Borosilicate glass Borosilicate
Silica-boria glass: about 80% SiO2, 13% B2O3, 4% Na2O or K2O and 2-3% Al2O3 for the low-expansion laboratory type
Borosilicate glass is a silica glass with about 13% boric oxide, which lowers its thermal expansion.
Evidence level
2 of 5
Lab-measured
- 0Predicted only. No one has made it, or no measurement exists.
- 1Calculated. Only calculated or predicted values from sources.
- 2Lab-measured. Measured properties with their conditions stated.
- 3Space-relevant tests. Measured in conditions relevant to space, such as vacuum or atomic oxygen.
- 4Ground-qualified. Qualified for an application on the ground.
- 5Flown or field-proven. Flight or field results exist.
Density
2.23 g/cm3
Working temperature
450 °C
Transformation point
525 °C
Optics
1.4714
Thermal expansion
3.25 x10^-6 K^-1
Recipe, share of the weight
- SiO2 80%Silica
- B2O3 13%Boric oxide
- Na2O or K2O 4%Alkali oxide
- Al2O3 2%Alumina
Best for
Laboratory glassware and optics that must survive temperature swings, where a low thermal expansion matters more than cost
Weakest at
Higher working temperature and processing cost than soda-lime glass; the expansion coefficient and hazard data could not be checked in this run
Risk at a glance
- People None found. Hazard data partly checked: boric oxide classes read from
- If it fails Moderate. Thermal shock
- Corrosion No data. No corrosion data recorded
Quick facts: Borosilicate
| Property | Value | Basis |
|---|---|---|
| Density | 2.23 g/cm³ | 1 sourced value |
| Young's modulus | 64 kN/mm2 | 1 sourced value |
| Thermal conductivity | 1.2 W/(m·K) | 1 sourced value |
| Thermal expansion (CTE) | 3.25 ppm/K | 1 sourced value |
| Specific heat | 830 J/(kg·K) | 1 sourced value |
What is the density of Borosilicate glass?
Borosilicate glass has a density of 2.23 g/cm³. The source and test conditions are listed in the properties table on this page.
What is the thermal conductivity of Borosilicate glass?
Borosilicate glass has a thermal conductivity of 1.2 W/(m·K). The source and test conditions are listed in the properties table on this page.
What is the coefficient of thermal expansion of Borosilicate glass?
Borosilicate glass has a coefficient of thermal expansion of 3.25 ppm/K. The source and test conditions are listed in the properties table on this page.
What is the Young's modulus of Borosilicate glass?
Borosilicate glass has a Young's modulus of 64 kN/mm2. The source and test conditions are listed in the properties table on this page.
What is Borosilicate glass used for?
Uses recorded in the sources include Reflecting telescope primary mirror; Hot and cold mirrors and dichroic filters; Telescope mirror substrates.
Why it exists
Borosilicate glass is a silica glass with boric oxide added. The boron lowers the thermal expansion, so the glass is less likely to crack when it is heated or cooled quickly. That property is why most modern laboratory glassware is made from it, and why the 200 inch mirror of the Hale Telescope was cast from it.
Where it's used
Optics
The 200 inch mirror of the Hale Telescope is made of borosilicate glass, according to Wikipedia. Low-expansion borosilicate is also used for reflecting telescope mirrors in general.
Laboratory glassware
Wikipedia states that virtually all modern laboratory glassware is made of borosilicate glass. No application from the approved list matches this use, so it is recorded here only.
Space and aviation
SCHOTT lists telescope and satellite mirror substrates, and aircraft components for illumination, navigation and displays, among its borosilicate applications.
Hot and cold mirrors
SCHOTT says BOROFLOAT 33 is the material of choice for high-temperature dichroic filters and hot and cold mirrors.
Recipe
Silica-boria glass: about 80% SiO2, 13% B2O3, 4% Na2O or K2O and 2-3% Al2O3 for the low-expansion laboratory type
Borosilicate glass is an amorphous silica network with boric oxide added. It is made by melting boric oxide, silica sand, soda ash and alumina together (Wikipedia). BOROFLOAT 33 is made by the float process, which SCHOTT describes as the world's first floated borosilicate glass. The composition below is the typical low-expansion laboratory type; the BOROFLOAT 33 recipe is not published as numbers.
| Ingredient | Share of the weight | What it does |
|---|---|---|
| Silica SiO2 | 80% | The main network former. It gives the glass its rigid, mostly silica framework. |
| Boric oxide B2O3 | 13% | Lowers the thermal expansion and the working temperature of the glass. |
| Alkali oxide Na2O or K2O | 4% | Helps the glass melt and adjusts its viscosity. |
| Alumina Al2O3 | 2% | Improves chemical durability. |
Weight shares are as printed in the sources.
What it can do
Density
2.23 g/cm3
measured
SCHOTT lists a density of 2.23 g/cm3 at 25 °C for BOROFLOAT 33, which is lighter than most soda-lime glass.
Working temperature
450 °C
measured
SCHOTT gives a maximum long-term operating temperature of 450 °C, and 500 °C for short exposures of less than 10 h.
Transformation point
525 °C
measured
The transformation temperature is 525 °C per ISO 7884, the point where the glass stops behaving like a fully elastic solid in cooling tests.
Optics
1.4714
measured
The refractive index at 587.6 nm is 1.4714, with an Abbe number of 65.41, which means low dispersion for a visible-light lens.
Thermal expansion
3.25 x10^-6 K^-1
measured
SCHOTT gives 3.25 x 10-6 K-1 for 20 to 300 °C (ISO 7991). SCHOTT says this is three times lower than soda-lime glass.
