Borosilicate Glass
Borosilicate replaces part of the soda in ordinary glass with about 13 % boron oxide, which cuts thermal expansion to 3.3 ppm/K — a third of soda-lime — and makes the glass far more resistant to acids and thermal shock. It is the laboratory, process and sight-glass standard (Borofloat 33 as float sheet, Duran and Pyrex 7740 as tubing and ware), usable to 450 °C continuously, and the family also gives the optical crown N-BK7, the default glass for visible lenses, prisms and windows.
Popular borosilicate grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|
| Borofloat 33 | 64 | 35 | 2.23 | Not recommended | Not recommended | Float sheet for viewports, wafers and covers |
| Duran | — | 35 | 2.23 | Not recommended | — | Tubing, rod and lampworked lab apparatus |
| Pyrex 7740 | — | 35 | 2.23 | Not recommended | — | US-standard 3.3 lab and process glass |
| N-BK7 | 60 | 50 | 2.51 | Poor | Not recommended | Default optical crown for visible lenses |
All 4 borosilicate grades
Choose borosilicate for
Laboratory and chemical-process glassware, sight glasses and viewports to 450 °C, lamp and tube envelopes, microscope slides and cover slips, display and sensor substrates that are anodically bonded to silicon, and visible-range optics (N-BK7) where cost and availability matter more than UV transmission or zero expansion.
Look elsewhere when
The part must transmit below 350 nm or survive above 500 °C (Fused Silica), it needs zero expansion for metrology (ZERODUR), the panel is large and impact is the risk (tempered Soda-Lime or Gorilla Glass — borosilicate does not temper well), it must resist hydrofluoric acid or hot caustics (no silicate glass does; use PTFE or a fluoropolymer lining), or the window must be scratch-proof (Sapphire).
Loading the library…
Borosilicate by application
- Sight glasses and viewports
- Borofloat 33 discs and plates, edges seamed, to 450 °C and modest pressure; annealed design stress about 7 MPa long-term, or specify a tempered borosilicate or Fused Silica for higher pressure and temperature.
- Process tubing and reactors
- Duran and Pyrex 7740 tubing, flanges and columns lampworked and annealed at 560 °C; hydrolytic class 1, acid class 1, alkali class 2 — fine for everything except HF and hot concentrated caustics.
- Visible optics
- N-BK7 for lenses, windows, prisms and beamsplitters from 350 nm to 2 µm — the reference against which every other optical glass is priced; move to Fused Silica only for UV, high laser power or thermal stability.
- Wafers and MEMS substrates
- Borofloat 33 wafers anodically bond to silicon and match its expansion closely enough for microfluidics, pressure sensors and MEMS caps; polished to a few nanometres roughness for bonding.
Working with borosilicate
- Melting & forming
- Sheet is float-formed (Borofloat 33) with a fire-polished surface; tubing and rod are drawn (Duran, Pyrex 7740); optical blanks are cast, pressed or drawn and fine-annealed (N-BK7). Lampworking and bending run at 1,200–1,270 °C working temperature; every hot-worked part is annealed at about 560 °C so it does not crack later.
- Grinding & lapping
- Diamond CNC grinding, core drilling, waterjet and score-and-break handle plates, discs, rings and holes to ±0.05 mm; optical polishing brings N-BK7 to λ/10 flatness and 20-10 scratch-dig. Seam or polish every edge — usable strength of a borosilicate part is set by its worst edge flaw, not by the glass.
- Joining & sealing
- Borosilicate fuses to itself and is sealed to tungsten or to Kovar through graded or intermediate sealing glasses; Borofloat 33 anodically bonds to silicon at 300–450 °C. UV-curing acrylic and epoxy adhesives, glass frit and optical contacting join glass to glass; compression fittings with PTFE or graphite gaskets connect tubing to metal.
- Tolerances & design rules
- Cut sheet holds ±0.2 mm, ground diameters ±0.05 mm, thickness ±0.1 mm on float sheet; as a design stress use about 7 MPa long-term tensile for annealed glass. Support on compliant gaskets, keep thermal gradients below about 100 K across a part, and avoid point loads, drilled holes near edges and sharp internal corners.
Other glass series
Borosilicate Glass — frequently asked
What is borosilicate?
Borosilicate replaces part of the soda in ordinary glass with about 13 % boron oxide, which cuts thermal expansion to 3.3 ppm/K — a third of soda-lime — and makes the glass far more resistant to acids and thermal shock. It is the laboratory, process and sight-glass standard (Borofloat 33 as float sheet, Duran and Pyrex 7740 as tubing and ware), usable to 450 °C continuously, and the family also gives the optical crown N-BK7, the default glass for visible lenses, prisms and windows.
Which grade of borosilicate should I use?
Borofloat 33 for flat parts: sight glasses, viewports, cover plates, wafers and substrates cut from float sheet 0.7–25 mm thick with a fire-polished surface. Duran for tubing, rod and lampworked apparatus — reactors, columns, level gauges — and Pyrex 7740 when a US drawing calls the same 3.3 expansion glass by that name; the three are interchangeable in chemistry and can be fused to one another. N-BK7 for anything optical: it is not a low-expansion glass (7.1 ppm/K) but a highly homogeneous crown with refractive index 1.517 and very low bubble and striae content, polished to a fraction of a wavelength for lenses, windows, prisms and beamsplitters in the visible and near infrared.
Which grade of borosilicate is the strongest?
Borofloat 33 has the highest yield strength in the series at 64 MPa; N-BK7 follows at 60 MPa. Values are for each grade's default condition.
Which grade of borosilicate is easiest to machine?
N-BK7 (machinability poor, 18/100), then Borofloat 33 (12/100).
Borofloat 33, Duran and Pyrex 7740 — are they the same glass?
Chemically yes: all three are 3.3-expansion borosilicates (about 81 % SiO₂, 13 % B₂O₃, 4 % Na₂O + K₂O, 2 % Al₂O₃) with the same 3.25–3.3 ppm/K expansion, 525 °C transformation temperature and class 1 hydrolytic resistance, and they fuse to one another. They differ by form and maker: Borofloat 33 is Schott float sheet with a fire-polished surface, Duran is Schott tubing and pressed ware, Pyrex 7740 is the Corning lab glass. Specify the form you need and accept any of the three as equivalent chemistry.
Why is N-BK7 the standard optical glass?
It is the cheapest glass that meets optical specifications: refractive index 1.5168, Abbe number 64.2 (low dispersion), transmission above 90 % from 350 nm to 2 µm, very low bubble and striae content and excellent homogeneity, so the same design works from every supplier. It polishes easily and is stable against humidity and staining. Its limits are UV cutoff near 350 nm, a 7.1 ppm/K expansion that rules it out for thermal stability, and softness (Knoop 610) compared with Sapphire — which is when Fused Silica or a harder crystal takes over.