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Fused Quartz Glass

Sight glasses made of molten natural quartz

Special Properties

  • Short-term application temperature up to 1100 °C
  • More economical than optical grade fused silica
  • High thermal shock resistance
  • Excellent chemical strength
  • Low coefficient of expansion
  • Good transmission in the UV range
  • Low absorption
  • Crystal clear appearance
  • Non-crystalline amorphous glass

Typical Applications

  • Heat-resistant sight glasses
  • Front windows for UV lamps
  • Sight-glass for flame monitoring
  • Material for industrial quartz parts
  • UV-LED covers
  • Protective sight glass windows
  • UV-disinfection systems for medical use
  • Heat-resistant viewing ports
  • UV-drying / curing systems
  • Quartz windows for the chemical industry

Related glass types

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48h-Express Service Partially available in 48 h Express-Service


Please visit our Quartz Glass FAQ web page, which will help you choose the suitable quartz glass material grade for your application.

Fused Quartz glass for sight glass applications

Natural fused quartz glass material is an ideal material for fabricating high-temperature sight glasses. Generally, any quartz glass type mainly contains silicon and oxygen, which also applies to fused quartz. Still, it has more impurities and inclusions than the more expensive optical grade quartz glass materials like fused silica. However, because purity is irrelevant for front cover glasses of UV lamps, high-temperature viewing ports, and many other less optically demanding industrial applications, natural fused quartz glass often is the perfect choice for such use cases.

Fused quartz glass is made by melting natural silicon dioxide crystals obtained from quartz sand. Therefore, it is much more economical than the synthetically produced high-end fused silica materials, especially at large dimensions. Still, the low expansion and the high thermal strength of fused quartz are comparable to fused silica. The chemical properties are outstanding and on the same level as the materials used for optics. The available high ultraviolet transmittance down to about 200 nm also is an essential property for various UV-lighting applications. Besides, fused quartz provides a similar electrical resistivity and excellent insulation properties.

Our fused quartz glass is made transparent by an economical mechanical polishing process. The polishing process does not achieve optical-grade finish quality regarding flatness and surface quality. However, the polished glass is clear and has almost no distortions or inhomogeneities when directly looked through with the naked eye. Caused by this cost-effective polishing process, the achievable thickness tolerances and the parallelism of sight-glass polished material are relatively coarse, which usually is not relevant for most application areas in which these sight glass materials are used.

Please click on the following button to request a quote for fused quartz glass windows with sight glass quality.


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48h-Express Service 48h-Express Service 

We offer 48h-Express fabrication of some fused quartz glass thicknesses. Please find the available glass thicknesses in this table.



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

T curve of natural fused quartz glass
Enlarge curve to show details

Glass thicknesses

Natural fused quartz glass materials are not available in the form of natural blank glass sheets. Hence, there are no standard thicknesses, and you must specify the glass thickness individually on request. However, we continually polish to several thicknesses available quickly to offer our customers the best possible service and availability. Even our Express Service is possible if required. Please find these glass thicknesses, polished transparent for sight glass applications, in the following table:

Please click on glass thickness to request:

Thickness (mm)


48h-Express Service
1,0 ±0,2
1,1 ±0,2
3,0 ±0,3
5,0 ±0,5
Custom thicknesses on request.
The thicknesses marked with "" are also available within 48–h Express Service.

Optical Properties

  • UV-Transmission: approximately 90 % at 260 nm
    (at 3 mm thickness)
  • 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

Please follow the link for information about our high purity quartz glass for optics, like our synthetic fused silica.


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 21.03.2023