing and con-sistent program of cleaning and maintenance inorder to minimize the potential for permanent fen-estration damage. In those situations where con-crete runoff has been allowed to amass on glass,any effective cleaning strategy that includes resto-ration will require specialized procedures andcleaning agents that also bear added costs. How-ever, there is no simple or fool proof approach tosuccessfully reversing the degradation of windowsthat have been the object of neglect. Once again,such projects are best addressed by an experiencedIWCA professional.

Hard Water Spots

Another topic that warrants consideration focuseson events that often affect glazing installations sit-uated in suburban settings where glass and decora-tive landscapes reside in close proximity. In thesesettings it is not uncommon to observe sprinklerheads that deliver tap water from public or privatesources to the ornamental flowers and shrubberyon a regular basis. An example of such an arrange-ment is shown in Figure 14 where the close prox-imity of glass and landscape components is clearlyillustrated. Hidden from view are pop-up sprinklerheads that rise periodically to irrigate the sur-rounding area. In these situations, it is not uncom-mon for the irrigation zone to also include thenearby windows. However, this seemingly

Figure 12. Concrete runoff deposits on coated glass unit removed from an existing glazing installation. Note the line of demarcation at point where glass was protected by framing.

Figure 13: Magnified view of silicate residue from con-crete runoff on a coated glass surface; shown at 10X.

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uneventful sequence of events can, in some cases,yield problems for subsequent attempts at remov-ing the hard water spots that form. The reason forsuch behavior emanates from the fact that not allhard water spots are alike. More specifically, thisdepends upon the chemical nature and concentra-tion of various minerals dissolved in the water.

A majority of individuals are likely to be familiarwith the impact that hard water deposits can haveon daily experience whether it be calcium carbon-ate build up (lime scale) in a tea kettle or theunsightly spots that accumulate in a shower enclo-sure. In addressing these situations, it is normalpractice to use weakly acidic household cleanersto remove water spots from bathroom surfaces oremploy vinegar to dissolve lime scale fromkitchen vessels. As a consequence, these limitedsuccesses serve to create the impression that hardwater deposits always respond to relatively simplecleaning procedures. However, in the case of win-dow glass surfaces, the removal of hard waterdeposits may not be so easy.

Typical household concerns regarding the miner-als commonly found in tap water focus on dis-solved calcium and magnesium salts andsometimes iron. Nevertheless, there is a less con-spicuous and often overlooked component in tapwater that, depending upon concentration levels,

can create significant difficulty when it comes toremoving spots from glass. The “often over-looked” material in question includes the entirefamily of dissolved silicates which behave quitedifferently from calcium and magnesium basedresidues.

In a manner similar to the process involved in for-mation of concrete runoff deposits, whenever hardwater droplets containing dissolved silicates evap-orate, these materials become increasingly moreconcentrated as the liquid volume decreases.5,6

Eventually, the silicates begin to bond with oneanother as well as the glass surface producingwhat is akin to a “glass-on-glass” deposit. Just likeconcrete runoff, these glass-like entities fail torespond to common household cleaning agents.Vinegar and other weakly acidic solutions will notdissolve the silicate deposits. A good example ofthis is shown in Figure 15 which presents a closeup view of the window shown in Figure 14. In thisinstance, repeated contact with hard water dropletsfrom an adjacent lawn sprinkler has created a sig-nificant build up of silicate material.

The general condition of the window under con-sideration here suggests that a regularly scheduledprogram of cleaning and maintenance had not

Fig. 14 Hotel window with adjoining decorative garden and hidden pop-up irrigation system.

Figure 15 Hard water spots containing silicate mate-rials after normal attempts at cleaning.

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been implemented by the time the photograph wastaken. Of course, the preferred course of action isto prevent such conditions from developing in thefirst place by consistently employing the servicesof a window cleaning professional.

As mentioned previously, the problems associatedwith removal of silicate deposits from glass are notrestricted to uncoated surfaces. Figure 16 presentsa view of a suburban office building with adjacentlawn and garden features similar to those observedin Figure 14. However in this instance the glazingsystem incorporates a reflective coating on theouter or No. 1 surface. Nevertheless, silicatedeposits that emanate from hard water spots areequally resistant to removal as illustrated in theclose up view shown in Figure 17.

With the aid of an electron microscope, one is ableto discern that silicate residue from hard water issimilar in physical appearance to deposits fromconcrete runoff. This characteristic is displayed inFigure 18 which presents a silicate deposit magni-fied 2000 times. The glass-like silicate residue lieson the substrate surface and extends a small dis-tance above the point of contact. There is no evi-dence that chemical erosion or etching hasoccurred.

