Volume 36, Issue 5, May 2001

How to Keep From Saying, "What Happened?" On the Job Site 

Performance efficiency and aesthetics are key characteristics usually sought in contemporary glazing systems. Quite often, this challenge is met through the creative genius of the design architect whose artistic insights become manifested in the form of structural expressions that embody both the high performance and classic durability of today’s flat glass products. While glass industry product literature aids in the selection phase of this creative process by emphasizing performance features, the elements of preserving “classic durability” are often lost in the common sense mindset that glass is immutable. However, the environments glass encounters during storage prior to installation and the construction-site hazards experienced after glazing pose a severe challenge to the “classic durability” of glass.

Preventing Surface Damage
Glass that is packed for transport and storage is vulnerable to permanent surface damage in the form of surface corrosion or, more commonly, staining. The key factor in the initiation of staining is the extended and undisturbed contact between glass and water that has been allowed to invade the pack as condensation, or a more overwhelming onslaught such as rainfall or sprinkler streams. Since the distance between stacked glass units is very small, the moisture becomes entrapped to the point that evaporation does not occur quickly as is customary in an open space. 

Within the confines of the spaces between stacked glass units, glass and moisture can engage in mutual contact for days on end. Without the benefit of evaporation or dilution, glass relinquishes sodium ions, Na+, to the water that creates a chemical imbalance, which is rectified by the glass in turn absorbing hydrogen ions, H+, from the water. Of course, this absorption of hydrogen ions generates a disruption in chemical equilibrium among the water molecules that also needs to be addressed. This is accomplished in a manner that is well-known to those familiar with elementary solution chemistry. More precisely, the water molecules engage in the balancing act by dissociating spontaneously in sufficient numbers to compensate for the hydrogen ions consumed by the glass. In the process, an excess of hydroxide ions, OH-, is created in the solution. In more familiar terms, if an aqueous solution exhibits a pH of 7.0, we describe it as being “neutral.” This means the concentration of hydrogen ions and hydroxide ions are equal. However, when the hydroxide ions predominate there is a commensurate increase in pH and the solution is described as being alkaline.

Studies have shown that soda lime glass in general, including commercial flat glass, is susceptible to damage by solutions that exhibit pH levels of 9.0 or greater. Unfortunately, such levels can be achieved in confined, stagnant environments where moisture and glass remain in mutual contact for days and even weeks on end. An example of a potentially stagnant environment includes the tiny spaces that exist between stacked glass. Accelerated by summer temperatures, this interaction can lead to permanent glass surface damage similar to that shown in Figure 1.

Due to the inherent characteristics of glass surface corrosion, it is critical that glass be stored carefully and protected properly prior to installation. Fabricated glass units should not be stacked outdoors where they can get wet, nor should they be stored for prolonged periods (six months or more) without instituting special protective measures. If an accidental occurrence results in a pack getting wet, it should be disassembled immediately and dried. These recommendations should be followed with respect to the protection and storage of glass prior to installation. Sometimes, a lack of proper attention frequently emanates from the common-sense view that glass is immutable and cannot be harmed in any situation that might unfold within the ambient environment. However, it only takes a few weeks of exposure to moisture during mid-summer’s heat to create damage similar to that shown in Figure 1. In fact, this glass was stored outdoors and installed in a building after the surface staining shown had occurred. Perhaps it was determined that the condition was transient in nature and could possibly have been addressed by normal window cleaning methods.

At times, the combined effects of a stagnant, warm and humid environment on glass can be devastating to a particular project. Clearly, an enhanced awareness of the practices and technologies applicable to the preservation of glass surface quality can reduce the potential for damage prior to installation. Towards this end, the professional design architects and contract glaziers loom as important allies in disseminating recommendations for the proper storage and protection of glass products that await installation. In view of their preeminent position in the selection and utilization of glass products, it is important to enlist the support of professional architects in pursuing the preservation of glass surface quality.

Preventing Hard-Water Spots
Another area worthy of consideration focuses on events that occur after installation, particularly in commercial structures situated in suburban settings. Typically, the accoutrements of beautifully-designed lawns and gardens accompany construct within these areas. Of course, both of these may require regular doses of water that, depending on the prevailing climate, are provided by means of sources other than rainfall. However, satisfying this relatively simple need can often lead to another form of glass surface damage that is illustrated in Figure 2, hard-water spots.

Most people are aware of the impact hard water has on their daily lives in some form or other whether it be calcium carbonate, lime, scale in a tea kettle or the deposits that need to be removed from a shower stall. In any event, household cleaners that cleanse the bathroom walls effectively or heated vinegar solutions that remove scale from tea kettles tend to create another common sense view of glass surface characteristics that can also lead to situations which result in damage. 

Both examples cited previously give the impression that hard-water spots, while being somewhat problematic, can nevertheless be removed from any surface around with the appropriate cleaning agent. However, this is not true for glass where hard-water spots can take up permanent residence and defy the most noble of cleansing efforts as is the case with the spots shown in Figure 2.
Of the total dissolved solids typically found in hard water, the majority of household and industrial concerns focus on calcium and magnesium-bearing constituents. However, there is another, less conspicuous component that gets little attention but nevertheless can be devastating to glass. More precisely, this includes the family of dissolved silicate materials that behave in a manner quite unlike calcium and magnesium carbonates, which can be removed with weak acidic cleaning solutions such as vinegar.

On the other hand, dissolved silicate species will bond chemically to one another when allowed to deposit from solution by evaporation. Even more disconcerting is the fact that these materials also form bonds to the glass surface producing a “glass-on-glass” deposit. Such deposits are extremely difficult to remove with normal cleaning agents. Vinegar and household cleaners will not dissolve the deposits. As such, this type of problem is one best left to the devices of the window maintenance professional. Once again awareness of the problem and prevention combine to form the best approach in dealing with this issue.
Within the realm of prevention, newly-installed glass or even pre-existing glazing systems should not be subjected repeatedly to over-spray from lawn sprinklers. 

As in the case of the proper care and storage of glass to prevent surface staining, the hard-water spotting issue is one that can be improved effectively by means of raising awareness and improving education. 

Paul Duffer is a senior research associate for PPG Industries, based in Pittsburgh.



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