A persistent question asks whether glass is a liquid or a solid. This confusion is often fueled by the appearance of very old windowpanes, which seem thicker at the bottom, suggesting a centuries-long flow. While the classification of glass is not as simple as a crystalline solid or a standard liquid, the evidence firmly places it in a single state of matter. Understanding this material requires examining its molecular structure and how scientists define the line between a fluid and a fixed solid.
Glass: The Amorphous Solid
The true nature of glass lies in its unique internal arrangement, classifying it as an amorphous solid. Unlike crystalline solids, such as quartz or salt, which have highly ordered, repeating lattice structures, glass lacks this long-range atomic order. Instead, the molecules in glass are arranged randomly, a structural characteristic it shares with liquids.
The term “supercooled liquid” is sometimes used to describe glass, but material scientists consider this a misleading historical misnomer. A supercooled liquid is cooled below its freezing point without solidifying, and its molecules are still able to move freely. When molten glass cools, it passes through the glass transition temperature (\(T_g\)), where it becomes rigid and molecular movement effectively stops.
Below this temperature, the material is considered configurationally “frozen” in its disordered state. While the atoms possess the random arrangement of a liquid, they are held in place with sufficient cohesion to maintain a rigid, fixed structure. This combination of structural disorder and mechanical rigidity is the definition of an amorphous solid.
Defining Flow and Viscosity
The distinction between a solid and a fluid depends on a material’s ability to flow under shear stress. True fluids deform and flow continuously when a force is applied, while solids resist deformation and maintain a fixed shape. Glass is mechanically rigid and holds its shape over any human timescale, demonstrating the behavior of a solid.
This physical property is quantified by viscosity. The viscosity of common window glass at room temperature is astronomically high, typically cited to be in the range of \(10^{17}\) to \(10^{20}\) pascal-seconds (Pa·s). For comparison, water has a viscosity of about \(10^{-3}\) Pa·s.
Even viscous substances like pitch or tar, which visibly flow over days or weeks, have a viscosity many orders of magnitude lower than room-temperature glass. Although glass molecules theoretically undergo slow rearrangement, the rate is so minimal that it does not constitute flow in any practical or scientific sense. The material is classified as a solid because its high viscosity places it far beyond the threshold for liquid behavior.
Addressing the Myth of Ancient Glass
The belief that glass is a flowing liquid is often supported by the observation that medieval church windows are thicker at the bottom. This unevenness, however, is a direct result of historical manufacturing limitations, not gravity. Before modern industrial processes, glass was not produced in perfectly uniform sheets.
One common technique was the Crown Glass method, where a large, blown glass sphere was spun rapidly to create a flat disc. This centrifugal force caused the glass to be thicker near the center and thinner toward the edges, creating an uneven sheet. Glaziers cut panes from this imperfect disc and habitually placed the thicker, heavier edge at the bottom for stability within the frame.
Manufacturing Methods
Another process was the Cylinder method, which involved blowing a long cylinder of glass that was then cut and flattened into a sheet. This also yielded non-uniform thickness and surface waviness.
Scientific measurements and analysis of ancient glassware, including Egyptian vessels that are thousands of years old, have shown no evidence of sagging or flow over millennia. Calculations confirm that it would take many millions of years for a glass pane to show even a slight, measurable increase in thickness at the base.