There is an opinion that glass is not actually a solid, but a so-called supercooled liquid, which is why glass in medieval buildings is thicker at the bottom edge. We decided to check whether glass is really a liquid that flows down over time.

The opinion that glass is actually a liquid can be found in posts in various social networks and forums. Various specialized resources also write about this, for example website glass factory in Gus-Khrustalny. Questions about the aggregate state of glass are asked by resource users with questions and answers "Mail.ru Answers" And "Yandex.Q". Basic argument in favor of the fact that glass is still a liquid, is The problem is that in medieval stained glass windows the glass is thicker at the bottom edge, that is, over time it has run off. Statements that glass is a liquid can be found even in authoritative scientific publications. For example, in Russian the article justifying this was published in the journal Science and Life in 1983.

The idea that glass might flow down over time appeared at the beginning of the 20th century. Experiments on glass flow in detail described Russian chemist, candidate of chemical sciences and popularizer of science Ilya Abramovich Leenson in his article “Do window glasses leak.” Physicist Robert Rayleigh was the first to conduct such an experiment when he noticed that his colleagues tried not to store glass tubes for experiments in a vertical position. They were guided by advice from the Nobel Prize winner's book laureate in chemistry by Wilhelm Ostwald “Physico-chemical research”. It noted that such tubes should be stored horizontally on a support, otherwise they will deform under their own weight. Rayleigh decided to experimentally test whether this was true. He took a glass rod about 1 m long and 5 mm in diameter, placed its ends on two pins driven into the wall, and suspended a load weighing 300 g from the center. Under the weight of the load, the rod immediately bent by 28 mm. Rayleigh left the structure in this state for seven years.

In parallel with Rayleigh, an experiment similar in design was launched by a laboratory assistant at the American company General Electric, K. Spencer. However, the load placed on the glass tube was almost three times heavier. In 1930, Rayleigh completed the experiment; the glass tube was bent by only 1 mm. Spencer's results were more clear: the deformation was 9 mm. However, in both experiments a load was used that was several times greater than the weight of the tube itself. It turns out that only under the influence of its weight, deformation, even minor, should not occur. Spencer offered a reasonable explanation for why some of the glass tubes for chemical experiments had a curvature visible to the naked eye. When choosing a tube from a rack, the scientists initially sought to choose the most straight one; those that remained unused were already slightly crooked. Moreover, while the chemists were choosing a straighter tube, they shifted the others in such a way that their deflection shifted to one side, and visually this could be mistaken for the result of glass flow.

In a similar way explained and why in ancient buildings the glass at the bottom is thicker than at the top. In the era of hand-made glass, it was impossible to achieve the same thickness across the entire glass layer, so craftsmen placed the glass in the frame with the thicker end down for normal stability. At the same time, restorers note, such placement was not a strict rule, and you can find buildings in the construction of which this was not done - in them the thicker edge of the glass is not necessarily located at the bottom.

Chemist Paddy Royal from the University of Bristol calls The idea of the fluidity of glass is an urban legend: “Glass, usually made from silicon dioxide, does not change its shape over periods of time comparable to a human life. If it changes shape, it will take billions of years.” Professor of Materials Science and Engineering and Faculty Director of the Glass Laboratory at the Massachusetts Institute of Technology Michael Cima speaks out It’s also clear: “If you leave the glass at room temperature, no matter how long, it will not change in any way.” Doctor of Science in physical chemistry Robert Brill, head of the Corning Museum of Glass (New York State, USA), leads a clear example: in ancient churches, stained glass windows are held in place by lead bindings, and the viscosity of lead is approximately a billion times less than the viscosity of glass. If the glass flowed over time, forming a thickening at the bottom, then the lead binding should have literally turned into a liquid state during the same time.

Brazilian scientists using a mathematical model calculated, how long would it take glass from a medieval cathedral to still flow over time, taking into account its viscosity at room temperature. According to their calculations, this period is 10³² years, despite the fact that the age of our Universe is only 10¹⁰ years! Similar results received Ozgur Gulbiten and John Mauro from the University of Pennsylvania. They examined not abstract glass, but stained glass windows of Westminster Abbey in London. Their measurements showed that ancient glass deforms by just 1 nanometer, or one billionth of a meter, over the course of one billion years.

Modern science found and an explanation for the slight deformation of glass over time under the weight of a load, observed at the beginning of the 20th century by Rayleigh and Spencer. The glass did not actually flow; the reason for the changes was the slow diffusion of positively charged sodium ions. Moreover, if the weight is removed, the ions will eventually return to their original position and the shape of the glass tube will be restored.

And although glass is definitely not a liquid, science didn't come to a single opinion, can glass be considered a solid? The Conversation Magazine describes it goes like this: “There are more theories about glass than there are theorists who put them forward.”