How Is Glass Made? Sand That Never Decides to Become a Crystal

Ordinary glass is made from three components with distinct jobs. Silica, meaning silicon dioxide and supplied as clean sand, forms the network, and on its own it melts around seventeen hundred degrees Celsius, which is impractically hot. Soda, meaning sodium carbonate, is added as a flux and breaks some of the links in that network, dropping the melting point by hundreds of degrees and making the process affordable, and that single substitution is why glass became a common material rather than a luxury. The problem is that soda-silica glass dissolves in water, which is useless for a container. Lime, meaning calcium oxide from limestone, is therefore added as a stabiliser, restoring chemical durability. The resulting soda-lime glass accounts for the overwhelming majority of what is manufactured, in windows, bottles and jars, and the recipe has been essentially stable for a very long time because it is a good solution to a tightly constrained problem.

Glass is properly described as an amorphous solid, meaning it has the rigidity of a solid and the disordered atomic arrangement of a liquid. Cooling it does not produce a sharp melting point but a gradual transition over a temperature range, above which it softens progressively and can be worked. That is what makes the material formable: there is a wide window in which it is stiff enough to hold shape and soft enough to blow, draw, press or float, whereas a metal goes from liquid to solid over a narrow range. A persistent myth holds that old window panes are thicker at the bottom because glass flows over centuries, and measurements show the flow rate at room temperature is so slow as to be irrelevant over any human timescale. The real explanation is manufacturing: hand-spun crown glass varied in thickness and glaziers generally installed the heavier edge downward.

Flat glass was the hardest form to produce well, and the methods used to make it changed the material's price and quality repeatedly:

• •Crown glass, spun from a blown bubble into a flat disc, which produced good surfaces and small panes with a distortion at the centre where the rod attached
• •Cylinder glass, blown as a long cylinder then cut and flattened, which gave larger panes with poorer flatness
• •Cast and polished plate, poured onto a table and ground and polished on both sides, which was optically excellent and extremely expensive in labour
• •Drawn sheet, pulled vertically from a bath, which was cheaper and left visible waviness that still identifies mid-century windows
• •The float process, developed in the late 1950s, in which molten glass is poured onto a bath of molten tin and spreads to a uniform thickness with both faces naturally flat
• •The float method made polished-quality flat glass at drawn-sheet cost, displaced every other method within about a decade and is how nearly all window glass is made today

Glass is strong in compression and weak in tension, and it fails from surface flaws rather than from bulk weakness, since a microscopic scratch concentrates stress at its tip and a crack once started runs. That is why a scored line lets a sheet snap cleanly and why handling damage matters so much. Two treatments exploit the same principle in reverse. Toughened or tempered glass is heated and then cooled rapidly at the surface, which leaves the outside in compression and the interior in tension, so an applied load must first overcome the built-in compression before any surface flaw can open, making it several times stronger and causing it to disintegrate into small blunt fragments when it does finally fail. Laminated glass bonds two sheets to a plastic interlayer, so breakage leaves the fragments stuck in place, which is why windscreens are laminated and side windows are usually toughened. Glass is also indefinitely recyclable without loss of quality, and remelting cullet uses less energy than raw batch, which is unusual among materials.

Sand melts into a liquid too tangled to crystallise, so it stiffens while still disordered, which is the amorphous state the material occupies. Soda lowers the melting point enough to be affordable and lime stops the result dissolving in water. The float process, pouring the melt onto molten tin, gave polished flatness at low cost. Failure starts at surface flaws, which is why toughening puts the surface into compression.