Is glass a solid or a liquid?
Window glass is an amorphous solid at room temperature. Its atoms do not form a regularly repeating pattern over long distances as in a crystal, but this disorder does not make it a flowing liquid on everyday timescales.
Structure and operating mechanism
1. A solid does not have to be crystalline In a salt crystal, the structural units repeat in an ordered pattern. Glass has relationships between nearby atoms, but the same pattern does not continue over long distances. This is called an amorphous structure. Assuming that every solid must be crystalline leads to the wrong conclusion. Mechanical response and atomic arrangement are separate properties. Under ordinary conditions glass retains its shape and resists shear. Atomic vibration does not prove that it is liquid either: atomic motion also occurs in solids above absolute zero. Microscopic movement and visible flow are different concepts. 2. Thinking about the liquid-to-glass transition As a molten glass composition cools, its molecular movements become progressively slower. If crystallization does not occur before these movements slow sufficiently, the disordered arrangement can become effectively arrested. Mechanical properties change greatly across the glass-transition region. This differs from a pure crystal melting at a sharply defined temperature. The observed transition depends on composition and cooling rate, so one glass-transition temperature cannot describe every glass. Ordinary window glass, laboratory borosilicate glass and amorphous polymer materials do not all have the same properties. 3. The limits of the supercooled-liquid description The phrase supercooled liquid, used when discussing glass formation, is sometimes interpreted to mean that room-temperature glass continuously drips. That conclusion does not follow. As a nonequilibrium structure, glass can undergo very slow structural relaxation, but this does not explain windows visibly flowing downward over a few centuries. Timescale matters. Engineering behavior depends on the measurable rate of change, not merely on a theoretically possible change. At higher temperatures viscosity decreases substantially, allowing glass to be shaped industrially. These high-temperature conditions should not be confused with normal service conditions.
Examples, measurement and limitations
4. Why old windows can have thick lower edges Uneven thickness in historic panes is associated with earlier production and shaping methods. If a pane was uneven from the beginning, present-day thickness differences are not evidence of flow. A thicker lower edge alone cannot establish the entire original shape. Testing the claim requires knowledge of the starting geometry, manufacturing technique and comparable examples. This illustrates why inferring a historical mechanism from a difference observed today needs care. A story involving shape change does not by itself demonstrate that the material actually flowed. 5. Hardness differs from brittleness Glass hardness describes a degree of resistance to scratching. Brittleness means that an advancing crack does not consume much energy through large plastic deformation. These properties do not contradict one another. A small surface defect can concentrate tensile stress and initiate fracture. An amorphous structure does not make every glass equally brittle or equally strong: composition, surface condition, production and strengthening treatments change the result. Flexible glass fibers and a seemingly rigid thick window also remind us that dimensions and geometry influence the behavior we observe. 6. Why the classification matters An explanation of glass should specify temperature, observation time and applied load. Comparing a window at room temperature with glass softened in a furnace as though they were under identical conditions is misleading. Nor does amorphous solid name a single chemical substance: there are different glass families. Heating glass at home to test its softening creates burn and fracture risks and is unnecessary. The scientific distinction separates atomic disorder from mechanical flow. Glass may share some structural features with liquids, while its everyday mechanical classification remains that of a solid.