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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 UV-transmission
  • Low absorption
  • Crystal clear appearance

Typical Applications

  • Heat resistant sight glasses
  • Front windows for UV lamps
  • Sight-glass for flame monitoring
  • Material for mechanical 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 ideal for the fabrication of sight glasses with a high thermal load. Generally, any quartz glass type mainly contains silicon and oxygen. The same applies to fused quartz, which has more impurities than optical grade quartz glass materials. For parts like front cover glasses for UV lamps, high temperature viewing ports, and many other less optically demanding applications, this is not relevant. The exceptional material grade and purity provided by Fused Silica materials used for optics are unnecessary in those cases, and high transparency below 200 nm is usually not required. Hence, fused quartz glass is an excellent material choice for the fabrication of less sophisticated components requiring heat resistance or a reasonable UV transmission at a lower cost.

Fused quartz glass is produced by the melting of natural silicon dioxide crystals obtained from quartz sand. Therefore it cannot achieve an optical performance and purity as provided by synthetic fused silica glass. On the other hand, it is much more economical, 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 for the materials used for optics. The available high ultraviolet transmittance down to about 200 nm is an essential property for various UV-lighting applications. Also, 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 an optical-grade finish 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. The possible thickness tolerances and the parallelism of sight-glass polished material are relatively coarse, but this usually is not a relevant factor in most application areas for which these sight glass grade fused quartz 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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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
at 3 mm thickness

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, to offer our customers the best possible service and availability, we continually polish to several thicknesses available quickly and even as part of our Express-Service. 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 x 1010
  • Poisson number: 0,17

Thermal Properties

  • Coefficient of thermal expansion:
    approx. 0,55 x 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 x 10-4 (20 °C ; 1 MHz)
  • Specific resistivity: 7,0 x 10Ω x cm (350 °C)
  • Insulation capacity 5 x 10-7 V/m

For information about our high purity quartz glass for optics like our synthetic fused silica, please follow this link.


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


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