Flat glass furnished for buildings is uniquely resistant to chemical attack and other degradation. Substances and conditions which may cause many building materials to deteriorate have no important effect on glass. The basic characteristics of glass are not altered by the principle environmental factors of solar radiation, rain, or wind. In view of this, it is surprising to find buildings with serious glass staining problems. On occasion, the staining evolves into etching of the glass surface, requiring replacement of the glass

Staining normally implies a chemical change or degradation of the glass surface. The discussion which follows is on this basis.

Glass is inert to most acids and a wide variety of other chemicals. Only phosphoric and hydrofluoric acid are harmful. Either of these in concentrated form will rapidly etch a glass surface and are commonly used for this purpose. Among many other uses, strong solutions of hydrofluoric acid are used for cleaning new concrete. There is a possibility that this acid may be splashed onto the glass during the construction of the building. The staining can become severe in an hour or less. The recorded cases of this occurring are rare, however.

Alkaline materials, unlike acids, will attack glass surfaces to varying degrees. In concentrated forms, etching of glass surfaces can occur very rapidly, within a few hours in some cases. Diluted alkaline solutions are often used as glass cleaners and attack on the glass surface is extremely slow. However, even though the concentration may be low, the glass surface should always be thoroughly rinsed after cleaning to avoid staining.

Some of the more common alkaline solutions are ammonia, trisodium phosphate (used in diluted form as a commercial glass cleaner), most cleaners and detergents and, run-off from construction materials such as concrete, mortar, plaster, and gravel.

Probably the most common cause of staining after the glass is installed is from the run-off of rain from unsealed concrete panels or mortar joints. The resulting alkaline solution can be concentrated and cause serious etching in a few days. Another common cause is the use of a concentrated liquid cleaner as a lubricant for gasket-glazed installations. In this case, even though the glass is rinsed, some cleaner is trapped between the gasket and the glass. This will seep out onto the glass over a period of time and a serious attack of the glass surface may result. Only lubricants recommended by the gasket manufacturer should be used.

Glass Staining in Storage or Shipment

Staining of glass in storage or shipment is almost always caused by an alkaline attack triggered by the presence of water. Special conditions often occur when glass is packed close together in crates or racks. These conditions do not exist for glass in practical uses, such as in windows, curtain walls, and automobiles.

For glass in storage, conditions are markedly different. Water may collect between the panes of glass. This can be from a leaky pipe or a roof for glass stored indoors and from rain, snow, or ground water for glass stored outdoors. Most often, however, it is caused by condensation of water vapor on the glass. This is possible whenever the glass temperature is below the dew point of the surrounding air. For glass stored outdoors or in an unheated warehouse, this is very likely to occur.

The glass temperature will change more slowly than the air temperature because of the bulk and consequent temperature lag of the glass. A crate of glass stored all night at a cool temperature may be close to the air temperature early in the morning. The air temperature and moisture vapor content often increase rapidly after sunrise. The glass will remain at a much lower temperature for a number of hours. Under these conditions, condensation of water vapor is likely. This water is trapped and usually cannot drain away. It evaporates slowly and often will only partially evaporate during a 24-hour period. If the same temperature cycle reoccurs a number of times, the glass surface may remain wet for a number of weeks or months. As a portion of the water evaporates, the alkaline solution becomes more and more concentrated. It will almost always become sufficiently concentrated to etch the glass.

Severity of Glass Staining

During the first stage of staining a transparent white film will appear on the glass. Under some lighting conditions, this is observable on installed glass which has not been washed in some time and almost always uniformly covers the entire glass surface. Visually, it cannot be readily separated from surface contamination from cigarette smoke, atmospheric dust and other materials.

The second stage of staining appears as an iridescence or discoloration of the glass surface and has the appearance of an oil film. This is multi-colored with a very irregular pattern. It is readily observable when daylight or other light sources are viewed by reflection from the glass surface.

The final stage of staining produces an irregular translucent white discoloration which is readily discernible under most lighting conditions. It can reach the stage where objects cannot be clearly distinguished through the glass.