What is Borosilicate Glass?
, 15 min reading time
It's no secret that we're big fans of borosilicate glass here at Handmade Glass Co.! From functional artwork to durable jewelry it's a unique and mesmerizing material that has limitless possibilities. This primer is for everyone curious about what we mean when we say "borosilicate glass" and what makes it different compared to other glass types. We hope you enjoy the read and let us know if you ever have any borosilicate questions or just want to talk glass!
What is Borosilicate Glass?
Borosilicate glass is a special type of glass made by mixing 70-80% silica sand with boron trioxide, the key ingredient that transforms its properties and makes it resistant to thermal shock. At first glance, you likely wouldn’t be able to tell the two-component materials apart, but when heated, boron trioxide absorbs massive amounts of heat before crystallizing into glass. Once the two materials fuse into the glass, boron trioxide slows the annealing process, producing a harder, more durable material. Thanks to this unique composition, borosilicate glass can move directly from a freezer to a hot oven without breaking. This ability is due to borosilicate’s low COE (coefficient of thermal expansion), which is 33.
What is the Coefficient of Thermal Expansion?
The coefficient of thermal expansion (COE) of glass is a measure of how much a material expands when it is heated. For borosilicate glass, the COE is about:
3.3 × 10⁻⁶ per °C (between 20°C and 300°C)
That means for every degree Celsius the temperature rises, one meter of borosilicate glass would only expand by about 3.3 micrometers, a very tiny amount. Thats where the COE rating of 33 comes from. The lower the COE rating, the less the glass expands when exposed to heat. This low expansion rate is what makes borosilicate glass so resistant to thermal shock!
Uses of Borosilicate Glass
Because borosilicate glass is strong and durable, it is often chosen for many industrial and commercial purposes. Some common uses include:
- Decorative: Our personal favorite, borosilicate glass is popular for household and decorative items because of its durability and heat resistance. It can be used to make decanters, bottles, tchotchkes, and lots of other cool stuff you can find throughout our website. Its ability to handle sudden temperature changes also makes it ideal for items that may come into contact with hot liquids, such as coffee or tea carafes, or for artisanal glasswork and craft projects.
- Scientific lenses and mirrors: Borosilicate glass expands very little when heated, keeping its optical clarity across a wide range of temperatures. This makes it ideal for precision lenses, laboratory instruments, and hot mirrors that protect sensitive equipment by reflecting infrared light.
- Thermal insulation: It can handle much higher temperatures than regular soda lime glass, making it suitable for demanding environments where heat resistance is critical.
- High-intensity lighting systems: Lamps that produce a lot of heat, such as high-intensity discharge (HID) lamps and stage or studio lights, often use borosilicate glass components. These parts withstand continuous high temperatures without losing strength.
- Sight glass: Borosilicate is used for strong viewing windows in tanks, boilers, and other hot industrial equipment. These windows allow workers to monitor processes safely without opening the system.
Borosilicate Glass vs Soda Lime or “Soft” Glass
How is Borosilicate Glass Worked?
Torch
The torch is the primary tool for working borosilicate glass (i.e. boro). Because boro has a higher melting point than softer glasses, it requires hotter flames to bring it into a workable state. Each torch has at least one set of oxygen and propane knobs. By adjusting the oxygen and propane mix, artists can control the size and temperature of the flame. A broad flame is often used to heat large sections evenly, while a sharp, focused flame is used for fine details. Once the glass is glowing and softened, it can be shaped, stretched, or fused with other pieces. The torch is also used to “anneal” smaller adjustments during shaping so the glass remains stress-free.
Graphite Tools
Graphite is heat-resistant and smooth, making it ideal for shaping hot glass without sticking to it. Tools like paddles, reamers, and rollers are used to flatten surfaces, open up hollow tubes, or create symmetry. For example, a graphite paddle can press a hot piece into a clean, flat base, while a graphite reamer can widen an opening by pushing gently into softened glass. Because graphite doesn’t pull or scratch the glass surface, it allows artists to maintain smooth finishes and precise shapes.
Sharp Edges
Sharp edges refer to cutting tools, such as tungsten carbide or diamond-tipped scribes, scoring knives, or even specialized shears. Instead of cutting through hot glass directly, these tools are often used on cooled or partially cooled glass to create a “score line.” The piece is then heated slightly or given a controlled tap to separate cleanly at that line. Sharp edges are also essential for trimming unwanted sections, preparing tubes for joining, or making break points for later shaping. For some pieces, blades or razors are used during the hot process to create crisp separations, decorative lines, or controlled breaks.
Eye Protection: B3 Borosilicate Glasses (Didymium Lenses)
When working with borosilicate glass, B3 safety shades are one of the most essential tools for protecting the eyes and for longevity in the craft. The intense yellow-orange glow known as sodium flare that appears when heating glass, can overwhelm natural vision and make it difficult to see fine details. B3 lenses are specially formulated to filter out this flare, allowing the glassworker to clearly observe the shape, heat, and movement of the material. At the same time, they provide protection from harmful ultraviolet (UV) and infrared (IR) radiation produced by the torch flame, which can cause long-term eye damage if left unchecked. Without these shades, glasswork becomes not only less accurate but also far more dangerous. By combining safety with improved visibility, B3 borosilicate shades make it possible to work with confidence and accuracy while preserving long-term eye health.
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, by Patrick Bitting What is Borosilicate Glass?
