Glass is an everyday material with an unusual structure. It is a solid, yet it lacks the orderly crystal pattern of most solids. That structure, set by how it is cooled, explains why glass is transparent, why it is brittle, and how factories make it so flat and so strong.
How it works
Most window and bottle glass is soda-lime glass: about 70–75% silica (silicon dioxide, SiO2), about 13–15% sodium oxide and about 10% calcium oxide, by mass.
In quartz, the crystalline form of silica, every silicon atom bonds to four oxygen atoms and the units repeat in a regular pattern. In glass the same bonds exist, but the network is irregular: there is no long-range order. A material like this is called an amorphous solid.
Watch it explained
Pilkington Float Glass Process
Pilkington North America, Inc.
Amazing Float Glass Production Process - Glass Manufacturing
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Simplified flat picture of atoms: in a crystal the atoms repeat in a regular pattern; in glass the same kind of bonds join the atoms but the pattern is irregular
The science
Transparency. Light is a stream of tiny packets of energy called photons. A material absorbs a photon only if the photon has enough energy to lift one of its electrons to a higher energy level. In silica, the electrons are held so tightly that it takes the energy of an ultraviolet photon to move them. Photons of visible light carry less energy, so they pass through. Because glass has no crystal grains, there are also no grain boundaries to scatter light. A faint green tint at the edge of a thick pane comes from traces of iron.
Bending light. Light slows down to about 2×108 m/s in glass. The refractive index is n=vc=2.0×1083.0×108=1.5. That change of speed is why lenses and the curved side of a jar bend light.
Melting point. Pure silica melts at about 1,700 °C. Adding soda ash (sodium carbonate) breaks up some of the silica network and lowers the working temperature to about 1,500 °C, saving fuel. In the furnace, carbonates decompose, releasing carbon dioxide:
CaCO3→CaO+CO2
Soda alone would make glass that dissolves slowly in water; the calcium oxide from limestone stops that.
No melting point at all. A crystal melts at one sharp temperature. Glass simply gets softer and softer as it is heated, which is exactly what lets a glassblower shape it.
How it's made
Sand, soda ash, limestone and crushed recycled glass (cullet) are weighed, mixed and fed continuously into a furnace at about 1,500 °C. A large furnace runs day and night for many years.
Float glass. Molten glass flows onto a bath of molten tin. Tin is denser than glass, so the glass floats and spreads into a flat ribbon, while gravity and surface tension give it a natural thickness of about 6 mm. The ribbon enters at about 1,100 °C and leaves stiff at about 600 °C. The bath is filled with nitrogen and hydrogen so the tin does not react with oxygen.
Annealing. If glass cools unevenly, the outside sets first and the inside pulls on it as it shrinks, locking in stress. An annealing oven (a lehr) cools the ribbon slowly and evenly so no stress remains.
Toughening. Cut panes are reheated to about 620 °C and then cooled quickly with jets of air. The surfaces set first and the core shrinks afterwards, pulling the surfaces into compression. Cracks grow only when a surface is stretched, so toughened glass is about four to five times stronger. When it does break, the stored energy shatters it into small, blunt cubes.
Shaping. Bottle machines drop gobs of molten glass into moulds and blow them into shape. Glassblowers gather glass on a hollow pipe and inflate it by mouth.
Recycling. Cullet melts at a lower temperature than the raw mix, so every 10% of cullet cuts furnace energy by roughly 2–3%. Glass can be recycled indefinitely.
Common misconceptions
"Glass is a very slow liquid." It is not. At room temperature glass is so stiff that any flow would take vastly longer than human history to see. Old windows are thicker at the bottom because old methods made uneven panes, and glaziers usually set the heavier edge down.
"Glass is transparent because it has no atoms in the way." Glass is full of atoms; they just cannot absorb visible photons.
"Toughened glass cannot break." It is stronger, but a hard knock on an edge or a sharp point can still shatter it.
Screens and TVsHow millions of tiny red, green and blue dots, switched on and off dozens of times a second, fool your eyes into seeing moving pictures.