How it compares
Borosilicate glass has a linear thermal expansion of 3.25 x10^-6 K^-1 (20 to 300 °C), which SCHOTT puts at about one third of soda-lime glass. That low expansion gives it the thermal-shock limits in the 0407 data sheet (RTS 140 to 175 K by thickness). Its density of 2.23 g/cm3 and long-term limit of 450 °C are the figures on the SCHOTT sheet.
The same measurements for each material, so you can compare them. Each row names the key condition under the value; the full conditions and methods are in the numbered notes under Technical details. Other materials are shown beside it; each was measured under its own conditions, so open its page before comparing closely. Compare every grade in the family.
| Measurement | Borosilicate | Soda-lime | Aluminosilicate |
|---|---|---|---|
| Thermal behavior and safety | |||
| specific heatValue printed in the thermal properties table | 0.83 kJ/(kg·K) | 0.88 kJ/(kg·K) | 0.768 J/(g.K) |
| Thermal conductivityValue printed in the thermal properties table | 1.2 W/(m·K) | 0.937 W/(m·K) | 0.0109 W/cm.K |
| Physical and electrical | |||
| Density | 2.23 g/cm³1 | 2500 kg/m3 | 2.38 g/cc |
| knoop hardnessValue printed in the mechanical properties table | 480 HK0.1/20 | 470 kg/mm2 | no data |
| Temperature behavior | |||
| transformation temperature | 525 °C2 | no data | 580 °C |
| max operating temperature short term | 500 °C3 | no data | no data |
| max operating temperature long term | 450 °C4 | no data | no data |
| Other measurements | |||
| youngs modulusValue printed in the mechanical properties table | 64 kN/mm2 | no data | no data |
| poissons ratioValue printed in the mechanical properties table | 0.2 dimensionless | 0.22 - | 0.23 - |
| refractive indexValue printed in the optical data table | 1.471 nd (587.6 nm) | 1.523 - | 1.51 - |
| abbe numberValue printed in the optical data table | 65.41 dimensionless | no data | no data |
| stress optical coefficientValue printed in the optical data table | 4 x10^-6 mm2/N | no data | no data |
| hydrolytic classClass printed in the chemical durability table | 1 class HGB (ISO 719 / DIN 12 111) | no data | no data |
| Linear thermal expansion coefficient | 3.25 x10^-6 K^-15 | 8.3 x10^-6 /°C | 31.7 x10^-7/C |
| Dielectric constant | 4.6 dimensionless6 | no data | 5.27 - |
| Loss tangent | 37 x10^-47 | no data | 0.3 % |
| Dielectric strength | 16 kV/mm8 | no data | no data |
| Bending strength, normal conditions | 25 MPa9 | 41 MPa | 800 MPa |
| Bending strength, safety conditions | 7 MPa10 | 41 MPa | 800 MPa |
| Annealing point (1013 dPa·s) | 560 °C11 | 548 °C | 722 °C |
| Softening point (10^7.6 dPa·s) | 815 °C12 | 715 °C | 971 °C |
Why choose it, and why not
Good for
- Lab glassware and optical parts that must tolerate temperature changes. SCHOTT's 450 °C long-term limit is the reference.
- Telescope mirrors and lenses, where low dispersion (Abbe number 65.41) and a known refractive index matter.
Watch out for
- The expansion coefficient (3.25 x10^-6 K^-1) is for BOROFLOAT 33. Other borosilicate grades, such as the 3.3 value on Wikipedia, can differ.
- Working limits are the manufacturer's reference values for BOROFLOAT 33, not guaranteed values for other borosilicate grades.
- Solarization and fluorescence are described as low by SCHOTT, but no numbers are printed on the text pages read.
Borosilicate is a sound choice where a low-expansion, durable glass is needed. This profile now has the thermal expansion, electrical and bending values, the chemical durability tests and the boric oxide hazard classes from sourced pages. Supply of silica and soda ash, the glass SDS and space testing are still open.
Technical details
For scientists and researchers. Everything above comes from the record below: every number with its method, its conditions and its source.
Notes
The method, full conditions and any caveat for the numbered rows above.
| # | Measurement | Method | Conditions | Caveat |
|---|---|---|---|---|
| 1 | Density | Value printed in the mechanical properties table of the SCHOTT BOROFLOAT 33 technical details page | temperature 25 °C | — |
| 2 | transformation temperature | Value printed in the thermal properties table of the SCHOTT page | standard ISO 7884 | — |
| 3 | max operating temperature short term | Value printed on the SCHOTT page, reference value | duration less than 10 h | — |
| 4 | max operating temperature long term | Value printed on the SCHOTT page, reference value | duration 10 h or more | — |
| 5 | Linear thermal expansion coefficient | Printed as 3.25 x 10-6 K-1 in the BOROFLOAT 33 brochure; same value on the 0407 data sheet and the Edmund Optics copy of the SCHOTT technical data sheet | temperature range 20 to 300 °Cstandard ISO 7991 | — |
| 6 | Dielectric constant | Printed in the electrical properties list of the SCHOTT 0407 data sheet | frequency 1 MHztemperature 25 °C | — |
| 7 | Loss tangent | Printed as 37 x 10-4 in the electrical properties list of the SCHOTT 0407 data sheet | frequency 1 MHztemperature 25 °C | — |
| 8 | Dielectric strength | Printed in the electrical properties list of the SCHOTT 0407 data sheet | frequency 50 Hztemperature 25 °C | — |
| 9 | Bending strength, normal conditions | Printed in the mechanical properties list of the SCHOTT 0407 data sheet | standard DIN 52 292 Part 1basis normal conditions | — |
| 10 | Bending strength, safety conditions | Printed in the mechanical properties list of the SCHOTT 0407 data sheet; the lower design value for safety use | standard DIN 52 292 Part 1basis safety conditions | — |
| 11 | Annealing point (1013 dPa·s) | Printed in the thermal properties list of the SCHOTT 0407 data sheet | viscosity 1013 dPa·s | — |
| 12 | Softening point (10^7.6 dPa·s) | Printed in the thermal properties list of the SCHOTT 0407 data sheet | viscosity 10^7.6 dPa·s | — |
Corrosion and compatibility
What happens when it meets other materials and environments.
