Borosilicate Glass
Glass made with boron trioxide in place of some of the soda and lime, which cuts its thermal expansion to roughly a third that of ordinary glass and makes it very hard to crack with a temperature change. It is the standard glass for laboratory beakers and for pharmaceutical vials (USP Type I), and in the kitchen it turns up in Chemex and pour-over carafes, glass kettles, French press beakers, measuring jugs, and Pyrex-branded bakeware made outside the US. The common 3.3 type is roughly 80% silica, 13% boric oxide, 4% sodium or potassium oxide and 2-3% alumina.
How it counts in our ratings
Not plastic in any sense — a part made of this never counts against a product's rating.
We only count what's in the contact path — the parts that touch your food, drink, or skin. How we rate →
Is it safe?
Widely regarded as safe for food and drink contact, and used as the reference material when other packaging is tested for leaching. Its resistance to attack by water is why: SCHOTT rates BOROFLOAT 33 at hydrolytic class HGB 1 and acid class 1, the top grades, meaning very little dissolves out of the surface even under hot acidic conditions. There is no plasticizer, no coating and no binder, so there is nothing to migrate. The boron is bound into the glass network rather than sitting on the surface, and pharmaceutical practice relies on exactly that when packaging injectable drugs. The real risk with borosilicate is mechanical, not chemical.
With heat
This is the material's whole point. Its coefficient of thermal expansion is about 3.3 x 10^-6 /K against 9-9.5 for soda-lime, which works out to a survivable temperature differential of roughly 330F / 183C versus about 100F / 55C for ordinary glass. Boiling water into a room-temperature borosilicate carafe is a non-event, and so is a hot pour-over cone or a dishwasher cycle. SCHOTT gives BOROFLOAT 33 a maximum service temperature of 450C for long exposure and 500C short-term, far above anything a kitchen does. Direct flame and localized hot spots are still a bad idea, because a steep gradient across a thin wall can beat even a low expansion coefficient.
How to tell it apart
The word "Pyrex" tells you nothing on its own - it is a brand, and the brand now covers two different glasses. On kitchenware the useful convention is case: all-caps PYREX generally means vintage Corning or current European production (Arc International), which is borosilicate, while lowercase pyrex is the modern US line and is tempered soda-lime. Otherwise look for numbers rather than adjectives: a maker actually using borosilicate will say "borosilicate 3.3" or name a glass such as DURAN or BOROFLOAT. If a spec sheet only says "heat-resistant glass" or "high borosilicate", ask for the coefficient of thermal expansion in ppm/K - 3.3 is borosilicate, 9 is not. Borosilicate is also noticeably lighter for its size, 2.23 g/cm3 against about 2.5 for soda-lime.
Care & durability
Chemically it does not wear out; a lab beaker from decades ago is still a lab beaker. What ends its life is impact and edge damage, and it is not a safety glass: it cracks and breaks into large sharp pieces rather than the blunt pebbles fully tempered glass produces. Chips and scratches on a rim or base are stress concentrators, so retire a chipped carafe instead of nursing it. Dishwashers are fine for the glass itself; the usual hand-wash advice is about protecting printed markings and avoiding knocks in the rack.
End of life
Durable, but not curbside-recyclable glass. Its higher melting point contaminates the soda-lime container stream, so most municipal programs ask you to keep bakeware and lab glass out of the glass bin and a broken carafe usually goes in the trash. It is inert in landfill. The honest environmental case for borosilicate is length of service, not recycling.
Pros
- Completely inert; no taste transfer, no coating to wear off
- Survives roughly a 330F temperature swing
- Transparent, so the contact surface is visible
- Effectively unlimited service life if it is not dropped
Cons
- Breaks into large sharp pieces, not blunt fragments
- Not accepted in curbside glass recycling
- Heavier and pricier than the plastic it replaces
- The word borosilicate on a listing is often unverified
Where you'll meet it
Every product we've broken down that uses Borosilicate Glass, and the exact parts made of it.
In the contact path
These parts touch your food, drink, or skin — so this material is part of the verdict.
Elsewhere in the product
Present, but not touching anything that matters — a housing, an exterior, or a part the recommended build replaces.
| Product | Parts made of it | Rating |
|---|---|---|
| Coletti Bozeman Percolator | lid knob |
Sources
- study https://ceramics.org/ceramic-tech-today/hells-kitchen-thermal-stress-and-glass-cookware-that-shatters/ American Ceramic Society coverage of Bradt and Martens 2012 - borosilicate tolerates about a 330F differential, soda-lime about 100F
- article https://www.chemedx.org/blog/pyrex-any-other-name Works the thermal-stress equation for both glasses, CTE 3 vs 9 x 10^-6 /K, and notes Pyrex is a brand name rather than a composition
- manufacturer https://www.schott.com/en-gb/products/borofloat-p1000314/technical-details BOROFLOAT 33 datasheet - density 2.23 g/cm3, service temperature 450-500C, hydrolytic class HGB 1 and acid class 1
- article https://en.wikipedia.org/wiki/Borosilicate_glass Composition of type 3.3 glass and its use as USP Type I pharmaceutical packaging
- article https://gizmodo.com/the-pyrex-glass-controversy-that-just-wont-die-1833040962 All-caps PYREX indicates vintage or European Arc International borosilicate, lowercase pyrex the US soda-lime line