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Fused Quartz Glass

Natural quartz materials for sight-glass applications

Special Properties

  • Heat-resistant up to over 1000 °C (short-term)
  • High thermal shock resistance
  • Excellent chemical strength
  • Low coefficient of expansion
  • Good transmission in the UV range
  • Low absorption
  • Crystal clear appearance
  • High uniform infrared transmission up to 2600 nm
  • Non-crystalline, amorphous silica glass
  • Nearly 100 % silicon dioxide material
  • More economical than fused silica

Typical Applications

  • Heat-resistant sight-glasses
  • Front glasses for UV lamps
  • Flame monitoring
  • Material for industrial quartz components
  • UV-LED covers
  • Protection windows
  • Medical UV-disinfection systems
  • Heat-resistant viewing ports
  • UV-drying and curing systems
  • Front glass for ultraviolet light sources
  • Quartz components for the chemical industry

Related glass types


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Visit our Quartz Glass FAQ web page to assist in choosing the suitable quartz material type for your application.

Fused quartz is ideal for sight-glass applications

Fused quartz is a non-crystalline amorphous glass fabricated by flame-melting natural silicon dioxide crystals from quartz sand with a SiO₂ content of 99,98 %. It is not as pure but much more economical than the synthetically produced high-end fused silica materials made by flame hydrolysis, especially at large dimensions.

Any quartz glass material mainly contains silicon and oxygen, which applies to fused quartz, too. Still, it has more impurities and inclusions than the much more expensive optical-grade materials like fused silica. However, because optical purity is irrelevant for components like the front cover glasses of UV lamps, high-temperature viewing ports, heat-resistant inspection windows, and many other less optically demanding industrial applications, natural fused quartz glass is often the perfect material for such use cases.

Fused quartz's low expansion and high thermal strength properties are comparable to those of optical-grade materials. Its chemical resistance is outstanding and also similar to that of the silicon dioxide materials used for optics. Moreover, the high ultraviolet light transmittance down to about 200 nm is essential for applications like UV curing or disinfection lamps. It is worth mentioning that natural quartz glass also has a high, uniform transmission up to 2600 nm. Besides, fused quartz provides similar electrical resistivity and excellent insulation properties similar to the mentioned high-end quartz glass materials.

We make our fused quartz glass transparent through an economical mechanical polishing process. Using this cost-effective process, the achievable thickness tolerances and parallelism are relatively coarse, which is usually irrelevant for most application areas where these sight-glass materials operate. This polishing process does not achieve an optical-grade finish regarding flatness and surface quality. Looking through the glass, however, you can see almost no distortions or inhomogeneities.

Please click the following button to request a quote for your sight-glass-grade fused quartz window:


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48-hour express optics Manufacturing and delivery within 48 hours 

For some thicknesses, we offer 48-hour express fabrication. Please find the available glass thicknesses for this option in the table.



Fused quartz glass with sight-glass quality
Transmission at 3 mm in the UV-NIR range

Transmittance of natural fused quartz
Zoom in to see more details

Glass thicknesses

Natural fused quartz glass material is usually not available as natural blank sheets. Hence, there is no standard thickness, and it must be specified individually on request. However, we continually polish to several thicknesses available quickly to offer our customers the best possible availability. Please find these fused quartz glass thicknesses, polished transparent for sight glass applications, in the following table. Even our 48-hour delivery option is viable for these thicknesses if required.

Please click on glass thickness to start an inquiry:

Thickness (mm)


48-hour express fabrication
1,0 ±0,2
1,1 ±0,2
2,0 ±0,3
3,0 ±0,3
5,0 ±0,5
Custom thicknesses are available on request.
For thicknesses marked with "" our optional 48-hour express fabrication is possible.

Optical Properties

  • UV-Transmission:
    Approximately 90 % at 260 nm (at 3 mm thick)
  • Refractive Index: nd = 1,459

Mechanical Properties

  • Density: 2,20 g/cm3
  • Young's Modulus: ca. 7,2 × 1010
  • Poisson's number: 0,17

Thermal Properties

  • Coefficient of thermal expansion:
    Approx. 0,55 × 10-6/K (25–300 °C)
  • Maximum operating temperature:
    • Short-term use: up to approx. 1100 °C
    • Long-term use: up to approx. 850 °C
  • Thermal conductivity: approx. 1,4 W/K m

Electrical Properties

  • Dielectric constant εr: 3,75 (1 MHz)
  • Loss factor: < 2 × 10-4 (20 °C ; 1 MHz)
  • Specific resistivity: 7,0 × 10Ω x cm (350 °C)
  • Insulation capacity 5 × 10-7 V/m

If you are searching for higher-quality quartz glass suitable for optics or semiconductor applications, please view our synthetic fused silica data sheet for detailed information.


All given details and specifications are mean reference values and are not guaranteed. Also, please consider the "Notes on specifications".

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Last update: 29.09.2023