Borosilicate Glass vs. Fused Silica: Which Should You Choose?
Borosilicate glass and fused silica both handle heat better than ordinary glass, but they're not interchangeable. Fused silica pushes every one of borosilicate's advantages further — at a meaningfully higher price.
Key Comparison
| Property | Borosilicate Glass | Fused Silica |
|---|---|---|
| Composition | SiO2 + B2O3 (+ minor oxides) | 99.9%+ pure SiO2 |
| Thermal expansion coefficient | ~3.3 × 10⁻⁶ /°C | ~0.5 × 10⁻⁶ /°C |
| Max use temperature | ~450–500°C | 1000°C+ |
| UV transmission | Limited below ~300 nm | Excellent down to ~185 nm (UV grade) |
| Relative cost | Moderate | High |
Where the Two Diverge
The gap between the two comes down to purity. Borosilicate glass still contains several oxide components alongside silica; fused silica is essentially pure silicon dioxide, melted and re-formed without the additives that lower soda-lime and borosilicate's melting points. That purity is what pushes fused silica's thermal and UV performance so far ahead — and what pushes its manufacturing cost up with it.
When Borosilicate Is Enough
If your application sits below roughly 450°C, stays within the visible-to-near-UV range, and doesn't demand extreme thermal shock resistance, borosilicate glass delivers most of the durability for a fraction of the cost of fused silica.
When You Need Fused Silica
Deep-UV transmission, laser systems with high damage-threshold requirements, or environments running well above 500°C are where fused silica earns its price premium — borosilicate simply isn't rated for that range. See our fused silica glass guides for grade-by-grade detail (JGS1/JGS2/JGS3 and UV performance).
FAQ
Is fused silica the same as quartz glass? Yes, in general usage "fused silica" and "quartz glass" refer to the same material — amorphous, high-purity silicon dioxide.
Can I substitute borosilicate glass for fused silica in a UV application? Only above roughly 300 nm. Below that, borosilicate's UV transmission drops off sharply, and fused silica is the only practical choice.
