Borosilicate glass

Lore & Background

Borosilicate glass is a glass family whose defining characteristic is the use of both silica and boron trioxide as its primary glass-forming ingredients. It is generally harder to break when dropped than regular soda-lime glass, more resistant to cracking from sudden temperature changes, and maintains a higher clarity over time, though it is typically more expensive. Its most notable physical property is a very low coefficient of thermal expansion—about one-third that of ordinary soda-lime glass—which allows it to withstand temperature differentials of approximately 165 °C without fracturing, whereas soda-lime glass can only handle a change of about 55 °C. This makes borosilicate highly resistant to thermal shock, explaining its common use in laboratory reagent bottles, flasks, cookware, lighting, and electronics. The glass is less dense than soda-lime glass, at roughly 2.23 g/cm³, due to boron’s low atomic mass, and has a mean specific heat capacity of 0.83 J/(g·K). Optically, it is a crown glass with low dispersion and a refractive index between 1.51 and 1.54. Its composition for common low-expansion types is approximately 80% silica, 13% boric oxide, 4% sodium or potassium oxide, and 2–3% aluminium oxide. Borosilicate glass melts at a higher temperature than ordinary silicate glass, requiring specialized industrial techniques, and is sold under trade names including Pyrex, Duran, Schott, Kimax, and Simax.

Reader's Guide

Borosilicate glass is significant for its very low coefficient of thermal expansion (≈3 × 10−6 K−1 at 20 °C), about one-third that of ordinary soda–lime glass, which makes it more resistant to thermal shock than any other common glass. Its legacy includes use in single-ended self-starting lamps such as sodium-vapor lamps for street lighting, and it is sold under many trade names including Borosil, Duran, Pyrex, Glassco, Supertek, Suprax, Simax, Bellco, and others. While more difficult to make than traditional glass due to its high melting temperature, it is economical to produce and remains a low-cost compromise between ordinary glass and more expensive fused quartzware.

Did You Know?

• Borosilicate glass was first developed by German glassmaker Otto Schott in the late 19th century in Jena, and was originally known as Jena glass.
• The common type of borosilicate glass used for laboratory glassware has a thermal expansion coefficient about one-third that of ordinary soda–lime glass.
• Borosilicate glass is less dense (about 2.23 g/cm³) than typical soda–lime glass due to the low atomic mass of boron.

Origins and the Birth of a Glass Family

In the late 1800s, a German glassmaker named Otto Schott working in Jena first developed a glass whose backbone was built from silica and boron trioxide rather than the soda-lime mix that dominated the industry. That pioneering material quickly earned the nickname "Jena glass," a label that stuck for decades. The story took a major turn in 1915 when Corning Glass Works launched Pyrex, a branded borosilicate product that would become virtually synonymous with the material across English-speaking markets. However, that identity has not been absolute: since the 1940s, a meaningful share of glass sold under the Pyrex name has actually been soda-lime, blurring the line for consumers. Today, "borosilicate" is best understood not as a single product but as a family of glasses, each tuned for a distinct purpose. The most widely encountered variants are the 3.3 and 5.0x grades, marketed under names like Duran, Corning33, Corning51-V (clear), Corning51-L (amber), NIPRO BSA 60, and BSC 51. Each of these carries a slightly different oxide recipe, yet all share the defining boron-silica architecture that sets them apart from ordinary window glass.

Thermal Resilience: The Numbers Behind the Reputation

The headline advantage of borosilicate glass is its remarkably low coefficient of thermal expansion, sitting at roughly 3 × 10⁻⁶ per kelvin at room temperature. That figure is approximately one-third the value of standard soda-lime glass, and the practical consequence is dramatic: a borosilicate vessel can tolerate a temperature swing of about 330 °F (166 °C) before it risks fracturing, whereas a soda-lime counterpart gives way at roughly 100 °F (40 °C). This is why a beaker of boiling water set on a cold bench will not shatter a borosilicate flask, yet will crack an ordinary glass tumbler. Fused quartz pushes even further, with a thermal expansion only one-fifteenth that of soda-lime, but the difficulty and cost of working with quartz make it impractical for most everyday uses. Borosilicate occupies the sweet spot: a low-cost compromise that delivers most of quartz's thermal resilience without the manufacturing headaches. It is worth noting, however, that "resistant" does not mean "immune." Rapid or uneven heating can still cause borosilicate to crack, and the softening point of a typical 7740 Pyrex formulation sits at 820 °C, with maximum service temperatures generally around 500 °C.

From Melt to Form: The Manufacturing Challenge

Producing borosilicate glass is a thermally demanding affair. The raw batch—boric oxide, silica sand, soda ash, and alumina—must be heated to roughly 1,650 °C (3,000 °F) before it fully melts, a temperature well above what ordinary silicate glass requires. That extra heat demanded new industrial techniques and equipment. The resulting composition of a typical low-expansion laboratory grade is approximately 80 percent silica, 13 percent boric oxide, 4 percent sodium or potassium oxide, and 2 to 3 percent aluminium oxide. Once molten, the material behaves differently from standard glass: its viscosity changes more steeply with temperature, a phenomenon described as super-Arrhenian, and the fragility index climbs as boron content rises. At the structural level, heating triggers a reversible depolymerisation of the glass network, and boron atoms shift from a tetrahedral four-coordinate arrangement at lower temperatures toward a trigonal planar three-coordinate geometry as the melt gets hotter. Depending on the final product shape, manufacturers choose among floating, tube drawing, or molding. The finished glass is less dense than soda-lime—about 2.23 g/cm³—thanks to boron's low atomic mass, and its specific heat capacity of 0.83 J/(g·K) is roughly one-fifth that of water.

Where Borosilicate Lives: Applications and Brand Identity

Borosilicate glass shows up in places far beyond the chemistry lab. Its extreme chemical resistance in corrosive environments makes it ideal for reagent bottles and flasks, while its thermal stability and clarity suit it for cookware, certain windows, and electronics. In street lighting, single-ended self-starting sodium-vapor lamps rely on a borosilicate gas-discharge arc tube paired with a mica disc insulator and a metal cap. The glass family's range of thermal expansion coefficients also enables direct seals with metals such as molybdenum (CTE 4.6), tungsten (CTE around 4.0), and Kovar (CTE around 5.0), which is critical in vacuum and high-temperature engineering. Optically, borosilicate glasses behave as crown glasses with low dispersion—Abbe numbers near 65—and refractive indices between 1.51 and 1.54 across the visible spectrum. Commercially, the material is sold under a long roster of trade names: Borosil, Duran, Pyrex, Glassco, Supertek, Suprax, Simax, Bellco, Marinex, BSA 60, BSC 51, Heatex, Endural, Schott, Refmex, Kimax, Gemstone Well, United Scientific, and MG. Because it is generally pricier than soda-lime glass, manufacturers reserve it for applications where its durability, clarity retention, and heat tolerance justify the extra cost.