Soda-lime glass
The commonest glass — silica with soda to lower its melting point and lime to stop it dissolving in water.
Soda-lime glass is around 70 percent silica, with roughly 15 percent sodium oxide and 10 percent calcium oxide. Each addition solves a specific problem. Pure silica melts around 1,700 °C, which is impractical; soda brings that down by hundreds of degrees. But soda-silica glass is water-soluble — genuinely so, which is what water glass is — and lime is what makes it durable.
The composition is therefore not a recipe someone happened on. It is the minimum set of ingredients that makes glass both meltable and permanent, and it has been essentially stable for two thousand years.
Why it behaves as it does
Soda-lime glass is transparent because it has no internal boundaries to scatter light and no electrons able to absorb it. Its atoms are arranged as a disordered network rather than as crystals, so there are no grain boundaries — which is the reason most ceramics of similar composition are opaque while glass is not. Light passing through meets a uniform medium and keeps going.
The absorption half matters just as much. The electrons in silica's bonds are held tightly enough that visible light does not carry the energy to lift them, so the photons pass instead of being absorbed. Add a little iron and the glass turns green; add cobalt and it turns blue — those ions do have absorbing transitions in the visible range, which is how glass is coloured at all. The same glass is opaque to ultraviolet, where the photons finally do carry enough energy.
Processing
Nearly all flat glass is now made by the float process, in which molten glass is poured onto a bath of molten tin. Glass floats on tin, spreads to a uniform thickness under its own weight, and cools with two perfectly flat surfaces that need no grinding or polishing. Before its introduction in 1959, flat glass had to be ground and polished mechanically at enormous cost.
Uses
Containers and flat glass account for nearly all of it. Bottles and jars exploit the fact that glass is inert enough to store food and drink indefinitely without affecting flavour, and can be recycled repeatedly without degrading.
Flat glass — windows, facades, vehicle glazing, mirrors — is made by the float process and is the form most people are surrounded by. Toughened and laminated variants extend it to safety applications. Soda-lime glass is not used where thermal shock or chemical attack is expected; borosilicate handles those, at greater cost, which is the practical division between the two.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is composed of
- Quartz — mineral · 68–74% · as silica sand, the principal former
is produced by
- Float glass process — process · drawn off the tin bath as a sheet flat on both surfaces
- Glass melting — process · sand, soda ash and limestone, and about ninety per cent of all glass made
is used in
- Construction — industry · glazing, facades and internal partitions
- Incandescent lamp — object · the envelope, which has to seal against metal without cracking as both expand
- Stained glass window — object · coloured in the melt by metal oxides rather than painted on the surface
- Smartphone — object · as an aluminosilicate cover glass with the surface sodium exchanged for larger potassium, which leaves it in compression — a crack cannot open against compression, which is why a screen survives being sat on
- Solar panel — object · low-iron, and about three quarters of the module by mass
is used as
- Glass and ceramics — application · the product itself — ninety per cent of all glass made, in containers and windows
- Packaging — application · inert, reusable and heavy, which is the whole trade-off — glass wins on reuse and loses on freight
- Tableware and vessels — application · cheap, inert and transparent, which is the whole argument for it
is an alternative to
- Polyethylene terephthalate — material · in bottles: a tenth the weight, unbreakable, and a worse oxygen barrier — glass keeps wine and beer, PET took soft drinks and water
- Polycarbonate — material · as glazing where breakage is unacceptable: polycarbonate absorbs perhaps two hundred times the impact energy and scratches far more easily
- PMMA — material · acrylic transmits more light, weighs half as much, and can be cast in thicknesses glass cannot — which is why large aquarium windows are acrylic. Glass wins on scratch resistance and cost
- Borosilicate glass — material · a third of the thermal expansion and several times the price. Soda-lime is what a window and a bottle are; borosilicate is what survives the oven
contains
- Selenium — element · added in traces as a decolouriser: iron in the sand makes glass green, and selenium's pink cancels it — which is why a clear bottle contains a few grams per tonne of a rare element
was succeeded by
- PMMA — material · in vehicle rear light lenses and in large aquarium windows, on weight and on how it fails
is produced at
- Murano — place · concentrated on one island from 1291, ostensibly for fire safety and effectively to keep the technique where it could be watched
is a source for
- Glass-ceramic — material · not this composition, but the same forming route: a glass-ceramic is made as a glass first and made into a ceramic afterwards
is commonly confused with
- Borosilicate glass — material · sold under one trade name in some markets: American kitchen 'Pyrex' was reformulated to tempered soda-lime, which is strong against impact and no better against heat
Sources
- Material WorldOur own writing