Borosilicate glass is thermal-resistant and safe for using when reheating food in the microwave or popping into the oven. — GRETA HOFFMAN/Pexels
A few days ago, I slid a glass plate of leftover pasta into the microwave, pressed the buttons, and waited. Moments later, a sharp crack emanated from the microwave. I opened the door to find a fractured plate, cradling hot pasta like a messy broken mosaic. And I found myself wondering, “Why did the glass in the plate shatter, especially as it was on the unaffected glass rotating base of the microwave?”
I also know that the melting point of glass is around 1,400°C, which is very much higher than any temperature possible in a microwave oven, or even a conventional oven.
It turns out the reason for the shattered glass lies not so much in heat, but more in the chemistry of the glass itself. It was an old plate hand-made traditionally from soda-lime glass. In fact, most glassware today is still made of soda-lime glass, a blend of silica, sodium, and calcium.
Inexpensive and clear, it is perfect for windows, bottles and jars – but it is also vulnerable to sudden temperature changes. When heated unevenly, its structure expands and contracts rapidly, creating stress fractures. My old glass dish simply could not handle the microwave’s heat gradient.
Birth of a miracle glass
In 1882, German chemist Otto Schott revolutionised glassmaking. Teaming with physicist Ernst Abbe and optician Carl Zeiss, Schott sought to make better lenses for microscopes. By replacing soda’s sodium with boron oxide, he crafted a glass that resisted heat and chemical corrosion.
This “borosilicate” glass had a thermal expansion coefficient (the measure of how much a material expands with temperature) three times lower than soda-lime, meaning it could endure drastic temperature swings without cracking. By 1893, Schott’s company mass-produced it, paving the way for scientific and culinary revolutions.
Pyrex glassware originally made use of borosilicate glass, which is essentially unbreakable and can withstand extremely high temperatures. — ROMAN ODINTSOV/Pexels
Schott’s innovation was no happy accident. His typically Germanic approach involved a team methodically testing over 100 chemical compositions in a small laboratory in Jena, Thüringen. The team discovered that boron oxide’s unique ability to form strong, flexible bonds with silica created a lattice-like structure. This atomic arrangement allowed the glass to absorb thermal stress without fracturing – a discovery that would redefine future industries.
The story of Pyrex
In 1915, Corning Glass Works rebranded borosilicate glass as “Pyrex”, marketing it to homemakers. Suddenly, cooks and bakers could move dishes from freezers to ovens safely. Pyrex became a kitchen staple, symbolising reliability and resilience. Curiously, Corning’s inspiration came from railroad lanterns as their borosilicate lenses survived all kinds of extreme weather conditions while housing very hot electric and gas lamps.
However, in the 1980s, some Pyrex lines switched to tempered soda-lime glass to cut costs, leading to unexpected shattering of cookware around the world. Even today, when buying glass cookware from any brand, always check for the word “borosilicate” in the labels. It makes a huge and important difference.
The shift from labs to kitchens was not just about marketing. During World War I, Pyrex found an unexpected role in hospitals. Its resistance to sterilisation made it ideal for medical equipment, and soldiers’ rations were even served in Pyrex containers. By the 1950s, Pyrex’s iconic measuring cups and casserole dishes became symbols of post-war domesticity, blending science with suburban life.
If you are curious, the name “Pyrex” was derived by combining the Greek word “pyr” (fire) and the suffix “-ex” to highlight the glass’s heat resistance. It probably also helped that the first Pyrex product launched was an oven pie plate.
The science of unbreakability
Borosilicate’s superpowers stem from its atomic structure. Boron atoms form strong triangular bonds with oxygen, creating a flexible 3D network. When heated, this lattice expands minimally (3.3×10 -6/°C vs soda-lime’s 9×10 -6/°C). This means the material is impervious to thermal shock. Add resistance to acids and alkalis, and we have a material that thrives in labs, kitchens, and beyond.
But why does this matter? Imagine pouring boiling water into a soda-lime glass teapot. The sudden heat causes the inner layer to expand faster than the outer layer, creating tension. In borosilicate, its uniform expansion prevents such stress. This principle is why borosilicate glass cookware can survive decades of intense kitchen use and freezer-to-oven transitions.
From beakers to spacecraft: Borosilicate’s many lives
1. Laboratories: Test tubes, beakers, and distillation columns rely on borosilicate to withstand searing Bunsen burners and corrosive chemicals. During WWII, it was crucial for producing penicillin and the Manhattan Project’s reactors.
2. Pharmaceuticals: Vaccine vials (including Covid-19 doses) use high-grade borosilicate glass to endure sterilisation and sub-zero storage. Modern mRNA vaccines, which require ultra-cold temperatures, depend on its resilience.
