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Material World
Material · Engineered

Fused silica

Glass made of nothing but silica — almost no thermal expansion, transparent into the ultraviolet, and difficult enough to make that it is used only where nothing else will do.

Optical medium Refractory

Fused silica is glass with essentially nothing in it but silicon dioxide. Removing every other oxide removes the things that ordinary glass depends on to melt at a workable temperature — so it is difficult and expensive to make — and it also removes almost everything that limits what glass can do.

Its thermal expansion is about a twentieth of soda-lime glass's, low enough that a red-hot piece can be dropped into water without breaking. It transmits ultraviolet that ordinary glass absorbs. It is chemically inert to nearly everything except hydrofluoric acid and hot alkali. And in its synthetic form it is transparent enough that light travels kilometres through it, which is what an optical fibre is.

Processing

Melted from high-purity quartz sand in an electric or flame furnace at above 1700 °C, or — for optical grades — deposited from silicon tetrachloride vapour, which leaves the transition metals that absorb light behind in the starting material rather than carrying them into the glass.

That second route is why an optical fibre is possible at all. No amount of melting natural quartz gives glass clean enough for a signal to survive a hundred kilometres of it.

Uses

Optical fibre. Semiconductor process equipment — crucibles, tubes and wafer carriers — where its purity and its temperature tolerance are both required. Ultraviolet optics and lamp envelopes. Precision mirror substrates and metrology, where a material that does not change size with temperature is the point. Laboratory ware for the most demanding work.

And, historically, a great deal of high-temperature apparatus for which borosilicate was not enough.

History

Made in small quantities from the 1830s and industrially from the early twentieth century. Its transformative use is recent: Corning demonstrated a fibre with low enough loss for telecommunications in 1970, and the global network followed.

Environmental impact

Energy intensive to produce and long-lived in use, and not recycled in any consumer stream — it appears in equipment and infrastructure rather than in products people throw away.

The optical fibre network it makes possible is, per unit of data moved, an enormous reduction in energy against any alternative, which is the honest way to account for 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.

contains

  • Silicon — element · and essentially nothing else — removing every other oxide is what the material is
  • Oxygen — element · two per silicon, in a continuous random network

is sourced from

  • Quartz — mineral · melted directly for the ordinary grades — and deliberately not, for optical fibre, because the transition metals that absorb light survive the melt

is used as

  • Telecommunications — application · as optical fibre, and the vapour route exists because no amount of melting sand gives glass clean enough

is used in

  • Electronics manufacture — industry · crucibles, tubes and wafer carriers, where purity and temperature tolerance are both required

is an alternative to

  • Borosilicate glass — material · a twentieth of soda-lime's expansion against borosilicate's third, transparent into the ultraviolet, and far harder to make

is produced by

  • Glass melting — process · 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

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 Fused silica 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

  • Fused silica → is produced by (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 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
  • Fused silica → is sourced from (melted directly for the ordinary grades — and deliberately not, for optical fibre, because the transition metals that absorb light survive the melt) → Quartz
  • Fused silica → is produced by (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 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
  • Fused silica → is produced by (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 melting → takes as input (as the stabiliser, and without it a soda-silica glass would slowly dissolve in water) → Limestone → is composed of → Calcite
  • Fused silica → is produced by (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 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
  • Fused silica → is produced by (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 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

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

Downstream — what it becomes

  • 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
  • Fused silica → is used as (as optical fibre, and the vapour route exists because no amount of melting sand gives glass clean enough) → Telecommunications