Material · Engineered

Glass-ceramic

Glass made and then deliberately crystallised — a material that can be shaped like glass and then made to behave like a ceramic.

A glass-ceramic is formed as a glass and then heat-treated until most of it crystallises. The trick is control: a nucleating agent is included in the batch so that crystals start everywhere at once and stay small, rather than growing into a few large ones that would make the material weak and opaque.

The result gets both halves of the bargain. It can be melted, poured, pressed or rolled with all the ease of glass, and then becomes a fine-grained ceramic that is stronger, tougher and far more thermal-shock resistant than the glass it started as.

Some compositions reach a thermal expansion close to zero, and a few are slightly negative — the crystal phase shrinks on heating while the residual glass expands, and the two cancel. That is why a ceramic hob can have a pan at 500 °C on one ring and a hand on the next.

Processing

Melted and formed as glass, then held through two temperature stages: one to nucleate, one to grow the crystals. The schedule is the design — the same composition treated differently gives different crystal phases and different properties.

It is machinable in the glass state and hard to machine afterwards, which is exactly the right way round for manufacturing.

Uses

Ceramic hob tops and woodburner windows, which is where most people meet it. Cookware. Telescope mirror blanks, where a substrate that does not change shape as the night cools is worth a great deal. Dental restorations, which are pressed or machined and then crystallised. Missile radomes. Architectural cladding.

And, in a different composition entirely, as a bone-bonding bioactive material for implants.

History

Discovered at Corning in 1953 by Donald Stookey, and by accident: a furnace overheated a photosensitive glass to around 900 °C instead of 600 °C, and the sample that came out was opaque, white and — when he dropped it — bounced rather than shattered.

CorningWare followed in 1958 and put the material in a very large number of kitchens.

Environmental impact

Not recyclable through the container glass stream, for the same reason borosilicate is not: it does not melt with the rest. It is exceptionally durable, and a hob top or a mirror blank is expected to outlast the appliance or instrument around it.

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.

is sourced from

Soda-lime glass — 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 an input to

Firing — process · in two stages: one to nucleate crystals everywhere at once, one to grow them — the schedule is the design

is an alternative to

Borosilicate glass — material · in cookware and hobs: a glass-ceramic is tougher and closer to zero expansion, and cannot be seen through

is produced by

Glass melting — process · 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

Sources

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

Questions this page answers

Where it comes from, and what it becomes

Follow Glass-ceramic 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-ceramic → is sourced from (not this composition, but the same forming route: a glass-ceramic is made as a glass first and made into a ceramic afterwards) → Soda-lime glass → is produced by (drawn off the tin bath as a sheet flat on both surfaces) → 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
Glass-ceramic → is produced by (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 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-ceramic → is produced by (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 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-ceramic → is sourced from (not this composition, but the same forming route: a glass-ceramic is made as a glass first and made into a ceramic afterwards) → Soda-lime glass → is composed of (as silica sand, the principal former) → Quartz
Glass-ceramic → is sourced from (not this composition, but the same forming route: a glass-ceramic is made as a glass first and made into a ceramic afterwards) → Soda-lime glass → is produced by (drawn off the tin bath as a sheet flat on both surfaces) → 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
Glass-ceramic → is sourced from (not this composition, but the same forming route: a glass-ceramic is made as a glass first and made into a ceramic afterwards) → Soda-lime glass → is produced by (drawn off the tin bath as a sheet flat on both surfaces) → Float glass process → takes as input (as the silica sand that forms the bulk of the batch) → Quartz

These are the most distinct paths back. Glass-ceramic can be traced through others besides.

Downstream — what it becomes

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-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-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 as (the same principle at a smaller and far cheaper unit size, which is why it outlasted stone for ordinary building) → Structural engineering
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 as (alumina and silicon carbide are the dominant manufactured abrasives) → Abrasive
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 as (kiln furniture and furnace components) → Refractory lining
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 as (the demanding end of the same industry, where the material is chosen for what it survives rather than what it looks like) → Glass and ceramics

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