Material · Engineered

Borosilicate glass

Glass with boron in it, which barely changes size when heated — the reason an oven dish does not shatter and a drinking glass does.

Borosilicate glass replaces much of the sodium and calcium of ordinary glass with boron oxide, and the effect is one number: its thermal expansion is roughly a third of soda-lime glass's.

That is the whole of why it survives thermal shock. Glass breaks on heating not because it is weak but because the hot face expands against the cold one behind it, and the stress that generates is proportional to how much it expands. Cut the expansion by three and the same temperature difference generates a third of the stress.

It is also more chemically resistant, which is why laboratory glassware is borosilicate. And it is harder to melt and work than soda-lime glass, which is why windows and bottles are not.

Processing

Melted at higher temperatures than soda-lime glass — around 1650 °C — and worked over a wider temperature range, which is what makes it the glassblower's material for scientific apparatus: it stays workable long enough to shape without a furnace at hand.

It cannot be float-formed as easily as soda-lime glass, so flat borosilicate is drawn or rolled and costs several times as much per square metre.

Uses

Laboratory glassware, above all, where both the thermal shock resistance and the chemical durability matter. Oven and cookware. Lighting, particularly where a lamp runs hot. Pharmaceutical vials and ampoules. Solar thermal receiver tubes. The mirror blanks of large telescopes, which is what Corning developed it further for.

And, importantly, not most modern kitchen 'Pyrex' sold in the United States, which was reformulated to tempered soda-lime glass. Tempering gives strength against impact and does nothing for thermal expansion, which is why reports of oven dishes shattering rose after the change and why the two products under one name are a genuine consumer confusion.

History

Developed by Otto Schott in Jena in the 1880s, working with Ernst Abbe and Carl Zeiss on glasses for optical instruments — the systematic study of what each oxide does to glass begins there.

Corning's Pyrex followed in 1915, and its first application was railway lantern globes, which cracked when rain hit hot glass. Kitchenware came afterwards and by accident, when a researcher's wife baked a cake in a cut-down battery jar.

Environmental impact

Recyclable in principle and a contaminant in practice. Borosilicate melts at a higher temperature than soda-lime glass, so a piece of it in a container-glass furnace does not melt with the rest and leaves an inclusion — which is why kerbside glass recycling asks for bottles and jars and not oven dishes.

Its long service life is the stronger argument: laboratory glassware is used for decades.

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 · as silica, still the network former and about 80 per cent of it
Oxygen — element · the other half of every oxide in it
Boron — element · the element the glass is named for, and the whole of why it barely expands

is sourced from

Quartz — mineral · as silica sand, still the bulk of the batch
Borax — mineral · the boron source, and the ingredient the whole material is named for

is an input to

Float glass process — process · with difficulty, which is why flat borosilicate costs several times what window glass does

is used as

Tableware and vessels — application · ovenware, and the reason it survives the oven is a thermal expansion a third of ordinary glass's

is used in

Chemical manufacture — industry · laboratory and plant glassware, on chemical durability as much as on thermal shock

is an alternative to

Soda-lime glass — material · a third of the thermal expansion and several times the price. Soda-lime is what a window and a bottle are; borosilicate is what survives the oven
Fused silica — material · a twentieth of soda-lime's expansion against borosilicate's third, transparent into the ultraviolet, and far harder to make
Glass-ceramic — material · in cookware and hobs: a glass-ceramic is tougher and closer to zero expansion, and cannot be seen through

is commonly confused with

Soda-lime glass — material · sold under one trade name in some markets: American kitchen 'Pyrex' was reformulated to tempered soda-lime, which is strong against impact and no better against heat

is produced by

Glass melting — process · 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

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 Borosilicate glass 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

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
Borosilicate glass → is sourced from (as silica sand, still the bulk of the batch) → Quartz
Borosilicate glass → is sourced from (the boron source, and the ingredient the whole material is named for) → Borax
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 (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
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 composed of → Calcite
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 composed of (in young limestone, before conversion to calcite is complete) → Aragonite

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

Downstream — what it becomes

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 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
Borosilicate glass → is used as (ovenware, and the reason it survives the oven is a thermal expansion a third of ordinary glass's) → Tableware and vessels
Borosilicate glass → is used in (laboratory and plant glassware, on chemical durability as much as on thermal shock) → Chemical manufacture
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
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 (glazing, facades and internal partitions) → Construction
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 as (the product itself — ninety per cent of all glass made, in containers and windows) → Glass and ceramics

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