| Environment | Concern | What happens |
|---|---|---|
| Water, 5 vol% hydrochloric acid and 0.02 n sulphuric acid (24 h at 95 °C) | not rated | Water and acids give negligible attack.1 |
| 5% sodium hydroxide (6 h at 95 °C), 0.02 n sodium hydroxide, 0.02 n sodium carbonate | not rated | Alkali attacks the surface more than acid does.2 |
| 10% hydrofluoric acid (20 min at 23 °C) and 10% NH4F x HF | not rated | Hydrofluoric acid stains the glass; the buffered mix does not.3 |
| High-intensity UV from a mercury vapour lamp (365 nm main line, 15 h) | not rated | Low solarization: the glass darkens little under UV.4 |
| # | Environment | Details | Test conditions |
|---|---|---|---|
| 1 | Water, 5 vol% hydrochloric acid and 0.02 n sulphuric acid (24 h at 95 °C) | Abrasion below 0.01 mg/cm2 and surface unchanged in each test. | time 24 htemperature 95 °C |
| 2 | 5% sodium hydroxide (6 h at 95 °C), 0.02 n sodium hydroxide, 0.02 n sodium carbonate | 5% NaOH: abrasion 1.1 mg/cm2 with white stains. 0.02 n NaOH: 0.16 mg/cm2 with white haze. 0.02 n Na2CO3: 0.16 mg/cm2, unchanged. | time 6 htemperature 95 °C |
| 3 | 10% hydrofluoric acid (20 min at 23 °C) and 10% NH4F x HF | 10% HF: abrasion 1.1 mg/cm2 with stained white haze. 10% NH4F x HF: 0.14 mg/cm2, unchanged. | time 20 mintemperature 23 °C |
| 4 | High-intensity UV from a mercury vapour lamp (365 nm main line, 15 h) | SCHOTT describes low solarization and low auto-fluorescence. The before-and-after transmission curves are graphs; no single number is printed. | source HOK 4/120 lamp, 850 W/cm2, 30 x 15 x 1 mm sample |
Risks and side effects
Hazards to people, what happens when a cell overheats, and how the material is handled and stored.
For people
PubChem's aggregated GHS classes for boron oxide, the main glass oxide, are read below. No SDS for the finished glass was opened in this run. Hazard data partly checked: boric oxide classes read from PubChem; no SDS for the glass read
- Sharp edges
- Broken glass is sharp. Handle broken pieces with gloves and dispose of them in a sharps container.
- Boric oxide
- PubChem's aggregated classes for boron oxide include H360 (may damage fertility or the unborn child, Danger) and H319 (eye irritation). These are for the powder, not for the finished glass.
- Sodium hydride
- Do not heat borosilicate glass in contact with sodium hydride; the reaction can form sodium borohydride.
Test details
The original wording and numbers from the sources, for readers who want them.
Failure risks
| Failure | Trigger and consequence |
|---|---|
| Thermal shockmoderate severity | Temperature difference across the glass, short exposure (1 h)SCHOTT rates resistance to temperature differences at 110 K for a 1 h exposure, 90 K for 1 to 100 h and 80 K for over 100 h. |
| Thermal shock by panel thicknessmoderate severity | Temperature difference for a thermal shock resistance (RTS) rating, by thicknessRTS is 175 K for under 4 mm, 160 K for 4 to 6 mm, 150 K for 6 to 15 mm and 140 K for over 15 mm. |
| Impactmoderate severity | Physical impact on an installed panelSCHOTT says impact resistance depends on how the glass is fitted, its size and thickness, and the type of impact. |
| Thermal shock (kitchenware comparison)moderate severity | Boiling vessel placed on iceWikipedia says borosilicate withstands a change of about 330 °F without fracturing, against about 100 °F for soda-lime glass, and that borosilicate can still crack under rapid or uneven temperature change. |
Handling and storage
- Keep long-term use at or below 450 °C (short exposures of less than 10 h up to 500 °C), and keep temperature differences within the RTD and RTS limits in failure_risks.
- Do not heat borosilicate glass with sodium hydride. The reaction can form sodium borohydride.
Supply and geography
How common the raw materials are and where they come from.
Boron (borates)
USGS: borates are mostly in volcanic and arid-climate deposits; world reserves of refined borates are adequate for the foreseeable future. Boron is also on the USGS 2025 critical minerals list.
- Top producers (2025)
- Turkey, Bolivia, Chile, Peru, China, Argentina, Russia, Germany
- Critical mineral lists
- US: listed
- Recycling
- Not sourced.
Boron import dependence (United States)
USGS: import sources for all forms of boron, 2021 to 2024, as printed.
- Top producers (2021)
- Turkey 90%, Bolivia 6%, Other 4%
- Recycling
- Not sourced.
Documents
Datasheets, safety data sheets, papers and standards you can open.
| Document | Type | Year |
|---|---|---|
| BOROFLOAT 33 technical details (SCHOTT) | datasheet | |
| Borosilicate glass (Wikipedia) | report | |
| BOROFLOAT 33 & Functional Coatings (SCHOTT) | datasheet | |
| SCHOTT BOROFLOAT data sheet 0407 | datasheet | |
| Mineral Commodity Summaries 2026: Boron (USGS) | report | |
| Boron oxide (B2O3), PubChem CID 11073337 | database | |
| Corning 7740 (Pyrex) property table, copy hosted by Valley Design | datasheet | 2023 |
| NASA Solar Power Satellite System Definition Study (NTRS 19800004887) | report | 1980 |
Full record
The complete spore, as stored. Free to reuse under CC-BY-4.0. Images keep the licence shown with each one.
