Glass is an amorphous solids and therefore are pseudo solids.Hence like liquids it has tendency to flow.therefore in old buildings the glass panes look milky in appearence.
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Heat difference of day&night which may convert amorphous to some crystalline form in glass.

It is a slow process, due to the flow of glass particles from top to bottom (very slightly).

Hence, it looks faded in an old building.  

Heat difference is due to continuous exposure to the sun in day& cooling process in nights.