Borosilicate glass is the most widely available heat-resistant glass, able to handle temperatures up to about 360°C (680°F) and rapid temperature swings without cracking. But it’s not the only option. Several types of glass offer meaningful heat resistance, each suited to different situations, from baking dishes to industrial furnaces. The key difference between them comes down to how much the glass expands when it gets hot.
Why Some Glass Shatters From Heat
All glass expands when heated. The problem is that glass conducts heat poorly, so when you pour boiling water into a cold glass or move a hot dish to a cool counter, one part of the glass expands while the rest stays the same size. That uneven expansion creates internal stress, and if the stress is strong enough, the glass cracks. This is thermal shock.
The rate at which glass expands is measured by its coefficient of thermal expansion (CTE). A lower number means less expansion per degree of temperature change, which means less internal stress and better resistance to thermal shock. Standard window and drinking glass (soda-lime glass) has a CTE of about 8.8 to 9.0 µm/m-K. Borosilicate glass comes in at roughly 3.3 to 5.1, nearly half that rate. That single difference is why one type survives the oven and the other doesn’t.
Borosilicate Glass
Borosilicate glass gets its heat resistance from boron oxide added to the glass formula. A German chemist first developed this approach in 1893, creating a glass called Duran. The addition of boron dramatically lowers the expansion rate, letting the glass tolerate temperature swings that would shatter ordinary glass. It can operate continuously at temperatures up to about 360°C and handles being moved from a hot oven to a room-temperature counter without cracking.
This is the glass used in laboratory beakers and flasks. The ASTM specification for laboratory glassware classifies low-expansion borosilicate as Type I, Class A, the highest grade for lab equipment. It’s also what many people think of when they picture classic oven-safe cookware. Borosilicate glass is clear, lightweight, and doesn’t absorb odors or flavors, which makes it popular for bakeware, coffee makers, and food storage containers.
Tempered Soda-Lime Glass
Tempered glass starts as regular soda-lime glass but goes through a rapid heating and cooling process that creates compressive stress on the surface. This makes it roughly four to five times stronger than untreated glass. It can handle moderate heat and is less likely to crack from everyday kitchen use, but it still has the same high expansion rate as regular soda-lime glass (around 8.8 to 9.0 µm/m-K). Its maximum mechanical temperature tops out at about 300°C (572°F), lower than borosilicate.
The trade-off is cost. Tempered soda-lime glass is cheaper to produce than borosilicate, which is why many manufacturers switched to it for consumer cookware. It holds up well for typical oven temperatures and resists impact better than borosilicate. But it’s more vulnerable to sudden temperature changes. Placing a hot tempered glass dish on a wet or cold surface is riskier than doing the same with borosilicate.
The Pyrex Confusion
The Pyrex brand illustrates this distinction perfectly. When Corning introduced Pyrex in 1915, starting with pie plates and loaf pans, the line was made from borosilicate glass. It became synonymous with heat-resistant cookware. But after Corning’s patent expired in 1936, competitors entered the market using soda-lime formulations that were less heat resistant than the original.
Pyrex itself eventually switched to tempered soda-lime glass for most of its U.S. products, likely because boron is expensive to handle and toxic to dispose of. The brand has recently reintroduced borosilicate glass into some products, but most Pyrex sold in North America is still tempered soda-lime. A rough shortcut: if the logo reads “PYREX” in all capital letters, it’s more likely borosilicate. Lowercase “pyrex” typically signals the tempered soda-lime version. European Pyrex products have generally stayed with borosilicate.
Fused Quartz (Fused Silica)
For applications far beyond kitchen temperatures, fused quartz is the standard. Made from nearly pure silicon dioxide, it has an extremely low expansion rate and a softening point of 1,683°C (3,061°F). It won’t even begin to show strain until around 1,120°C. This makes it essential in semiconductor manufacturing, telescope mirrors, fiber optics, and high-temperature laboratory work where borosilicate would fail.
You won’t find fused quartz in consumer products. It’s expensive to produce, difficult to shape, and completely unnecessary for cooking or household use. But if you’re working with industrial furnaces, UV lamps, or scientific instruments, fused quartz handles temperatures that no other glass type can match.
Glass-Ceramic
Glass-ceramics sit between true glass and ceramic materials. They’re made by heating glass until crystals form within it, creating a material with near-zero thermal expansion. Cooktop surfaces are the most familiar example. The flat black cooking surface on an electric or induction stove is typically a glass-ceramic that can withstand direct contact with heating elements reaching several hundred degrees while the edges of the panel stay cool.
Glass-ceramics also show up in fireplace doors, woodstove windows, and some high-performance cookware. They handle sustained high heat and extreme temperature gradients better than borosilicate, though they tend to be heavier and less transparent.
How to Spot Heat Stress in Glass
Even heat-resistant glass has limits. The early warning signs include small cracks near the edges (where temperature differences are greatest), visible warping, and discoloration. Framed glass is especially vulnerable because the edges are shielded from heat while the center absorbs it, creating uneven expansion. Tinted or solar-control glass absorbs more heat than clear glass, which increases the risk further.
For cookware, the most common cause of failure isn’t exceeding the maximum temperature. It’s thermal shock from sudden changes: placing a hot dish on a cold granite counter, adding cold liquid to a hot pan, or moving glass directly from the freezer to the oven. Even borosilicate glass can crack under extreme enough temperature swings. Letting glass warm or cool gradually is the simplest way to extend its life, regardless of the type.
Choosing the Right Glass for Your Situation
- Everyday baking and food storage: Borosilicate glass offers the best thermal shock resistance for home use. Look for brands that specifically label their products as borosilicate.
- Budget-friendly cookware: Tempered soda-lime glass works fine for standard oven use up to about 300°C, as long as you avoid sudden temperature changes.
- Stovetop surfaces and fireplace doors: Glass-ceramic handles sustained direct heat and extreme temperature gradients.
- Laboratory and industrial work: Borosilicate for routine lab use, fused quartz for anything above 500°C or requiring optical clarity at extreme temperatures.