Show the full YAML
spore: profile
id: borosilicate-glass
spore_version: 0.2
family: glass-and-composites
name: Borosilicate glass
abbreviation: Borosilicate
role: optical and laboratory glass
category: glasses-optics
type: borosilicate
summary: |
Borosilicate glass is a silica glass with about 13% boric oxide, which lowers its thermal expansion. SCHOTT's BOROFLOAT 33 sheet lists a density of 2.23 g/cm3, a transformation temperature of 525 °C and a long-term maximum operating temperature of 450 °C. Wikipedia reports that virtually all modern laboratory glassware is made of borosilicate, and that the 200 inch mirror of the Hale Telescope is borosilicate. The thermal expansion coefficient and thermal-shock ratings come from the SCHOTT technical-details page; see the property list for the values and conditions.
best_for: Laboratory glassware and optics that must survive temperature swings, where a low thermal expansion matters more than cost
weakest_at: Higher working temperature and processing cost than soda-lime glass; the expansion coefficient and hazard data could not be checked in this run
identity:
formula: 'Silica-boria glass: about 80% SiO2, 13% B2O3, 4% Na2O or K2O and 2-3% Al2O3 for the low-expansion laboratory type'
source: wikipedia-borosilicate
where: Wikipedia, Borosilicate glass, Manufacturing process section (composition quoted as approximately 80% silica, 13% boric oxide, 4% sodium or potassium oxide and 2 to 3% aluminium oxide). Composition is a typical example, not a single specification; products such as BOROFLOAT 33 are defined by the manufacturer.
tags:
environments: []
stresses: []
applications:
- optics-and-cameras
- electronics-and-chips
properties:
- benchmark: density
value: 2.23
unit: g/cm3
scope: material
conditions:
temperature: 25 °C
method: Value printed in the mechanical properties table of the SCHOTT BOROFLOAT 33 technical details page
status: ground
source: schott-borofloat33
where: Mechanical properties table, density row
- benchmark: transformation_temperature
value: 525
unit: °C
scope: material
conditions:
standard: ISO 7884
method: Value printed in the thermal properties table of the SCHOTT page
status: ground
source: schott-borofloat33
where: Thermal properties table, Tg row
- benchmark: max_operating_temperature_short_term
value: 500
unit: °C
scope: material
conditions:
duration: less than 10 h
method: Value printed on the SCHOTT page, reference value
status: ground
source: schott-borofloat33
where: Thermal properties table, maximum operating temperature row
- benchmark: max_operating_temperature_long_term
value: 450
unit: °C
scope: material
conditions:
duration: 10 h or more
method: Value printed on the SCHOTT page, reference value
status: ground
source: schott-borofloat33
where: Thermal properties table, maximum operating temperature row
- benchmark: specific_heat
value: 0.83
unit: kJ/(kg·K)
scope: material
conditions:
temperature_range: 20 to 100 °C
method: Value printed in the thermal properties table
status: ground
source: schott-borofloat33
where: Thermal properties table, specific heat row
- benchmark: thermal_conductivity
value: 1.2
unit: W/(m·K)
scope: material
conditions:
temperature: 90 °C
method: Value printed in the thermal properties table
status: ground
source: schott-borofloat33
where: Thermal properties table, thermal conductivity row
- benchmark: youngs_modulus
value: 64
unit: kN/mm2
scope: material
conditions:
standard: DIN 13316
method: Value printed in the mechanical properties table
status: ground
source: schott-borofloat33
where: Mechanical properties table, Young's modulus row
- benchmark: poissons_ratio
value: 0.2
unit: dimensionless
scope: material
conditions:
standard: DIN 13316
method: Value printed in the mechanical properties table
status: ground
source: schott-borofloat33
where: Mechanical properties table, Poisson's ratio row
- benchmark: knoop_hardness
value: 480
unit: HK0.1/20
scope: material
conditions:
standard: DIN ISO 9385
method: Value printed in the mechanical properties table
status: ground
source: schott-borofloat33
where: Mechanical properties table, Knoop hardness row
- benchmark: refractive_index
value: 1.4714
unit: nd (587.6 nm)
scope: material
conditions:
wavelength: 587.6 nm
note: reference value, not guaranteed by the manufacturer
method: Value printed in the optical data table
status: ground
source: schott-borofloat33
where: Optical data table, refractive index nd row
- benchmark: abbe_number
value: 65.41
unit: dimensionless
scope: material
conditions:
wavelength: nu_e
note: reference value, not guaranteed by the manufacturer
method: Value printed in the optical data table
status: ground
source: schott-borofloat33
where: Optical data table, Abbe number row
- benchmark: stress_optical_coefficient
value: 4
unit: x10^-6 mm2/N
scope: material
conditions:
note: reference value, not guaranteed by the manufacturer
method: Value printed in the optical data table
status: ground
source: schott-borofloat33
where: Optical data table, stress-optical coefficient K row
- benchmark: hydrolytic_class
value: 1
unit: class HGB (ISO 719 / DIN 12 111)
scope: material
conditions:
standard: ISO 719 / DIN 12 111
method: Class printed in the chemical durability table
status: ground
source: schott-borofloat33
where: Chemical durability table, hydrolytic resistance rows
- benchmark: cte
label: Linear thermal expansion coefficient
value: 3.25
unit: x10^-6 K^-1
scope: material
conditions:
