Fused quartz glass picture
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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

Typical Applications

  • Temperature resistant sight glasses
  • Front windows for UV lamps
  • Sight glass for flame monitoring
  • Material for mechanical quartz components
  • Protective sight glass windows
  • Glass for UV-disinfection applications
  • UV-drying / curing systems
  • Windows for the chemical industry

Related glass types

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

Fused Quartz glass for sight glass applications

Made of natural fused quartz glass material we produce sight glasses for high thermal load used for front cover glasses of UV-lamps and similar less optically demanding components. Since for such kind of applications the very high optical quality and purity as provided by Fused Silica glass materials are of secondary concern and a high transmissivity below 200 nm usually is not required, fused quartz glass is an excellent material choice for the fabrication of less sophisticated parts at comparatively lower cost.

Fused quartz glass materials are produced by the melting of natural quartz (silicon dioxide) crystals. Therefore they cannot achieve such a high optical performance and don't have such a very high purity as our synthetic optics grade silica glass. On the other hand they are much more economical materials, especially at larger dimensions. Still, the low coefficient of expansion and the high thermal strength are comparable to those of optical quartz glass material. Also, the chemical properties are excellent and on the same level as for the materials used for optics. Besides that, the available high ultraviolet transmission down to about 200 nm is an essential property for various UV-lighting applications. In addition fused quartz also provides a high specific electrical resistance.

Our fused quartz glass materials are made transparent by a more economical mechanical polishing process. The polishing finish does not offer 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 this polished material are relatively coarse, but this usually is not an important factor in most areas of application for which these sight glass grade materials are used for.

To request a quote for windows of fused quartz glass with sight glass quality, please click on the following button:


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For optics-grade high purity quartz glass materials like our synthetic fused silica, please follow this link.


48h-Express Service 48h-Express Service 

For some fused quartz glasses thicknesses, we offer our 48h-Express Service for the fabrication and shipping within two working days. Below please find the available thicknesses within the express service.

Express service thicknesses:
3 mm ; 5 mm

Dimensions within express-service are limited to 500 mm x 500 mm.




Transmission curve of natural fused quartz glass

Show optical transmissivity of Fused Quartz Glass
(Sight glass grade material)



  • 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

Additional information is available on request.


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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