Introduction

“A seemingly ordinary set of numbers defines the reliability boundaries of everything from laboratory precision instruments to heat-resistant glass kettles used at home.”
In the field of glass materials, terms such as BF33, Borosilicate 3.3, or high borosilicate glass are often associated with BOROFLOAT® 33 from SCHOTT. This material is widely used, ranging from high-end optical components to heat-resistant cookware.
But what exactly is Borosilicate 3.3? How does it relate to high borosilicate glass as a broader category? And why does it exhibit outstanding thermal stability and chemical inertness?
This article explores the unique characteristics of this classic material through its history, chemical composition, and microscopic structure.


 

1. Historical Origins of BF33

The origin of Borosilicate 3.3 can be traced back to late 19th-century Jena, Germany:
1884: Otto Schott founded a laboratory dedicated to glass research together with physicist Ernst Abbe and optician Carl Zeiss, focusing on the development of new glass materials.
1891: Boric acid was introduced into glass formulations for the first time, resulting in borosilicate glass capable of withstanding temperatures up to 500 °C, enabling its use in professional thermometers.
1897: Borosilicate glass with a linear thermal expansion coefficient of 3.3 × 10⁻⁶/K was developed—this became the prototype of modern Borosilicate 3.3 glass.
1993: SCHOTT established the world’s first micro-float production line, enabling mass production of BOROFLOAT® 33, which complies with ISO 3585 and EN 1748, setting an industry benchmark.
It is important to note that Borosilicate 3.3 is not a proprietary concept but a generic classification of materials. BF33 is simply the most well-known representative, distinguished by its successful combination of low thermal expansion and exceptional surface flatness and optical homogeneity achieved through the float glass process.
 

2. The Essence and Core Properties of Borosilicate 3.3

The defining characteristic of Borosilicate 3.3 glass is its linear thermal expansion coefficient of (3.3 ± 0.1) × 10⁻⁶/K.
This extremely low value allows the glass to maintain dimensional stability under rapid temperature changes.

Key properties include:

High boron and high silica content:
B₂O₃ ≈ 12–15%, SiO₂ ≈ 78–80%, forming a robust glass network
Excellent thermal shock resistance:
Withstands temperature differentials of 200–300 °C, such as transferring directly from ice water to boiling water without cracking
High softening point and working temperature:
Softening point above 820 °C, ensuring stability in high-temperature environments
Outstanding chemical stability:
Highly inert to water, acids, alkalis, salt solutions, and most organic substances
High optical transmittance:
90%, suitable for optical and precision instrument applications

Classification of Borosilicate Glass by Boron Content

Borosilicate glass can be broadly divided into high-, medium-, and low-boron types, primarily differentiated by B₂O₃ content and thermal expansion coefficient:

 

Type	B₂O₃ Content	Thermal Expansion (×10⁻⁶/K)	Key Characteristics	Typical Applications
High borosilicate	12–13%	~3.3	Excellent thermal shock resistance, high chemical stability	High-end labware, heat-resistant cookware, optical components
Medium borosilicate	8–12%	4.0–5.0	Balanced properties, good thermal and chemical resistance	Pharmaceutical glass, chemical packaging, premium lighting
Low borosilicate	5–8%	6.0–8.0	Moderate performance, easy processing	Everyday glassware, standard lighting, low-demand pharma packaging

The thermal expansion coefficient is the key indicator of thermal shock resistance:
the lower the value, the stronger the resistance to thermal stress.
 

3. Chemical Composition Determines Performance

The superior performance of high borosilicate glass originates from its “strong network, minimal disruption” chemical design.
 

Component	High Borosilicate Glass	Soda-Lime Glass
SiO₂	~78–80%	65–70%
B₂O₃	12–15%	5–8%
Na₂O / K₂O	Low	High
CaO / MgO	Low	High

High borosilicate glass maximizes the content of network formers (SiO₂ + B₂O₃) while limiting network modifiers such as Na₂O. This results in a strong, three-dimensionally connected microscopic structure.
In contrast, soda-lime glass relies on large amounts of fluxes to reduce cost and melting temperature, sacrificing thermal and chemical stability.
 

4. The Microscopic Role of Boron Oxide

B₂O₃ plays a critical role in optimizing the glass network through two mechanisms:
Flexible buffering by [BO₃] triangular units
When incorporated into the Si–O tetrahedral network, these units can internally adjust under thermal stress, reducing the thermal expansion coefficient.
Network repair via [BO₃] → [BO₄] transformation
In the presence of alkali ions (e.g., Na⁺), B₂O₃ undergoes the so-called “boron anomaly”, converting [BO₃] units into [BO₄] tetrahedra. This enhances three-dimensional connectivity, increasing strength and stability.
This dual mechanism of network repair and reinforcement allows high borosilicate glass to maintain relatively low melting temperatures and good processability, while approaching the low thermal expansion and chemical inertness of fused silica.
 

5. High Borosilicate Glass vs. Soda-Lime Glass

In both fundamental and critical performance aspects, high borosilicate glass clearly outperforms soda-lime glass:
Lower thermal expansion → superior thermal shock resistance
Higher chemical stability → suitable for harsh chemical and high-temperature environments
Greater mechanical reliability → applicable to precision instruments and industrial systems
Although soda-lime glass is inexpensive and easy to process, it is prone to deformation, cracking, and corrosion under high-temperature or chemically aggressive conditions.

Conclusion

The outstanding performance of **high borosilicate glass—especially Borosilicate 3.3—**is rooted in its high-silica, high-boron composition and its uniquely engineered microscopic network structure.
Its advantages include low thermal expansion, excellent thermal shock resistance, and high chemical inertness, while its production presents challenges such as high melting temperatures, boron volatilization control, and compositional uniformity.
As industrial technologies continue to advance, Borosilicate 3.3 glass will remain an irreplaceable material in high-temperature applications, precision optics, chemical laboratories, and heat-resistant consumer products.