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Material World
Process

Firing

Heating shaped clay until its minerals break down irreversibly — the transformation that separates pottery from mud, and why it survives in the ground.

Clay is plastic because its mineral plates slide over one another with water between them. Dried clay gives that water back and becomes plastic again; fired clay does not, because above roughly six hundred degrees the chemically bound water is driven out of the mineral structure and the structure itself breaks down.

That step is irreversible, and it is the whole point. What comes out of the kiln will not soften in water however long it is soaked, which is why pottery survives in the archaeological record when almost nothing else made by the same people does. Fire it further and the particles begin to fuse and the body vitrifies, becoming denser, stronger and eventually impermeable.

Uses

Brick, roof tile, sanitaryware, tableware and porcelain, and — as sintering, with refined powders rather than clay — the whole family of technical ceramics.

The schedule matters as much as the peak temperature. Ware is heated slowly through the early stages so that steam escaping from the body does not crack it, and slowed again through 573 degrees in both directions because quartz in the clay changes volume abruptly at that point. Firing too fast through the quartz inversion cracks the piece, which is a constraint imposed by a mineral rather than by the process.

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

  • Clay — material · shaped and dried, then heated past the point where its minerals break down irreversibly
  • Quartz — mineral · present in most clay bodies, and the reason firing schedules slow through 573 degrees in both directions
  • Stoneware — material · to 1200–1300 °C, which is the vitrification that separates it from earthenware — and it is a temperature rather than a recipe
  • Earthenware — material · to 1000–1150 °C, below vitrification, which is why it stays porous and why it was achievable before anybody could build a hotter kiln
  • Refractory brick — material · or installed unfired as a monolithic castable and cured by the furnace's own first heat
  • Glass-ceramic — material · in two stages: one to nucleate crystals everywhere at once, one to grow them — the schedule is the design

produces

  • Brick — material · fired in a continuous tunnel kiln, the atmosphere governing colour as much as the temperature
  • Technical ceramic — material · as sintering, from refined powders rather than clay
  • Porcelain — material · at around 1,300 °C — the temperature European potters could not reach, which is half the reason porcelain stayed a Chinese secret for a thousand years
  • Phosphor — material · the host lattice and the activator ion are fired together at high temperature, so the rare earth is built into the crystal rather than mixed with it
  • Bone china — material · twice, and in the reverse of the usual order: a high biscuit firing around 1,250 °C which sets the body, then a lower glost firing around 1,080 °C for the glaze — the opposite of the porcelain sequence, because a fired bone china body would slump if it were taken back up
  • Ferrite — material · sintered at 1,100 to 1,300 °C, and the hard grades are pressed in a magnetic field first so every particle is aligned before the firing locks it in
  • Pencil — object · the core, at least: graphite and clay are fired like any ceramic, which is Conté's 1795 method and is still how every pencil is made
  • Stoneware — material · to 1,200 °C or so, hot enough to vitrify the body so it holds water without a glaze
  • Earthenware — material · to around 1,000 °C, which is not hot enough to vitrify — earthenware stays porous, which is why it must be glazed to hold anything
  • Refractory brick — material · and the point of the firing is that the brick has already been to a higher temperature than the furnace it will line

Sources

  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Firing 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

  • Firing → takes as input (or installed unfired as a monolithic castable and cured by the furnace's own first heat) → Refractory brick → is composed of (calcined, as the alumina source for the high-alumina grades) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt → is sourced from (partial melting of mantle peridotite is what basalt magma is — the residue left behind stays peridotite, depleted of what went into the melt) → Peridotite → is composed of (the mineral that defines the rock and gives it its name — a peridotite is olivine-dominant by definition) → Olivine
  • Firing → takes as input (in two stages: one to nucleate crystals everywhere at once, one to grow them — the schedule is the design) → 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 sourced from (as silica sand, still the bulk of the batch) → Quartz
  • Firing → takes as input (to 1200–1300 °C, which is the vitrification that separates it from earthenware — and it is a temperature rather than a recipe) → Stoneware → is sourced from (a clay that survives 1200 °C and above, which an earthenware clay does not) → Clay → is sourced from (clay is chiefly what feldspar becomes when it weathers) → Orthoclase
  • Firing → takes as input (to 1000–1150 °C, below vitrification, which is why it stays porous and why it was achievable before anybody could build a hotter kiln) → Earthenware → is sourced from (almost any clay, which is why it is the ceramic every early culture reached first) → Clay → is sourced from (clay is chiefly what feldspar becomes when it weathers) → Orthoclase
  • Firing → takes as input (shaped and dried, then heated past the point where its minerals break down irreversibly) → Clay → is sourced from (clay is chiefly what feldspar becomes when it weathers) → Orthoclase
  • Firing → takes as input (present in most clay bodies, and the reason firing schedules slow through 573 degrees in both directions) → Quartz

These are the most distinct paths back. Firing can be traced through others besides.

Downstream — what it becomes

  • Firing → produces (the host lattice and the activator ion are fired together at high temperature, so the rare earth is built into the crystal rather than mixed with it) → Phosphor → is used in (in the display, where rare earth emitters convert the backlight or drive the pixel directly) → Smartphone → is used in (and it is where four separate raw-material arguments meet — the 3TG conflict minerals, cobalt, the rare earths, and everything too dispersed to recover) → 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
  • Firing → produces (sintered at 1,100 to 1,300 °C, and the hard grades are pressed in a magnetic field first so every particle is aligned before the firing locks it in) → Ferrite → is used in (and its most consequential use is historical: core memory was the working memory of computers for twenty years, which is why a memory image is still called a core dump) → 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
  • Firing → produces (to around 1,000 °C, which is not hot enough to vitrify — earthenware stays porous, which is why it must be glazed to hold anything) → Earthenware → is associated with (fired clay figurines are around 29,000 years old and predate pottery vessels by some fifteen thousand years — people fired clay into shapes long before using it to hold anything) → Stone Age complete chain
  • 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
  • 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
  • Firing → produces (at around 1,300 °C — the temperature European potters could not reach, which is half the reason porcelain stayed a Chinese secret for a thousand years) → Porcelain → is used as (the demanding end of the trade, and the reason the industry buys feldspar at all) → Glass and ceramics

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