temperature_range: 20 to 300 °C
standard: ISO 7991
method: Printed as 3.25 x 10-6 K-1 in the BOROFLOAT 33 brochure; same value on the 0407 data sheet and the Edmund Optics copy of the SCHOTT technical data sheet
status: ground
source: schott-media-datasheet
where: 'Thermal properties table, Linear Thermal Expansion (C.T.E.) row, footnote *: according to ISO 7991'
- benchmark: dielectric_constant
label: Dielectric constant
value: 4.6
unit: dimensionless
scope: material
conditions:
frequency: 1 MHz
temperature: 25 °C
method: Printed in the electrical properties list of the SCHOTT 0407 data sheet
status: ground
source: schott-datasheet-0407
where: Electrical Properties, Dielectric Constant row
- benchmark: dissipation_factor
label: Loss tangent
value: 37
unit: x10^-4
scope: material
conditions:
frequency: 1 MHz
temperature: 25 °C
method: Printed as 37 x 10-4 in the electrical properties list of the SCHOTT 0407 data sheet
status: ground
source: schott-datasheet-0407
where: Electrical Properties, Loss Tangent row
- benchmark: dielectric_strength
label: Dielectric strength
value: 16
unit: kV/mm
scope: material
conditions:
frequency: 50 Hz
temperature: 25 °C
method: Printed in the electrical properties list of the SCHOTT 0407 data sheet
status: ground
source: schott-datasheet-0407
where: Electrical Properties, Dielectric Strength row
- benchmark: flexural_strength
label: Bending strength, normal conditions
value: 25
unit: MPa
scope: material
conditions:
standard: DIN 52 292 Part 1
basis: normal conditions
method: Printed in the mechanical properties list of the SCHOTT 0407 data sheet
status: ground
source: schott-datasheet-0407
where: Mechanical Properties, Bending Strength, For Normal Conditions
- benchmark: flexural_strength
label: Bending strength, safety conditions
value: 7
unit: MPa
scope: material
conditions:
standard: DIN 52 292 Part 1
basis: safety conditions
method: Printed in the mechanical properties list of the SCHOTT 0407 data sheet; the lower design value for safety use
status: ground
source: schott-datasheet-0407
where: Mechanical Properties, Bending Strength, For Safety Conditions
- benchmark: annealing_point
label: Annealing point (1013 dPa·s)
value: 560
unit: °C
scope: material
conditions:
viscosity: 1013 dPa·s
method: Printed in the thermal properties list of the SCHOTT 0407 data sheet
status: ground
source: schott-datasheet-0407
where: Thermal Properties, Annealing Point row
- benchmark: softening_point
label: Softening point (10^7.6 dPa·s)
value: 815
unit: °C
scope: material
conditions:
viscosity: 10^7.6 dPa·s
method: Printed in the thermal properties list of the SCHOTT 0407 data sheet
status: ground
source: schott-datasheet-0407
where: Thermal Properties, Softening Point row
corrosion:
- environment: Water, 5 vol% hydrochloric acid and 0.02 n sulphuric acid (24 h at 95 °C)
headline: Water and acids give negligible attack.
effect: Abrasion below 0.01 mg/cm2 and surface unchanged in each test.
conditions:
time: 24 h
temperature: 95 °C
source: schott-borofloat33
where: Chemical durability, Resistance to selected chemicals table
- environment: 5% sodium hydroxide (6 h at 95 °C), 0.02 n sodium hydroxide, 0.02 n sodium carbonate
headline: Alkali attacks the surface more than acid does.
effect: '5% NaOH: abrasion 1.1 mg/cm2 with white stains. 0.02 n NaOH: 0.16 mg/cm2 with white haze. 0.02 n Na2CO3: 0.16 mg/cm2, unchanged.'
conditions:
time: 6 h
temperature: 95 °C
source: schott-borofloat33
where: Chemical durability, Resistance to selected chemicals table
- environment: 10% hydrofluoric acid (20 min at 23 °C) and 10% NH4F x HF
headline: Hydrofluoric acid stains the glass; the buffered mix does not.
effect: '10% HF: abrasion 1.1 mg/cm2 with stained white haze. 10% NH4F x HF: 0.14 mg/cm2, unchanged.'
conditions:
time: 20 min
temperature: 23 °C
source: schott-borofloat33
where: Chemical durability, Resistance to selected chemicals table
- environment: High-intensity UV from a mercury vapour lamp (365 nm main line, 15 h)
headline: 'Low solarization: the glass darkens little under UV.'
effect: SCHOTT describes low solarization and low auto-fluorescence. The before-and-after transmission curves are graphs; no single number is printed.
conditions:
source: HOK 4/120 lamp, 850 W/cm2, 30 x 15 x 1 mm sample
source: schott-media-datasheet
where: Solarization test figure and text
risks:
hazards:
- text: 'PubChem aggregated GHS for boron oxide (B2O3), the main oxide in the glass: H360 (95.1% of reports, Danger) May damage fertility or the unborn child; H360FD (13.1%) May damage fertility and the unborn child; H319 causes serious eye irritation; H335 may cause respiratory irritation; H303 may be harmful if swallowed. These describe the powder, not the solid glass.'
ghs:
- H360
- H360FD
- H319
- H335
- H303
source: pubchem-boric-oxide
where: Safety and Hazards, GHS Classification (CID 11073337)
failure_risks:
- mode: Thermal shock
trigger: Temperature difference across the glass, short exposure (1 h)
consequence: SCHOTT rates resistance to temperature differences at 110 K for a 1 h exposure, 90 K for 1 to 100 h and 80 K for over 100 h.
severity: moderate
source: schott-datasheet-0407
where: Resistance to Temperature Differences (RTD1) list
- mode: Thermal shock by panel thickness
trigger: Temperature difference for a thermal shock resistance (RTS) rating, by thickness
consequence: RTS is 175 K for under 4 mm, 160 K for 4 to 6 mm, 150 K for 6 to 15 mm and 140 K for over 15 mm.
