PrepTest 75, Section 4, Question 26

To glass researchers it seems somewhat strange that many people throughout the world share the persistent belief that window glass flows slowly downward like a very viscous liquid. Repeated in reference books, in science classes, and elsewhere, the idea has often been invoked to explain ripply windows in old houses. The origins of the myth are unclear, but the confusion probably arose partly from a misunderstanding of the fact that the atoms in glass are not arranged in a fixed crystal structure. In this respect, the structure of liquid glass and the structure of solid glass are very similar, but thermodynamically they are not the same. Glass does not have a precise freezing point; rather, it has what is known as a glass transition temperature, typically a range of a few hundred degrees Celsius. Cooled below the lower end of this range, molten glass retains an amorphous atomic structure, but it takes on the physical properties of a solid.

However, a new study debunks the persistent belief that stained glass windows in medieval cathedrals are noticeably thicker at the bottom because the glass flows downward. Under the force of gravity, certain solid materials including glass can, in fact, flow slightly. But Brazilian researcher Edgar Dutra Zanotto has calculated the time needed for viscous flow to change the thickness of different types of glass by a noticeable amount, and, according to his calculations, medieval cathedral glass would require a period well beyond the age of the universe.

The chemical composition of the glass determines the rate of flow. Even germanium oxide glass, which flows more easily than other types, would take many trillions of years to sag noticeably, Zanotto calculates. Medieval stained glass contains impurities that could lower the viscosity and speed the flow to some degree, but even a significant difference in this regard would not alter the conclusion, since the cathedrals are only several hundred years old. The study demonstrates dramatically what many scientists had reasoned earlier based on information such as the fact that for glass to have more than a negligible ability to flow, it would have to be heated to at least 350 degrees Celsius.

The difference in thickness sometimes observed in antique windows probably results instead from glass manufacturing methods. Until the nineteenth century, the only way to make window glass was to blow molten glass into a large globe and then flatten it into a disk. Whirling the disk introduced ripples and thickened the edges. To achieve structural stability, it would have made sense to install these panes in such a way that the thick portions were at the bottom. Later, glass was drawn into sheets by pulling it from the melt on a rod, a method that made windows more uniform. Today, most window glass is made by floating liquid glass on molten tin. This process makes the surface extremely flat.

To glass researchers it seems somewhat strange that many people throughout the world share the persistent belief that window glass flows slowly downward like a very viscous liquid. Repeated in reference books, in science classes, and elsewhere, the idea has often been invoked to explain ripply windows in old houses. The origins of the myth are unclear, but the confusion probably arose partly from a misunderstanding of the fact that the atoms in glass are not arranged in a fixed crystal structure. In this respect, the structure of liquid glass and the structure of solid glass are very similar, but thermodynamically they are not the same. Glass does not have a precise freezing point; rather, it has what is known as a glass transition temperature, typically a range of a few hundred degrees Celsius. Cooled below the lower end of this range, molten glass retains an amorphous atomic structure, but it takes on the physical properties of a solid.

However, a new study debunks the persistent belief that stained glass windows in medieval cathedrals are noticeably thicker at the bottom because the glass flows downward. Under the force of gravity, certain solid materials including glass can, in fact, flow slightly. But Brazilian researcher Edgar Dutra Zanotto has calculated the time needed for viscous flow to change the thickness of different types of glass by a noticeable amount, and, according to his calculations, medieval cathedral glass would require a period well beyond the age of the universe.

The chemical composition of the glass determines the rate of flow. Even germanium oxide glass, which flows more easily than other types, would take many trillions of years to sag noticeably, Zanotto calculates. Medieval stained glass contains impurities that could lower the viscosity and speed the flow to some degree, but even a significant difference in this regard would not alter the conclusion, since the cathedrals are only several hundred years old. The study demonstrates dramatically what many scientists had reasoned earlier based on information such as the fact that for glass to have more than a negligible ability to flow, it would have to be heated to at least 350 degrees Celsius.

The difference in thickness sometimes observed in antique windows probably results instead from glass manufacturing methods. Until the nineteenth century, the only way to make window glass was to blow molten glass into a large globe and then flatten it into a disk. Whirling the disk introduced ripples and thickened the edges. To achieve structural stability, it would have made sense to install these panes in such a way that the thick portions were at the bottom. Later, glass was drawn into sheets by pulling it from the melt on a rod, a method that made windows more uniform. Today, most window glass is made by floating liquid glass on molten tin. This process makes the surface extremely flat.