the glassy state

Picture a liquid caught in the act of freezing. In an ordinary crystal, the atoms have time to line up into a neat, endlessly repeating pattern, like oranges stacked in a crate. In a glass, the melt was chilled so fast that the atoms seized up before they could ever get into line. The result is a solid that has the frozen, jumbled arrangement of a liquid: rigid to the touch, yet internally as disordered as the syrup it came from. That is the glassy, or amorphous, state.

Amorphous does not mean random in every sense. Zoom in on a silica glass and each silicon is still neatly caged by four oxygens in an SiO4 tetrahedron, and each Si-O bond is still the right length. This is called short-range order, and it survives intact. What is missing is long-range order: the tetrahedra are tilted and twisted at every joint, so if you march a few atoms away the pattern has drifted and there is no repeating unit cell. You can see this fingerprint directly. Shine X-rays on a crystal and you get a set of sharp, needle-thin diffraction spots; do the same to a glass and the spots smear into a few broad, fuzzy halos, the signature of order that fades within a nanometre.

Because there is no crystal lattice to melt, a glass has no sharp melting point. Instead it softens gradually over a range of temperature as it is heated, a fact that governs how all glass is shaped. It also means glass is a genuine solid, not a slow-moving liquid: the old story that medieval church windows are thicker at the bottom because the glass has flowed downhill over centuries is a myth (they were simply made with uneven thickness and installed heavy-side-down). At room temperature a window is as rigid and static as any crystal.