1. Why Some Old Windows Are Thicker at the Bottom
Medieval glassmakers could not make perfectly flat panes. In the "crown" method, they blew a hollow globe of glass, flattened it, and spun it out into a flat disk. A disk made this way is not the same thickness everywhere, so panes cut from it were uneven from the start [1].
When workers set such a pane into a frame, the thicker edge was often placed at the bottom. Mauro, one of the researchers behind the study below, calls that the natural way to orient an uneven pane [1]. A newspaper report on the same question puts it plainly: the windows "were just as unevenly thick when new" [3].
So the story that glass sagged downward is a tidy explanation for a lopsided pane. But a tidy explanation can still be wrong. The better test is to ask how fast glass could flow at room temperature, if it flows at all.
2. How Much Could a 750-Year-Old Window Have Flowed?
Researchers at Corning, a glass company in the United States, and Penn State University studied glass from windows in Westminster Abbey in London, dated to 1268. They used the glass's chemical makeup to estimate its viscosity, which is how strongly a material resists flowing. Honey has a high viscosity; water has a low one [1] [2].
Their result was a twist. According to the news summaries, earlier estimates of how this kind of glass flows were off by about 16 orders of magnitude, a factor of ten million billion: the glass flows far more easily than those estimates said. The older estimates had been based on a different kind of glass, soda-lime silicate glass [1] [2].
Even so, according to the news summary, the team calculated that the window would flow only about 1 nanometer in one billion years. A nanometer is one millionth of a millimeter. The window is only about 750 years old, so any flow would be far too small to see or to explain a thick bottom edge [1] [2]. The "flowing window" story does not hold up.
3. What Makes Glass Different From Ice
That settles the windows. It does not settle the title question, because "does not flow in a visible way" is not the same as "is a solid."
Water freezes at one definite temperature, and its molecules snap into a neat, repeating pattern: ice. Glass does not freeze like that. As melted glass cools, its molecules "just move slower and slower and slower," as one report put it. There is no single temperature where it suddenly turns solid [3].
That is why glass is hard to classify: its atoms stay jumbled, like those of a liquid, but they have almost stopped moving. Penn State's John Mauro describes it this way: glass has a liquid-like atomic structure, but mechanically it responds as a solid [2].
4. What Something That Really Flows Slowly Looks Like: The Pitch Drop
To see how different "very thick" and "glass" are, look at a real slow flow. In 1927, physics professor Thomas Parnell at the University of Queensland in Australia set up an experiment with pitch, a black, tar-like substance. He heated it, poured it into a glass funnel, and let it cool for three years. In October 1930 he cut the bottom of the funnel, and the pitch began to ooze out [4].
To the eye it looks like a solid. Yet the first drop took eight years to fall after the funnel was cut, and the next five drops took seven to nine years each. The eighth drop took more than 12 years to fall. A 1984 paper put the pitch's viscosity at about 250 billion times water's viscosity [4].
Here is the surprising part. In all those years, nobody has ever seen a drop fall; the university notes that Guinness World Records recognises it as the longest-running laboratory experiment [4]. Pitch really does flow, but only on a timescale of decades, while a medieval window shows nothing even after centuries. The comparison helps show why any possible flow in glass would be hard to notice.
5. Why Scientists Still Disagree About What Glass Is
So, liquid or solid? A chemistry professor at the University of Sydney, Peter Harrowell, once summed it up as "the thickest and gooiest of liquids, and the most disordered and structureless of rigid solids" [3].
Penn State's Mauro says that because glass sits between the two, it "has eluded definition, even among experts," and he has argued for changing how glass is defined [2]. In 2008 a news report noted that scientists still disagreed, "with some vehemence," about the nature of glass. It also quoted the physicist Philip Anderson, who wrote in 1995 that the nature of glass might be the deepest and most interesting unsolved problem in solid state theory [3].
For everyday purposes, the answer to the title is this: glass is not a liquid that creeps down windows. At normal temperatures it acts as a solid. Yet its atoms are arranged like a frozen liquid, and exactly how it "freezes" is still debated. That is why both "liquid" and "solid" feel slightly wrong.
6. A Safe Way to See Thickness Change With Temperature
You cannot make glass at home, but you can feel how temperature changes how thick a liquid is. Ask an adult to help. Put a closed jar of honey in the refrigerator overnight. The next day, scoop the same amount from the cold jar and from a jar kept at room temperature onto two spoons, and tilt both at the same angle. Which honey moves more slowly? If neither moves, wait a few minutes and try again.
Honey in a refrigerator does not become glass. This only shows that cooling makes a liquid slow down. If you pass an old building, look at its windows from outside and notice any waviness, but remember that waviness alone does not tell you why the glass is uneven.