Skip to content
Material World
Process

Glass melting

Melting sand with a flux and a stabiliser, and cooling it fast enough that it never gets the chance to crystallise.

Glass is not a substance but a state, and melting is where it is produced. Silica on its own melts at about 1,700 °C, which is hotter than any furnace could reach for most of history and is expensive now, so almost every glass is silica with something added to bring the melting point down.

Soda — sodium carbonate — does that, and drops it to around 1,500. Soda glass then dissolves in water, so lime is added to stabilise it, and soda-lime glass is the result and is about ninety per cent of all glass made. Substitute boron oxide for most of the soda and the glass expands far less when heated, which is borosilicate. Leave the additives out entirely and melt pure silica, and the result is fused silica, which is the best of them and costs what the temperature costs.

What makes any of them a glass is the cooling. A melt cooled slowly enough will crystallise; cooled fast enough, the atoms are frozen in the disordered arrangement they had as a liquid. Glass is a liquid's structure with a solid's rigidity, and every glass property follows from that.

Processing

The batch — sand, soda ash, limestone, and usually twenty to sixty per cent recycled cullet — is charged into a continuous tank furnace that runs for years without being allowed to cool, because letting it cool would crack the refractory lining.

Melting is followed by fining, which matters more than it sounds: bubbles must be removed, and they are, by adding a salt that releases gas at high temperature so that small bubbles are swept out by large ones rising through the melt.

The melt is then formed while it is workable, and the forming method is what distinguishes the products. Floated on molten tin for flat glass. Blown into a mould for containers. Drawn through a bushing of hundreds of platinum-alloy holes for fibre. Spun off a rotating dish for mineral wool. Drawn as tube for laboratory ware.

Annealing is the last step and is not optional. Glass cooled unevenly is left with internal stress and will crack later, apparently spontaneously, so every piece is taken slowly back down through its annealing range in a lehr.

Economic significance

A glass furnace is the reason the industry looks the way it does. It costs a great deal, must run continuously for ten to fifteen years, and cannot easily be turned down when demand falls — so glassmaking is a high-volume, low-margin, regionally concentrated business with heavy incentives to keep the tank full.

It is also energy-intensive, and among the harder industries to decarbonise: the heat is needed at 1,500 °C, which rules out most electrification approaches, and electric and hydrogen-fired furnaces are being demonstrated rather than deployed.

Cullet is the lever that works. Recycled glass melts at a lower temperature than raw batch, so every ten per cent of cullet saves around two to three per cent of the energy, and container glass is one of the few materials where recycling is straightforwardly better on both cost and emissions rather than being an argument.

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.

produces

  • Soda-lime glass — material · sand, soda ash and limestone, and about ninety per cent of all glass made
  • Borosilicate glass — material · 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
  • Fused silica — material · pure silica with no flux at all, which means melting at around 1,700 °C — the best of the glasses, and it costs what the temperature costs
  • Glass-ceramic — material · melted as a glass and then deliberately crystallised by a second heat treatment, which is the opposite of what every other glass process is trying to avoid
  • Glass fibre — material · drawn through a bushing of hundreds of platinum-alloy holes, at a diameter of a few micrometres and a speed of tens of metres a second
  • Mineral wool — material · spun off a rotating dish so the melt is thrown out as fibres — the same principle as candyfloss, at 1,400 °C
  • Optical fibre — material · drawn from a preform of ultrapure fused silica, at a purity where a kilometre of it is as clear as a window pane

takes as input

  • Sand — material · the largest ingredient by mass, and it must be low in iron because iron colours glass green
  • Limestone — rock · as the stabiliser, and without it a soda-silica glass would slowly dissolve in water

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)

Questions this page answers

Where it comes from, and what it becomes

Follow Glass melting 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

  • 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
  • Glass melting → takes as input (the largest ingredient by mass, and it must be low in iron because iron colours glass green) → 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
  • Glass melting → takes as input (as the stabiliser, and without it a soda-silica glass would slowly dissolve in water) → Limestone → is composed of → Calcite
  • Glass melting → takes as input (as the stabiliser, and without it a soda-silica glass would slowly dissolve in water) → Limestone → is composed of (in young limestone, before conversion to calcite is complete) → Aragonite
  • Glass melting → takes as input (as the stabiliser, and without it a soda-silica glass would slowly dissolve in water) → Limestone → is composed of (in nodules and cavities, strontium substituting for calcium and then separating out — the mineral's commonest sedimentary occurrence) → Strontianite
  • Glass melting → takes as input (the largest ingredient by mass, and it must be low in iron because iron colours glass green) → 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. Glass melting can be traced through others besides.

Downstream — what it becomes

  • Glass melting → produces (drawn through a bushing of hundreds of platinum-alloy holes, at a diameter of a few micrometres and a speed of tens of metres a second) → Glass fibre → is a component of (the reinforcement, and the fibre's strength is not the laminate's) → Glass fibre composite → is used in (the FR-4 laminate itself: woven glass cloth in flame-retardant epoxy, stiff, dimensionally stable when heated, and self-extinguishing) → Printed circuit board → is used in (and it is the change that made electronics manufacturable: assembly stopped being a wiring job and became a printing one) → Electronics manufacture → is associated with (and the supply chain it produced is the deepest and most concentrated in the world economy) → The semiconductor era complete chain
  • Glass melting → produces (pure silica with no flux at all, which means melting at around 1,700 °C — the best of the glasses, and it costs what the temperature costs) → Fused silica → is used in (crucibles, tubes and wafer carriers, where purity and temperature tolerance are both required) → Electronics manufacture → is associated with (and the supply chain it produced is the deepest and most concentrated in the world economy) → The semiconductor era complete chain
  • Glass melting → produces (sand, soda ash and limestone, and about ninety per cent of all glass made) → 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
  • Glass melting → produces (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) → Borosilicate glass → is an input to (with difficulty, which is why flat borosilicate costs several times what window glass does) → 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
  • Glass melting → produces (melted as a glass and then deliberately crystallised by a second heat treatment, which is the opposite of what every other glass process is trying to avoid) → 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 (as sintering, from refined powders rather than clay) → Technical ceramic → is used in (the honeycomb monolith the metals are dispersed across, which supplies the surface area and survives the thermal cycling) → Catalytic converter → is used as (the largest catalytic application by metal value there is) → Catalysis
  • Glass melting → produces (spun off a rotating dish so the melt is thrown out as fibres — the same principle as candyfloss, at 1,400 °C) → Mineral wool → is used as (and it does not burn, which after 2017 is the property regulation cares about most on a tall building) → Thermal insulation

These are the most distinct paths onward. Glass melting ends up in others besides.