severity: moderate
source: schott-datasheet-0407
where: Resistance to Thermal Shock (RTS) list
- mode: Impact
trigger: Physical impact on an installed panel
consequence: SCHOTT says impact resistance depends on how the glass is fitted, its size and thickness, and the type of impact.
severity: moderate
source: schott-tech-data-edmund
where: Physical Impact paragraph
- mode: Thermal shock (kitchenware comparison)
trigger: Boiling vessel placed on ice
consequence: Wikipedia says borosilicate withstands a change of about 330 °F without fracturing, against about 100 °F for soda-lime glass, and that borosilicate can still crack under rapid or uneven temperature change.
severity: moderate
source: wikipedia-borosilicate
where: Thermal shock section
side_effects: []
handling:
- text: Keep long-term use at or below 450 °C (short exposures of less than 10 h up to 500 °C), and keep temperature differences within the RTD and RTS limits in failure_risks.
source: schott-borofloat33
where: Maximum operating temperature; RTD1 list
- text: Do not heat borosilicate glass with sodium hydride. The reaction can form sodium borohydride.
source: wikipedia-borosilicate
where: Laboratory section
mitigations: []
uses:
- application: optics-and-cameras
device: Reflecting telescope primary mirror
example: The 200 inch mirror of the Hale Telescope is made of borosilicate glass.
source: wikipedia-borosilicate
where: Optics section
- application: optics-and-cameras
device: Hot and cold mirrors and dichroic filters
example: SCHOTT says BOROFLOAT 33 is the material of choice for high-temperature-resistant dichroic filters and hot and cold mirrors, because its high boron content gives a low thermal expansion.
source: schott-media-datasheet
where: BOROFLOAT 33 & Functional Coatings, mirrors section
- application: optics-and-cameras
device: Telescope mirror substrates
example: SCHOTT lists telescope mirror substrates and glass-ceramics among its aviation, astro and space applications.
source: schott-borofloat33
where: Aviation, Astro and Space navigation list
- application: optics-and-cameras
device: Satellite mirror substrates
example: SCHOTT lists satellite mirror substrates and electronic packaging in its aviation, astro and space applications.
source: schott-borofloat33
where: Aviation, Astro and Space navigation list
- application: electronics-and-chips
device: Electronic packaging
example: SCHOTT lists electronic packaging alongside satellite mirror substrates in its space applications.
source: schott-borofloat33
where: Aviation, Astro and Space navigation list
- application: solar-arrays
device: Front-surface cover glass on silicon solar cells
example: The NASA 1980 solar power satellite study says cerium-doped borosilicate meets the front-cover requirements if it can be bonded to the cell without thermal-mismatch damage; Corning 7070 borosilicate was used as a 250 um integral cover that survived annealing.
source: nasa-sps-1980
where: Cover glass section, text on front-surface cover requirements
supply:
- subject: Boron (borates)
abundance: 'USGS: borates are mostly in volcanic and arid-climate deposits; world reserves of refined borates are adequate for the foreseeable future. Boron is also on the USGS 2025 critical minerals list.'
producers:
- country: Turkey
production: refined borates 1,500 thousand t (2025)
reserves: 950,000 thousand t
year: 2025
- country: Bolivia
production: ulexite 380 thousand t (2025)
year: 2025
- country: Chile
production: ulexite 300 thousand t (2025)
reserves: 35,000 thousand t
year: 2025
- country: Peru
production: crude borates 220 thousand t (2025)
reserves: 4,000 thousand t
year: 2025
- country: China
production: boric oxide equivalent 230 thousand t (2025)
reserves: 9,100 thousand t
year: 2025
- country: Argentina
production: crude ore 170 thousand t (2025)
year: 2025
- country: Russia
production: datolite ore 80 thousand t (2025)
reserves: 40,000 thousand t
year: 2025
- country: Germany
production: compounds 40 thousand t (2025)
year: 2025
critical_lists:
- list: USGS 2025 Final List of Critical Minerals (published 7 Nov 2025)
listed: true
recycling: Not sourced.
substitutes: []
source: usgs-boron
where: Salient statistics and World Production and Reserves table; critical minerals paragraph; the USGS table gives no country shares, so none are shown
- subject: Boron import dependence (United States)
abundance: 'USGS: import sources for all forms of boron, 2021 to 2024, as printed.'
producers:
- country: Turkey
share_percent: 90
year: 2021
basis: 'year as printed: 2021-2024 imports'
- country: Bolivia
share_percent: 6
year: 2021
basis: 'year as printed: 2021-2024 imports'
- country: Other
share_percent: 4
year: 2021
basis: 'year as printed: 2021-2024 imports'
critical_lists: []
recycling: Not sourced.
substitutes: []
source: usgs-boron
where: 'Import Sources (2021-24): All forms'
documents:
- title: BOROFLOAT 33 technical details (SCHOTT)
type: datasheet
url: https://www.schott.com/en-us/products/borofloat/technical-details
- title: Borosilicate glass (Wikipedia)
type: report
url: https://en.wikipedia.org/wiki/Borosilicate_glass
- title: BOROFLOAT 33 & Functional Coatings (SCHOTT)
type: datasheet
url: https://media.schott.com/api/public/content/32b935120d7343109caa36fb70f40ced?v=541db792&download=true
- title: SCHOTT BOROFLOAT data sheet 0407
type: datasheet
url: https://iosoptics.com/pdf/Schott-Datasheet-Borofloat-33.pdf
- title: 'Mineral Commodity Summaries 2026: Boron (USGS)'
type: report
url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-boron.pdf
- title: Boron oxide (B2O3), PubChem CID 11073337
type: database
url: https://pubchem.ncbi.nlm.nih.gov/compound/11073337
- title: Corning 7740 (Pyrex) property table, copy hosted by Valley Design
type: datasheet
url: https://valleydesign.com/wp-content/uploads/2023/11/corning-pyrex-7740.pdf
year: 2023
- title: NASA Solar Power Satellite System Definition Study (NTRS 19800004887)
type: report
url: https://ntrs.nasa.gov/api/citations/19800004887/downloads/19800004887.pdf
year: 1980
gaps:
- 'Silica sand and soda ash supply: no USGS MCS text read in this run; only the boron chapter was read.'
