Ordinary window glass is an amorphous solid, not a slowly flowing liquid.

Amorphous describes atomic arrangement rather than visible shape. A crystalline material has a repeating structural pattern extending over long distances on the atomic scale. Glass lacks that long-range periodicity, although neighboring atoms still have local arrangements and chemical bonds rather than being randomly disconnected.

Disorder does not automatically imply liquidity. At room temperature, ordinary window glass retains its shape and resists deformation as a solid. Descriptions of glass as a frozen or supercooled liquid can refer to its formation or structure, but become misleading when used to claim that household windows noticeably flow.

Old panes with uneven thickness do not demonstrate centuries of downward movement. Historical glassmaking processes produced sheets that were not uniformly thick to begin with. Manufacturing variation is a sufficient explanation for those observations, without requiring room-temperature glass to behave like a slowly pouring fluid.

When a glass-forming melt cools without crystallizing, its structure can become arrested before a regular crystal lattice develops. Atomic rearrangement becomes increasingly slow as temperature falls. The resulting material preserves structural disorder while acquiring the mechanical behavior associated with a rigid solid.

This transition differs from the sharp melting behavior associated with a pure crystalline substance at a specified pressure. Glass softens through a temperature range, and its response depends on composition and thermal history. There is no single universal temperature at which every material called glass changes from completely rigid to freely flowing.

Traditional windowmaking helps explain the persistence of the flow story. Methods that involved blowing, spinning, or flattening glass could leave variations in thickness and optical distortion. Those inherited irregularities remain visible in old buildings. Observing a thick region today does not establish that the glass migrated there after installation.

The distinction between structure and behavior is useful beyond windows. The term glass can describe a broader class of noncrystalline materials, not just transparent household panes. Visible optical transparency itself does not define the category, and an amorphous material need not have the same composition or mechanical properties as soda-lime window glass.

Heating does change the situation. At sufficiently high temperatures, glass becomes workable and can be shaped, drawn, or blown because its viscosity decreases. That familiar hot-working behavior should not be extrapolated to ordinary room temperature. Temperature changes the timescale of structural rearrangement enormously, so hot glass and a cool window are not mechanically equivalent.

A solid need not contain an orderly crystal lattice. Glass separates mechanical behavior from atomic arrangement: it resists ordinary flow while lacking the long-range repeating structure of a crystal. Calling it amorphous describes that arrangement, not a claim that a window is secretly a room-temperature liquid.