Borosilicate glass
Selecting borosilicate glass by named grade, supplied form, thermal conditions, surface quality and component qualification.
Tags: materials, glass, borosilicate
On this page
SCHOTT lists 150 MPa typical bending strength for the float surface of 2.75 mm BOROFLOAT 33 under DIN EN ISO 1288-5. The surface deliberately damaged with 220 sandpaper gives about 25 MPa under the former DIN 52292 Part 1. Use a design basis that accounts for edge and surface damage, geometry, loading and environment. SCHOTT BOROFLOAT 33 mechanical data
Name the grade and supplied form
SCHOTT describes BOROFLOAT 33 as floated borosilicate flat glass. Its general-information sheet illustrates a composition of approximately 81% silica, 13% boron oxide, 4% sodium/potassium oxides and 2% aluminum oxide. These proportions describe that product, rather than every glass sold as borosilicate. The sheet distinguishes standard sheet dimensions, thickness tolerances and subsequent cutting and edge processing. An order for a finished window therefore needs more than a composition name. SCHOTT, BOROFLOAT 33 General Information
SCHOTT's DURAN Technical Data, with a printed Business Unit Tubing / 02/2026 footer, identifies DURAN as borosilicate glass 3.3 according to ISO 3585. Its approximate mean expansion coefficient is 3.3 × 10⁻⁶ K⁻¹ between 20 and 300 °C, measured according to ISO 7991. The interval and measurement basis belong with the number. A matching nominal expansion class does not prove that two suppliers' tubes have interchangeable dimensions, processing history or finished-component approvals. SCHOTT, DURAN Technical Data
Keep product codes distinct from brand shorthand. Corning's selection guide identifies Code 7740 PYREX labware, low-actinic 7740 with a surface modification, and PYREXPLUS 7740 with a vinyl coating. The coated or modified article has additional application constraints. Do not assign properties from Code 7740 laboratory glass to an unidentified article merely because its marking includes a familiar brand. Corning, Glass and Reusable Plastic Product Selection Guide, Glass Technical Data
Expansion, transition and operating temperature differ
SCHOTT's BOROFLOAT 33 thermal sheet lists the following reference data and explicitly says its values are not guaranteed:
| Quantity | BOROFLOAT 33 value and basis |
|---|---|
| Mean linear expansion coefficient | 3.25 × 10⁻⁶ K⁻¹, 20–300 °C, ISO 7991 |
| Glass transition temperature | 525 °C, ISO 7884 |
| Maximum operating temperature, short term | 500 °C, less than 10 hours |
| Maximum operating temperature, long term | 450 °C, at least 10 hours |
These entries answer different questions. The transition temperature is not permission to operate a loaded assembly at that temperature; the short-term entry does not qualify a seal, coating or mounting arrangement. A stated operating temperature also does not establish resistance to an abrupt temperature difference across a pane. SCHOTT, BOROFLOAT 33 Thermal Properties
Illustrative free-expansion calculation: for a 100 mm length heated uniformly from 20 to 120 °C, using the BOROFLOAT coefficient above as constant gives ΔL = 100 × 3.25 × 10⁻⁶ × 100 = 0.0325 mm. This calculation assumes unconstrained, uniform heating. It predicts neither stress in a rigid metal frame nor a safe thermal-shock temperature difference.
For thermal cycling, specify initial temperature, heating/cooling medium, ramp or transfer conditions, dwell, wall thickness, geometry and cycles. ROBU's VitraPOR guidance specifically notes the effects of thick walls, uneven temperature distributions and surface damage on thermal resilience. Those dependencies explain why a thermal-shock number without its specimen and procedure is incomplete. Do not transfer a porous-glass product's stated temperature capability to a solid window or pressure vessel. ROBU, VitraPOR temperature resistance
Strength depends on surfaces and installation
SCHOTT's mechanical sheet gives a useful bounded comparison: BOROFLOAT 33 with a typical float surface has a normal bending-strength value of 150 MPa, determined according to DIN EN ISO 1288-5 on 2.75 mm glass. The approximately 25 MPa entry concerns surfaces deliberately damaged with 220 sandpaper, based on former DIN 52292 Part 1. These are different surface/test conditions, not competing universal strengths.
The sheet explains that strength is statistically distributed and depends on processing, holes and edges, surface condition, load type/duration, environment and installation geometry. Its values are not guaranteed. Use a qualified design basis for the finished component rather than treating a pristine-surface result as an allowable stress throughout service. SCHOTT, BOROFLOAT 33 Mechanical Properties
Tubing needs its own assessment. SCHOTT's DURAN page restricts its pressure-calculation guidance to straight, stress-free tubes with uniform wall thickness and open ends. It is not a ready-made rating for a vessel with a closed end, branch, flange or damaged surface. Specify the complete assembly and its applicable pressure/temperature qualification. SCHOTT, DURAN tubing technical details
Chemical resistance has test conditions
High chemical durability does not mean universal chemical compatibility. SCHOTT's BOROFLOAT chemical sheet reports 1.1 mg/cm² surface loss and white staining for 5% NaOH after 6 hours at 95 °C; its 10% HF, 20 minutes at 23 °C entry also shows 1.1 mg/cm², with stained white haze. Concentration, temperature and duration are essential parts of both examples. Hydrolytic and acid-resistance classes must not be read as guarantees for every cleaning chemical or process mixture. SCHOTT, BOROFLOAT 33 Chemical Properties
For optical service, retain wavelength and thickness with transmission requirements. Corning lists 92% visible transmission over 400–760 nm for 2 mm Code 7740; this is not a universal UV or infrared window specification. Confirm required transmission, surface finish, coatings and permissible contamination on the actual finished part. Corning, Code 7740 technical information
Processing, handling and acceptance
Define cutting, holes, edge finish and any strengthening treatment on the drawing. SCHOTT's processing guide offers several routes but explicitly limits thermal/chemical toughening suitability to particular glass types. A generic processing menu does not prove that every operation is suitable for every borosilicate grade or geometry. Agree the manufacturing sequence and final inspection with the processor. SCHOTT, Processing Options for Special Glass
Protect finished surfaces and edges during handling. BOROFLOAT's general-information sheet advises original packaging protected from moisture and rejects abrasive sponges, scouring powders and corrosive or abrasive cleaners that damage the surface. Select cleaning agents for the actual process rather than introducing scratches during maintenance. SCHOTT, BOROFLOAT general information and care
Record the grade, dimensions, edge and hole quality, surface defects, processing condition and applicable component tests.