- 'Corning PYREX 7740: corning.com returns HTML, not a PDF, in this run. A Valley Design copy of the Corning 7740 property table is cited instead (source valley-7740); its values are not yet used in properties.'
- No safety data sheet for the finished glass was read. The hazard entry uses PubChem's classes for boron oxide, not for the glass.
- No open peer-reviewed paper on borosilicate properties was found; no flight, vacuum or radiation test was found.
- 'Source disagreement: the 0407 SCHOTT data sheet gives a transformation temperature of 530 °C and a Young''s modulus of 63 kN/mm2. The current SCHOTT technical page gives 525 °C and 64 kN/mm2. Both are kept with their own sources.'
- 'Lab glassware: Wikipedia states it as virtually all modern lab glassware, but the application is not on the approved list, so it stays in story only.'
- Wikipedia gives a CTE of 3.3 x10^-6 K^-1 at 20 °C for a generic lab borosilicate. That is not used as the BOROFLOAT value; it is recorded here only.
sources:
schott-borofloat33:
title: BOROFLOAT 33 technical details
id: SCHOTT AG product page
url: https://www.schott.com/en-us/products/borofloat/technical-details
kind: datasheet
license: manufacturer web page, cited not reproduced
wikipedia-borosilicate:
title: Borosilicate glass
id: Wikipedia, Borosilicate glass
url: https://en.wikipedia.org/wiki/Borosilicate_glass
kind: web
license: CC BY-SA 4.0
schott-media-datasheet:
title: BOROFLOAT 33 & Functional Coatings (SCHOTT brochure)
id: SCHOTT AG, BOROFLOAT 33 & Functional Coatings
url: https://media.schott.com/api/public/content/32b935120d7343109caa36fb70f40ced?v=541db792&download=true
kind: datasheet
license: manufacturer document, cited not reproduced
schott-datasheet-0407:
title: SCHOTT BOROFLOAT data sheet 0407 (SCHOTT North America)
id: SCHOTT North America, data sheet 0407 (copy on IOS Optics site)
url: https://iosoptics.com/pdf/Schott-Datasheet-Borofloat-33.pdf
kind: datasheet
license: manufacturer document, cited not reproduced
schott-tech-data-edmund:
title: BOROFLOAT 33 Technical Data (SCHOTT, copy on Edmund Optics)
id: SCHOTT AG technical data sheet, Edmund Optics copy
url: https://www.edmundoptics.com/ViewDocument/schott-borofloat-technical-data-sheet-english.pdf
kind: datasheet
license: manufacturer document, cited not reproduced
usgs-boron:
title: 'Mineral Commodity Summaries 2026: Boron'
id: USGS, MCS 2026
url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-boron.pdf
kind: report
license: US government work, public domain
pubchem-boric-oxide:
title: Boron oxide (B2O3), PubChem CID 11073337
id: PubChem, GHS classification of boric oxide
url: https://pubchem.ncbi.nlm.nih.gov/compound/11073337
kind: database
license: PubChem, public domain
valley-7740:
title: 'Glass Silicon Constraint Substrates: Corning 7740 (Pyrex) and 7070 property table'
id: Valley Design copy of the Corning 7740 and 7070 property table
url: https://valleydesign.com/wp-content/uploads/2023/11/corning-pyrex-7740.pdf
year: 2023
kind: datasheet
license: copy of a Corning property table, cited not reproduced
nasa-sps-1980:
title: Solar Power Satellite System Definition Study (NASA contract NAS9-15636)
id: NASA NTRS 19800004887
url: https://ntrs.nasa.gov/api/citations/19800004887/downloads/19800004887.pdf
year: 1980
kind: report
license: NASA technical report, public distribution
license: CC-BY-4.0
updated: '2026-10-19'
review:
status: draft
method: assistant
checked_values: 21
unreachable: 4
caveats: 2
note: 'Written by an AI assistant on 2026-10-19. Deepen pass: added CTE, electrical and bending values, annealing and softening points (SCHOTT data sheets), corrosion from the SCHOTT chemical durability table, hazard classes (PubChem), supply (USGS MCS 2026), uses, failure and handling sections.'
story:
recipe:
line: Borosilicate glass is an amorphous silica network with boric oxide added. It is made by melting boric oxide, silica sand, soda ash and alumina together (Wikipedia). BOROFLOAT 33 is made by the float process, which SCHOTT describes as the world's first floated borosilicate glass. The composition below is the typical low-expansion laboratory type; the BOROFLOAT 33 recipe is not published as numbers.
parts:
- name: Silica
symbol: SiO2
share: about 80% (approximately; Wikipedia gives 'approximately 80%')
role: The main network former. It gives the glass its rigid, mostly silica framework.
share_percent: 80
- name: Boric oxide
symbol: B2O3
share: about 13%
role: Lowers the thermal expansion and the working temperature of the glass.
share_percent: 13
- name: Alkali oxide
symbol: Na2O or K2O
share: about 4%
role: Helps the glass melt and adjusts its viscosity.
share_percent: 4
- name: Alumina
symbol: Al2O3
share: 2 to 3% (Wikipedia gives the range; share_percent uses the low end)
role: Improves chemical durability.
share_percent: 2
source: wikipedia-borosilicate
where: Wikipedia, Borosilicate glass, composition paragraph and Manufacturing process section
risk_groups:
- title: For people
level: 0
verdict: 'Hazard data partly checked: boric oxide classes read from'
lead: 'PubChem''s aggregated GHS classes for boron oxide, the main glass oxide, are read below. No SDS for the finished glass was opened in this run. Hazard data partly checked: boric oxide classes read from PubChem; no SDS for the glass read'
points:
- label: Sharp edges
line: Broken glass is sharp. Handle broken pieces with gloves and dispose of them in a sharps container.
