N-BK7 · heat-treated state · default condition
N-BK7 Annealed Glass
Properties of the Annealed Glass condition, compared with the other N-BK7 tempers.
Default choice for general lenses, windows and prisms where standard index tolerance (±0.001) and normal stress birefringence (≤10 nm/cm) are acceptable.
What is N-BK7 Annealed Glass?
N-BK7 Annealed Glass is N-BK7 heat treated to Annealed Glass — default choice for general lenses, windows and prisms where standard index tolerance (±0.001) and normal stress birefringence (≤10 nm/cm) are acceptable. N-BK7 Annealed Glass has a tensile strength of 50 MPa (7.25 ksi), stronger than 57% of glass grades. Elongation at break is 0% and the elastic modulus is 82 GPa (11.9 Msi). N-BK7 Annealed Glass has a density of 2.51 g/cm³ (0.0907 lb/in³), lighter than 71% of glass grades.
Advantages
- Low embodied carbon — 2 kg CO₂/kg, better than 82% of glass grades
N-BK7 Annealed Glass properties
Typical room-temperature values for N-BK7 Annealed Glass — 8 properties are specific to this condition; the rest are grade-level values shared by every N-BK7 temper. Each bar shows where the value sits among the glass grades in FabDigit's library — further right is higher.
Physical4
N-BK7 Annealed Glass has a density of 2.51 g/cm³ (0.0907 lb/in³), lighter than 71% of glass grades.
Mechanical12
N-BK7 Annealed Glass has a tensile strength of 50 MPa (7.25 ksi), stronger than 57% of glass grades. Elongation at break is 0% and the elastic modulus is 82 GPa (11.9 Msi).
Thermal5
N-BK7 Annealed Glass is rated for continuous service to 500°C (932 °F). It conducts heat at 1.1 W/m·K (0.644 BTU/hr·ft·°F), worse than 67% of glass grades. Thermal expansion is 7.1 µm/m·K (3.94 µin/in·°F).
Electrical4
N-BK7 Annealed Glass is an electrical insulator with a dielectric strength of 15 kV/mm (381 V/mil) and a resistivity of 1×10¹² Ω·m.
Chemical & Environmental2
Corrosion resistance is very good (78/100), worse than 51% of glass grades.
Sustainability4
Producing a kilogram of N-BK7 Annealed Glass takes about 30 MJ of energy and emits 2 kg of CO₂ — less than 80% of glass grades. Typical recycled content is 5%.
Manufacturability2
N-BK7 Annealed Glass's machinability is poor (18/100, better than 53% of glass grades).
Common Calculations4
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Other N-BK7 tempers and conditions
N-BK7 is also supplied in 2 other conditions. The full side-by-side table is on the N-BK7 overview.
- Annealed GlassDefaultDefault choice for general lenses, windows and prisms where standard index tolerance (±0.001) and normal stress birefringence (≤10 nm/cm) are acceptable.50 MPa tensile
Working with N-BK7 Annealed Glass
Is N-BK7 easy to machine?
No conventional cutting — shape by diamond sawing, curve generating, loose-abrasive lapping and CNC grinding with copious coolant; core-drill from both sides to avoid exit chipping.
Can N-BK7 be welded?
Not weldable; join by optical contacting, UV-cure or epoxy cementing, or frit bonding — CTE-match partners to ~7×10⁻⁶/K to avoid stress birefringence.
Can N-BK7 be formed, bent or molded?
Hot slumping/pressing above Tg (557 °C) is possible for blanks but requires re-annealing; precision glass molding is uncommon for N-BK7 (high Tg wears molds) — low-Tg molding glasses are preferred.
What surface finishes work on N-BK7?
Pitch or synthetic-pad polishing with ceria gives λ/10 surfaces and 20-10 to 40-20 scratch-dig; edges must be beveled, and AR / metallic / dielectric coatings adhere well after standard clean.
N-BK7 chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| B2O3 | 9 – 12 | 10.5 |
| Na2O | 8 – 11 | 9.5 |
| K2O | 5 – 8 | 6.5 |
| BaO | 2 – 4 | 3 |
| Sb2O3fining agent; N-glass is arsenic- and lead-free | 0 – 0.6 | 0.3 |
| SiO2network former | ≥ 68 (balance) | — |
N-BK7 Annealed Glass — frequently asked
What is N-BK7 Annealed Glass used for?
N-BK7 Annealed Glass is typically used for camera and imaging lenses, telescope optics, laser and instrument windows, prisms and beamsplitter substrates, achromat doublet crown elements and optical filter and mirror substrates. In short: premium optical glass.
What is the tensile strength of N-BK7 Annealed Glass?
N-BK7 Annealed Glass has a typical tensile strength of 50 MPa (7.25 ksi) — stronger than 57% of glass grades. Strength varies by condition: see the 3 listed tempers.
Is N-BK7 Annealed Glass easy to machine?
Not especially — machinability is rated poor (18/100, better than 53% of glass grades). No conventional cutting — shape by diamond sawing, curve generating, loose-abrasive lapping and CNC grinding with copious coolant; core-drill from both sides to avoid exit chipping.
Can N-BK7 Annealed Glass be welded?
Not weldable; join by optical contacting, UV-cure or epoxy cementing, or frit bonding — CTE-match partners to ~7×10⁻⁶/K to avoid stress birefringence.
What surface finishes work on N-BK7 Annealed Glass?
Pitch or synthetic-pad polishing with ceria gives λ/10 surfaces and 20-10 to 40-20 scratch-dig; edges must be beveled, and AR / metallic / dielectric coatings adhere well after standard clean.
Can N-BK7 Annealed Glass be formed, bent or molded?
Hot slumping/pressing above Tg (557 °C) is possible for blanks but requires re-annealing; precision glass molding is uncommon for N-BK7 (high Tg wears molds) — low-Tg molding glasses are preferred.
What is the maximum service temperature of N-BK7 Annealed Glass?
N-BK7 Annealed Glass is rated for continuous use to about 500°C (932 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
Can FabDigit make parts in N-BK7 Annealed Glass?
Yes — N-BK7 Annealed Glass is available for CNC machining and sheet metal with instant online pricing. Upload a STEP file to get a price and a DFM check.
Sources
- N-BK7 Optical Glass Data Sheet — SCHOTT AG (2023)
- Optical Glass Catalogue / TIE-19, TIE-27, TIE-33 technical information — SCHOTT AG (2022)
- Engineered Materials Handbook Vol. 4: Ceramics and Glasses — ASM International (1991)
- GB/T 7962 / ISO 12123 optical glass — SAC / ISO (2010)
Property data is compiled from published supplier and standards handbooks and normalised for comparison. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.