3. Nuclear reactors: Borosilicate glass is used in nuclear reactors for neutron- absorbing control rods, radiation shielding windows, and encapsulation of radioactive waste (vitrification). Its chemical stability resists corrosion from radioactive materials, and its ability to incorporate neutron-absorbing elements (like boron) makes it ideal for controlling nuclear reactions. It can also withstand high temperatures and radiation without degrading.
4. Space exploration: The Hubble Space Telescope’s mirrors and spacecraft sensors depend on its stability in extreme temperatures. Nasa’s Mars rovers even use borosilicate components to shield electronics from the planet’s -80°C nights.
What about ceramics?
Researching borosilicate glass also made me wonder about my collection of various pieces of ceramic cookware. There are certainly some advantages of ceramic cookware, such as:
• Non-stick properties: Ceramic cookware often has a natural non-stick coating, making it easier to cook with less oil and simpler to clean compared to borosilicate glass, which lacks inherent non-stick qualities.
• Durability against physical impact: Ceramic cookware is generally more resistant to drops and impacts than borosilicate glass, which can crack or shatter if mishandled.
• Aesthetic variety: Ceramic cookware comes in a wide range of colours and designs, offering more aesthetic options for kitchen decor compared to the typically clear or frosted appearance of borosilicate glass.
• Heat distribution: Ceramic cookware provides more even heat distribution for stovetop cooking with gas or heat rings, reducing hot spots, whereas borosilicate glass is less efficient at conducting heat evenly on direct heat sources.
• Versatility: Some ceramic cookware is rated for both stovetop and oven use (depending on the product), while borosilicate glass is designed for oven or microwave use and is not typically stovetop-safe.
However, borosilicate glass cookware wins on the following points:
• Thermal shock resistance: Borosilicate glass is highly resistant to thermal shock, meaning it can handle rapid temperature changes (e.g., from freezer to oven) without cracking. Ceramic cookware is much more prone to cracking under extreme temperature shifts.
• Transparency: Borosilicate glass is transparent, allowing you to monitor food while cooking or baking. Ceramic cookware is opaque, so you cannot see the contents without lifting the lid.
• Chemical inertness: Borosilicate glass is highly non-reactive and does not leach chemicals or flavours into food, even with acidic ingredients. While high-quality ceramic cookware is safe, lower-quality ceramics may have glazes that could leach into food under certain conditions.
• Microwave safety: Borosilicate glass is consistently microwave-safe, making it ideal for reheating. Ceramic cookware may not always be microwave-safe, depending on the glaze or construction.
• Longevity of coating: The non-stick coating on ceramic cookware can wear off over time with heavy use or improper care, reducing its effectiveness. Borosilicate glass does not rely on coatings, maintaining its properties indefinitely with proper handling.
I should emphasise that the major safety issue with ceramics is that they are unreliable under rapid temperature changes. A ceramic Dutch oven might survive a steady oven roast, but transferring it from a 200°C oven to a cold countertop risks it cracking apart. This is why one should always rest hot ceramic cookware on heat-resistant mats, and never on cold surfaces or sinks. There are no such concerns with borosilicate glass cookware.
A society built on borosilicate
The modern world is silently depending more and more on new uses for borosilicate. For example, borosilicate glass is used in microfluidic chips for lab-on-a-chip devices, which are employed in biomedical research, DNA analysis, and drug testing. Its chemical inertness prevents reactions with biological samples or reagents, and its transparency allows for precise optical monitoring. Its ability to be etched into precise microchannels is also critical.
Not all cookware can be used in ovens and microwave ovens. Ceramic for instance is generally not recommended for microwave ovens. — Filepic
Borosilicate glass is used in high-voltage electrical insulators, such as those in power lines or particle accelerators. Its high dielectric strength prevents electrical breakdown, and its thermal resistance ensures stability under the heat generated by high currents. It also resists environmental degradation in outdoor settings. For example, specialised insulators in CERN’s Large Hadron Collider or high-voltage transmission lines incorporate borosilicate glass.
Recent innovations push its boundaries further. Scientists are embedding nanoparticles into borosilicate to create “smart glass” that changes opacity with electricity. Even your smartphone’s screen is probably borosilicate-derived. Its strength, inertness and durability imparts the solidity and ruggedness one expects from such personal electronic devices.
Conclusion
I am now much more diligent to ensure that borosilicate kitchenware is used in the microwave and conventional oven wherever possible. It is fortunate that curiosity after a silly kitchen accident made me realise that there is a miracle material, invented over 140 years ago, hiding in plain view in front of everyone.
Borosilicate glass is, in many ways, a perfect example of human ingenuity. It is profoundly functional, aesthetically pleasing, and non-polluting – all it does is simply make our lives better and safer.
The views expressed here are entirely the writer’s own.
