Glass is what you get when a liquid is cooled so that its particles cannot organise into a crystal before they lose the ability to move. Its physics, from optics to fracture, follows from that frozen disorder and from the stresses created as it cools.
How it works
As molten soda-lime glass cools, its viscosity (resistance to flow) rises enormously. Below the glass transition, around 550 °C for soda-lime glass, the atoms can no longer rearrange on any practical timescale, and the material behaves as a rigid, elastic solid. There is short-range order (each silicon is still bonded to four oxygens) but no long-range order. The sodium and calcium ions sit in gaps in the network, breaking some Si–O–Si links, which lowers the working temperature.
The physics
Absorption. A photon of wavelength λ has energy
E=λ
Watch it explained
Pilkington Float Glass Process
Pilkington North America, Inc.
Amazing Float Glass Production Process - Glass Manufacturing
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hc
For visible light (λ from about 400 to 700 nm) that is about 1.8 to 3.1 eV. The electrons in silica need about 9 eV to cross its band gap, so visible photons are not absorbed; soda-lime glass starts absorbing only in the ultraviolet, below about 320 nm. That is why ordinary windows block much UV-B.
Reflection. At an interface between media of refractive index n1 and n2, the fraction of light reflected at normal incidence is
R=(n1+n2n1−n2)2
Worked example. For air (n1=1.0) to glass (n2=1.5):
R=(1.0+1.51.0−1.5)2=(2.5−0.5)2=0.04
So 4% reflects at the front surface. Of the 96% that enters, another 4% reflects at the back, so a single clear pane transmits about 0.96×0.96≈0.92, or 92%, ignoring small absorption. Anti-reflection coatings on spectacles and camera lenses reduce this loss.
Residual stress. Glass is strong in compression but weak in tension, because microscopic surface flaws open up and grow into cracks when stretched. In toughening, a pane is heated to about 620 °C and its surfaces are quenched by air jets. The surfaces become rigid while the core is still soft; when the core later cools and contracts, it pulls the surfaces into compression and is itself left in tension. With no outside load the forces must balance, so the stress averaged across the thickness is zero.
Stress across the thickness of toughened glass: a parabola with compression of about 100 megapascals at both surfaces and tension of about 50 megapascals in the centre, the two areas balancing
For a parabolic profile like this one, the surface compression is twice the central tension, which makes the areas above and below the axis equal. A crack must first overcome about 100 MPa of built-in compression before the surface is in tension at all. Once a crack reaches the core, though, the stored elastic energy drives it through the whole pane, which is why toughened glass shatters completely into small, blunt fragments.
How it's made
A float plant feeds sand, soda ash, limestone, dolomite and cullet (recycled glass) into a regenerative furnace at about 1,500 °C. Gas bubbles from decomposing carbonates, such as CaCO3→CaO+CO2, must rise out of the melt before it leaves, a stage called fining.
The melt flows onto molten tin, which is denser than glass and has a perfectly flat surface. Gravity spreads the glass and surface tension resists it; the balance gives an equilibrium thickness of about 6 mm. Rollers at the edges stretch the ribbon thinner or hold it back to make it thicker. A nitrogen–hydrogen atmosphere keeps the tin from oxidising. The ribbon leaves at about 600 °C, stiff enough to touch rollers without marking.
In the annealing lehr, a tunnel over 100 m long, the temperature is lowered slowly through the glass transition so that stresses relax before they freeze in. Only then can the glass be scored and cut cleanly.
Container glass is formed by machines that shear gobs from a stream of melt and press or blow them in moulds; studio glassblowing uses the same physics at a gentler pace.
Limits and trade-offs
Brittleness. Glass has no way to deform plastically and blunt a crack, so its real strength is set by its worst surface flaw, not by its strong bonds.
Thermal shock. Soda-lime glass expands by about 9×10−6 per kelvin. Sudden heating sets up stresses that can crack it. Borosilicate glass, used for lab glassware, expands about a third as much.
Energy. Furnaces run continuously at high temperature and release carbon dioxide from both fuel and carbonates. More cullet and electric melting reduce this.
Toughened glass cannot be cut after toughening; it must be shaped first. Laminated glass, with a plastic layer, is chosen where fragments must stay in place, as in windscreens.
It is not a liquid. Calculations from measured viscosity show that medieval window glass would need far longer than the age of the universe to sag visibly at room temperature.
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.