It is a little known fact, because of its apparent durability to weather, that glass is actually attacked by water.Ordinary window glass is correctly described as soda-lime-silicate glass.  It is a matrix of silica interspersed with sodium and calcium ions.  Sodium and calcium salts are added to the sand (silica) to make the glass more easily workable in the molten state.

If the glass surface is wet, the water leaches out the sodium ions.  In the normal course of events, the water is washed away in the rain, or dries off, or the windows are cleaned regularly, so the effect goes unnoticed – it is, literally, only skin deep and not visible or damaging.

However, if the water remains in contact with the glass then the sodium ions remain in solution and turn the water alkaline.  This is very diluted in large quantities of water, such as aquaria, but if the quantity of water is very small, such as can happen with water wicking in between plates of stacked glass, or trapped under dust particles which are not cleaned off, then it can become quite strongly alkaline.

Strongly alkaline water attacks glass.  Wet cement, or even the water running off fresh cement can be regular causes of surface damage and discoloration of glass in new buildings.  The alkaline water starts to attack the silica matrix and the first appearance of this is an ‘oily’ or ‘rainbow’ stain on the glass surface, noticed usually when the reflection off the glass is of a uniform background (e.g. the sky).  Further attack develops this into a ‘chalky’ deposit which is much more obvious. 

Wet stacked glass with no interleavant can, at this stage, actually ‘weld’ the glass together, as the deposits on adjacent surfaces become intermeshed.  Dust particles (which themselves often have high silica content) can also be ‘welded’ on to the surface – old glass which has never been cleaned may prove impossible to clean satisfactorily.

The stain, be it ‘oily’ or ‘chalky’ often cannot be removed by ordinary cleaning methods.  It has to be polished off, using rouge or some similar abrasive polish, to remove the damaged surface and get back to undamaged glass.

The effect is purely visual.  It is limited to a few microns depth of the surface and does not reduce the glass strength to a level which would make it unable to withstand the service loads (wind etc.).

Many standards, codes of practice and product brochures give advice on storage and maintenance of glass.  These usually contain instructions to store the glass in a dry environment and to clean it regularly once glazed.  While the reason for these instructions is not normally explicitly stated, it is to prevent water attack or severe water attack on glass and its effects as described.

When the major glass manufacturers deliver glass, the glass is separated by an interleavant, be it paper or powder.  One of the properties of the interleavant material is that it is mildly acidic (acids, apart from hydrofluoric acid, have little or no effect on glass), a mechanism to counter the potential effects of water wicking in between the plates by neutralising the alkali.  However, its effect is limited in duration and extended exposure to trapped water will overcome the interleavant.