When it comes to heat resistance, the clear winner is quartz glass. It can withstand temperatures up to 1,100°C for continuous use and 1,300°C for short periods, far surpassing borosilicate glass, which softens around 800-820°C. This difference comes down to their chemical composition and atomic structure.

The Chemistry Behind the Difference

Quartz glass is made of nearly pure silicon dioxide (SiO₂), with a content of over 99.9%. Its structure is a continuous, unbroken network of silicon and oxygen atoms, with no additives or network modifiers. This purity gives it an extremely high melting point and thermal stability.

Borosilicate glass, on the other hand, contains about 80% SiO₂, with the rest being boron trioxide (B₂O₃), alumina (Al₂O₃), and sodium oxide (Na₂O). These additives, especially the sodium oxide, act as network modifiers that lower the glass's melting point and thermal stability. While borosilicate glass is still far more heat-resistant than ordinary soda-lime glass, it cannot match the performance of quartz.

Thermal Expansion: A Crucial Difference

The difference in heat resistance is also reflected in their coefficients of thermal expansion. Quartz glass has a remarkably low coefficient of thermal expansion of 0.54 × 10⁻⁶ per °C. This means it expands and contracts very little with temperature changes. Borosilicate glass has a coefficient of approximately 3.3 × 10⁻⁶ per °C, about six times higher than quartz.

This difference in expansion directly affects resistance to thermal shock. A material with a lower expansion coefficient can withstand rapid temperature changes without cracking. Quartz glass can be heated to red-hot and then plunged into cold water without breaking, while borosilicate glass, though resistant, will shatter under such extreme conditions.

The Bottom Line

If your application involves temperatures above 500°C, frequent thermal cycling, or extreme thermal shock, quartz glass is the better choice. For general laboratory use at moderate temperatures, borosilicate glass offers an excellent balance of heat resistance, chemical durability, and cost-effectiveness. The higher cost of quartz is justified only when its superior temperature performance is required.