- label: Boric oxide
line: PubChem's aggregated classes for boron oxide include H360 (may damage fertility or the unborn child, Danger) and H319 (eye irritation). These are for the powder, not for the finished glass.
- label: Sodium hydride
line: Do not heat borosilicate glass in contact with sodium hydride; the reaction can form sodium borohydride.
why: Borosilicate glass is a silica glass with boric oxide added. The boron lowers the thermal expansion, so the glass is less likely to crack when it is heated or cooled quickly. That property is why most modern laboratory glassware is made from it, and why the 200 inch mirror of the Hale Telescope was cast from it.
uses:
- where: Optics
line: The 200 inch mirror of the Hale Telescope is made of borosilicate glass, according to Wikipedia. Low-expansion borosilicate is also used for reflecting telescope mirrors in general.
- where: Laboratory glassware
line: Wikipedia states that virtually all modern laboratory glassware is made of borosilicate glass. No application from the approved list matches this use, so it is recorded here only.
- where: Space and aviation
line: SCHOTT lists telescope and satellite mirror substrates, and aircraft components for illumination, navigation and displays, among its borosilicate applications.
- where: Hot and cold mirrors
line: SCHOTT says BOROFLOAT 33 is the material of choice for high-temperature dichroic filters and hot and cold mirrors.
can_do:
- label: Density
benchmark: density
value: 2.23 g/cm3
line: SCHOTT lists a density of 2.23 g/cm3 at 25 °C for BOROFLOAT 33, which is lighter than most soda-lime glass.
- label: Working temperature
benchmark: max_operating_temperature_long_term
value: 450 °C
line: SCHOTT gives a maximum long-term operating temperature of 450 °C, and 500 °C for short exposures of less than 10 h.
- label: Transformation point
benchmark: transformation_temperature
value: 525 °C
line: The transformation temperature is 525 °C per ISO 7884, the point where the glass stops behaving like a fully elastic solid in cooling tests.
- label: Optics
benchmark: refractive_index
value: '1.4714'
line: The refractive index at 587.6 nm is 1.4714, with an Abbe number of 65.41, which means low dispersion for a visible-light lens.
- label: Thermal expansion
benchmark: cte
value: 3.25 x10^-6 K^-1
line: SCHOTT gives 3.25 x 10-6 K-1 for 20 to 300 °C (ISO 7991). SCHOTT says this is three times lower than soda-lime glass.
compare: Borosilicate glass has a linear thermal expansion of 3.25 x10^-6 K^-1 (20 to 300 °C), which SCHOTT puts at about one third of soda-lime glass. That low expansion gives it the thermal-shock limits in the 0407 data sheet (RTS 140 to 175 K by thickness). Its density of 2.23 g/cm3 and long-term limit of 450 °C are the figures on the SCHOTT sheet.
compare_rows:
- scope: material
label: Density
benchmark: density
- scope: material
label: Long-term working temperature
benchmark: max_operating_temperature_long_term
- scope: material
label: Transformation temperature
benchmark: transformation_temperature
- scope: material
label: Linear thermal expansion
benchmark: cte
handling:
- Keep long-term use at or below 450 °C, and short exposures at or below 500 °C (under 10 h). SCHOTT gives these as the maximum operating temperatures.
- 'Keep temperature differences within the SCHOTT RTD limits: 110 K for 1 h, 90 K for 1 to 100 h, 80 K for over 100 h.'
- Do not heat in contact with sodium hydride; sodium borohydride can form.
- Broken glass is sharp; handle it with gloves.
good_for:
- Lab glassware and optical parts that must tolerate temperature changes. SCHOTT's 450 °C long-term limit is the reference.
- Telescope mirrors and lenses, where low dispersion (Abbe number 65.41) and a known refractive index matter.
watch_out:
- The expansion coefficient (3.25 x10^-6 K^-1) is for BOROFLOAT 33. Other borosilicate grades, such as the 3.3 value on Wikipedia, can differ.
- Working limits are the manufacturer's reference values for BOROFLOAT 33, not guaranteed values for other borosilicate grades.
- Solarization and fluorescence are described as low by SCHOTT, but no numbers are printed on the text pages read.
verdict: Borosilicate is a sound choice where a low-expansion, durable glass is needed. This profile now has the thermal expansion, electrical and bending values, the chemical durability tests and the boric oxide hazard classes from sourced pages. Supply of silica and soda ash, the glass SDS and space testing are still open.
References
Every number above carries a reference number. Click one to see the source and where in it the value comes from.
| # | Source | Type | Year | Used for |
|---|---|---|---|---|
| BOROFLOAT 33 technical details | datasheet | 20 values | ||
| Borosilicate glass | web | 4 values | ||
| BOROFLOAT 33 & Functional Coatings (SCHOTT brochure) | datasheet | 3 values | ||
| SCHOTT BOROFLOAT data sheet 0407 (SCHOTT North America) | datasheet | 9 values | ||
| BOROFLOAT 33 Technical Data (SCHOTT, copy on Edmund Optics) | datasheet | 1 value | ||
| Mineral Commodity Summaries 2026: Boron | report | 2 values | ||
| Boron oxide (B2O3), PubChem CID 11073337 | database | 1 value | ||
| Glass Silicon Constraint Substrates: Corning 7740 (Pyrex) and 7070 property table | datasheet | 2023 | 0 values | |
| Solar Power Satellite System Definition Study (NASA contract NAS9-15636) | report | 1980 | 1 value |