In a manner similar to concrete runoff, the taskpresented in removing silicate deposits caused byhard water sources is even more difficult forcoated surfaces than in the case of non-coatedglass. This situation arises from the fact that thestructural layers within the family of first surfacecoatings are quite thin and often vulnerable to theextraordinary mechanical and chemical cleaningprocedures required to remove silicate bearing res-idues. As a consequence, implementation of suchcleaning procedures is best left to the skilled pro-fessional.

Figure 16 Suburban office building with glass coated on #1 surface.

Figure 17: Close up view of silicate deposits on a coated glass surface arising from repeated contact with hard water droplets.

Figure 18: Glass-like silicate deposit on glass magnified 2000 times with an electron microscope.

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Frequency of Cleaning and Maintenance

This cursory overview of three of the most com-mon sources of soiling and fouling of windowglass obviously begs the question as to how oftenone must engage in cleaning and maintenance inorder to maintain glazing system aesthetics andprevent potential glass surface damage. Theanswer to this query will necessarily depend uponfactors that characterize the particular environ-ment in which the glass is located. For example,the rate at which dust particles accumulate onglass, among other influences, may hinge uponintensity of prevailing winds and the concentrationof air borne particulates. Thus the frequency ofnoticeable soiling will differ from one location toanother.

The time frame over which concrete runoff pre-sents a problem can be affected by factors such asthe age of masonry-based building components aswell as average annual rainfall. In a similar vein,issues related to hard water spots can be influ-enced by the frequency of sprinkler head operationand the nature of dissolved minerals in the water.Here again, the degree and rate of soiling will varyfrom site to site.

In view of the location-dependent influences citedin the previous paragraphs, the answer to the origi-nal question regarding the frequency of cleaningand maintenance may appear to remain somewhatelusive. However the solution to this quandary canperhaps be found by taking a second glance at Fig-ures 2,5,10,12,14 and 16. Each of these imagespresents an example of severe glass surface soilingdue to either particulate build up, concrete runoffor hard water spotting. Each of these also repre-sent a situation where the glass in question oughthave been subjected to cleaning and maintenancebefore the conditions portrayed in the photographshad developed.

It should be evident at this point that an effectiveprogram for the care and preservation of glass atany given building site have as a primary objectivethe prevention of the conditions presented in the

several examples offered during the progression ofthe current discussion. Of course, the actual timeframes that elapse between necessary repetitionsof washing and upkeep will vary. However, someexcellent insight regarding the topic of “frequencyof cleaning” can be found in a recent e-book pub-lished by the Building Owners and ManagersAssociation International (BOMA). Bearing thetitle, “BOMA International Guide to ExteriorMaintenance Management,”7 this guidebookpoints out that the necessity of exterior glass sur-face cleaning may vary from two to four, or more,iterations per year depending upon the specificenvironment. In view of this advice, an operativeapproach to identifying reasonable time framesbetween cleaning events can be facilitated bydirect involvement of building management per-sonnel engaging in regular site evaluations. Thisprocess can be further enhanced by including awindow cleaning and maintenance professionalwho has access to the full array of resources andsupport offered by International Window CleaningAssociation (IWCA)

1. Garris, Leah Grout. “Pay Now or Pay Later.” The BOMAMagazine, May/June 2012: 40-42.2. “Construction Site Protection and Maintenance ofGlass.” Glass Association of North America. TD 03-1003.

2010. <www.glasswebsite.com> 3. Düffer, Paul F. “Where Masonry, Glass and ConcreteMeet; The Interactions and Ramications of Concrete Runoffon Glass”, US Glass, Metal & Glazing, May 2008: 52-62 4. Düffer, Paul F. “How to Prevent Glass Corrosion,” GlassDigest 65 [12](1986): 77- 845. Düffer, Paul F. “Glass Reactivity and Surface Corrosion.”Tech Note Book for Glass and Ceramic Decorators. 1994Ed. Andrew Bopp. 2nd ed. Vol.1. Washington: Society ofGlass and Ceramic Decorators, 1997.6. Düffer, Paul F. “The Surface Chemistry of Float and LeadCrystal Glass,” Glass, March/April 2008: 32-34 7. P. Marc Fischer et. al., “BOMA International Guide toExterior Maintenance Management.” BOMA International.Washington, D.C. 2012: (31-32) <http://shop.boma.org/showItem.aspx?product=EXMM12&session764C6CF20A9D4B63A4ABD4A46133DD09>

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This bulletin has been commissioned by the Glass Committee of the International Window Cleaning Associating and 

authored by Paul F. Düffer Ph.D., Educational Program Adjunct to the Glass Committee; Paul D. West, Chairman. 

International Window Cleaning Association1100-H Brandywine BoulevardZanesville, Ohio 43701-7303(800) 875-4922 Fax: (740) 452-2552www.iwca.org

Copyright 2012 Paul F. Düffer licensed to International Window Cleaning Association