Borosilicate glass

Adding boron oxide drops the thermal expansion coefficient from 9 to 3.3 µm/(m·K), and that single number is the entire reason it survives thermal shock. Thermal stress is proportional to expansion coefficient times temperature difference, so borosilicate tolerates roughly three times the temperature gradient of window glass before it cracks — which is why a Pyrex dish goes from oven to worktop and a drinking glass does not. Older Pyrex kitchenware was borosilicate; much of the modern US-sold product is tempered soda-lime instead, which fails differently.

Properties

Density

2,230 kg/m³ — 2.23 g/cm³

Tensile strength

30–70 MPa

Young's modulus

64 GPa

Thermal conductivity

1.14 W/(m·K)

Hardness

6 Mohs

Thermal expansion

3.3 µm/(m·K)

Floats in water

No

Magnetic

No

What it is used for

• Laboratory glassware
• Oven dishes and coffee pots
• Stage and vehicle lighting
• Telescope mirror blanks

How it compares

Does it rust?

Borosilicate glass contains no iron, so it cannot rust. Whether it degrades depends on the material, not on oxidation.

Is it magnetic?

Borosilicate glass is not a metal and has no ferromagnetic response.