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Material World
Process

Float glass process

Forming flat glass by floating molten glass on molten tin, giving both surfaces a perfect finish without polishing.

Molten glass is poured onto a bath of molten tin, on which it floats and spreads to an even thickness. Because both the tin surface and the free upper surface are perfectly flat, the glass emerges needing no grinding or polishing at all.

Alastair Pilkington developed it through the 1950s, and it displaced every earlier method of making flat glass within about a decade. Nearly all window glass made anywhere is made this way.

Uses

Almost all flat glass in the world is made this way. Molten glass is poured onto a bath of molten tin, where it spreads and settles into a sheet with both surfaces naturally flat — one against the perfectly level tin, the other under gravity — and is drawn off and annealed.

Windows, facades, vehicle glazing, mirrors, and the substrates for display panels and solar modules all begin as float glass. The process replaced grinding and polishing plate glass, which was slow and wasted a great deal of material, and the cost collapse that followed is why large areas of glazing became ordinary in buildings rather than exceptional.

Medium confidence Weak evidence

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.

takes as input

  • Quartz — mineral · as the silica sand that forms the bulk of the batch
  • Borosilicate glass — material · with difficulty, which is why flat borosilicate costs several times what window glass does
  • Sand — material · the batch is essentially sand, with soda ash to lower the melting point and limestone to stop the result dissolving in water

produces

  • Soda-lime glass — material · drawn off the tin bath as a sheet flat on both surfaces

Sources

  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Float glass process back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Float glass process → takes as input (with difficulty, which is why flat borosilicate costs several times what window glass does) → Borosilicate glass → is produced by (with boron oxide replacing most of the soda, which is what drops the thermal expansion to a third and lets a hot dish go into water) → Glass melting → takes as input (as the stabiliser, and without it a soda-silica glass would slowly dissolve in water) → Limestone → is sourced from (precipitated from it, mostly by organisms building shells and skeletons out of dissolved calcium and carbonate) → Seawater → is composed of (about 96.5 per cent by mass — seawater is 3.5 per cent dissolved solids and the rest of it is this) → Water
  • Float glass process → takes as input (the batch is essentially sand, with soda ash to lower the melting point and limestone to stop the result dissolving in water) → Sand → is sourced from (the ultimate source of most of it — granite weathers, the feldspar becomes clay and washes out, and the quartz survives as grains) → Granite → is composed of (by volume, alongside feldspar and lesser mica) → Quartz
  • Float glass process → takes as input (as the silica sand that forms the bulk of the batch) → Quartz
  • Float glass process → takes as input (with difficulty, which is why flat borosilicate costs several times what window glass does) → Borosilicate glass → is sourced from (as silica sand, still the bulk of the batch) → Quartz
  • Float glass process → takes as input (with difficulty, which is why flat borosilicate costs several times what window glass does) → Borosilicate glass → is sourced from (the boron source, and the ingredient the whole material is named for) → Borax
  • Float glass process → takes as input (the batch is essentially sand, with soda ash to lower the melting point and limestone to stop the result dissolving in water) → Sand → is composed of (almost everywhere, because quartz is what is left when everything else in a rock has weathered away — though a volcanic beach is basalt grains and a tropical one is broken shell) → Quartz

These are the most distinct paths back. Float glass process can be traced through others besides.

Downstream — what it becomes

  • Float glass process → produces (drawn off the tin bath as a sheet flat on both surfaces) → Soda-lime glass → is a source for (not this composition, but the same forming route: a glass-ceramic is made as a glass first and made into a ceramic afterwards) → Glass-ceramic → is an input to (in two stages: one to nucleate crystals everywhere at once, one to grow them — the schedule is the design) → Firing → produces (fired in a continuous tunnel kiln, the atmosphere governing colour as much as the temperature) → Brick → is used in (load-bearing walls and cladding) → Construction
  • Float glass process → produces (drawn off the tin bath as a sheet flat on both surfaces) → Soda-lime glass → is used in (the envelope, which has to seal against metal without cracking as both expand) → Incandescent lamp → is used as (and almost all of the energy leaves as heat, which is inherent rather than a design failure and is why it was legislated away) → Lighting
  • Float glass process → produces (drawn off the tin bath as a sheet flat on both surfaces) → Soda-lime glass → is used in (glazing, facades and internal partitions) → Construction
  • Float glass process → produces (drawn off the tin bath as a sheet flat on both surfaces) → Soda-lime glass → is used as (the product itself — ninety per cent of all glass made, in containers and windows) → Glass and ceramics
  • Float glass process → produces (drawn off the tin bath as a sheet flat on both surfaces) → Soda-lime glass → is used as (inert, reusable and heavy, which is the whole trade-off — glass wins on reuse and loses on freight) → Packaging
  • Float glass process → produces (drawn off the tin bath as a sheet flat on both surfaces) → Soda-lime glass → is used as (cheap, inert and transparent, which is the whole argument for it) → Tableware and vessels

These are the most distinct paths onward. Float glass process ends up in others besides.