For definitive evidence that glass has not flowed in old windows we must examine the oldest examples. Early glass used to make bottles and windows was usually formed by adding soda and lime to silicates. Sometimes potash was added instead. Usually there were other impurities which made it softer than modern soda-lime glass. Other compounds were often added to give colour or to improve its properties. The Romans were making glass objects of this sort in the 1st century AD, and despite being very delicate, some examples remain--such as the elaborately decorated Portland Vase kept at the British Museum. Roman glassware provides some of the best available evidence that types of soda-lime glass are not fluid, even after nearly 2000 years. The oldest remaining examples of stained glass windows that remain in place have lasted since the 12th century. The oldest of all are the five figures in the clerestory of Augsburg Cathedral in Germany, which are dated to between 1050 to 1150. Many other early examples are found in France and England including the magnificent North Rose window of Notre Dame, Paris dating from 1250.
There have been many claims (especially by tour guides) that such glass is deformed because the glass has flowed slowly over the centuries. This has become a persistent myth, but close inspection shows that characteristic signs of flow, such as flowing around, and out of the frame, are not present. The deformations are more consistent with imperfections of the methods used to make panes of glass at the time. In some cases gaps appear between glass panes and their frames, but this is due to deformations in the lead framework rather than the glass. Other examples of rippling in windows of old homes can be accounted for because the glass was imperfectly flattened by rolling before the float glass process came into use.
It is difficult to verify with absolute certainty that no examples of glass flow exist, because there are almost always no records of the original state. In rare cases stained glass windows are found to contain lead which would lower the viscosity and make them heavier. Could these examples deform under their own weight? Only careful study and analysis can answer this question. Robert Brill of the Corning glass museum has been studying antique glass for over 30 years. He has examined many examples of glass from old buildings, measuring their material properties and chemical composition. He has taken a special interest in the glass flow myth and has always looked for evidence for and against. In his opinion, the notion that glass in Mediaeval stained glass windows has flowed over the centuries is untrue and, he says, examples of sagging and ripples in old windows are also most likely physical characteristics resulting from the manufacturing process. Other experts who have made similar studies agree. Theoretical analysis based on measured glass viscosities shows that glass should not deform significantly even over many centuries, and a clear link is found between types of deformation in the glass and the way it was produced.
i will look for the real term for it 
