Fused Quartz

Natural fused quartz 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
  • UV transparent
  • Low absorption
  • Crystal clear appearance
  • High uniform infrared transmission up to 2600 nm
  • Non-crystalline, amorphous silica glass
  • 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
  • Protective windows
  • Medical UV disinfection systems
  • Heat resistant viewing ports
  • UV drying and curing systems
  • Front glass for ultraviolet light sources
  • Quartz parts for the chemical industry

Related Glass Types


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Visit our Quartz Glass FAQ to help you choose the right type of quartz material for your application.

Fused Quartz:

The best choice for sight glasses

Fused quartz is a non-crystalline amorphous glass made by flame melting natural silica crystals with a SiO2 content of 99,98 %. Any quartz glass material contains primarily silicon and oxygen. This is also the case with fused quartz. However, it is less pure and contains more inclusions than the much more expensive optical grade fused silica materials. Since optical purity is irrelevant for the front cover glasses of UV lamps, high-temperature viewing ports, heat-resistant inspection windows, or other industrial applications, fused quartz glass is the ideal material choice for such applications.

The low expansion, high thermal resistance, and excellent chemical properties of fused quartz are comparable to those of optical-quality silicon dioxide materials. In addition, the high transmission of ultraviolet light down to about 200 nm is essential for applications such as UV curing or disinfection systems. Furthermore, fused quartz has a high, uniform transmission up to 2600 nm. It also offers high electrical resistivity and outstanding insulation properties.

Our fused quartz is made transparent by a low cost polishing process. The thickness tolerance and parallelism achieved by this polishing process are relatively coarse, and flatness and surface quality are not on optics level. However, in most applications where these sight glass materials are used, this is not an issue. Nevertheless, the glass is clear, and the view through the glass shows almost no distortions or inhomogeneities.

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


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48-hour express optics 48-hour manufacturing and delivery

For some thicknesses, we offer 48-hour express fabrication.

Please see the table for available glass thicknesses for this option.



Sight glass quality Fused Quartz
Transmission at 3 mm in the UV-NIR range

Transmittance of fused quartz
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Glass Thicknesses

Fused Quartz glass is normally not available as blank sheet. Therefore, there is no standard thickness, and it must be specified individually. However, we continuously produce and polish several common thicknesses to offer our customers the best possible availability. Please refer to the table below for the most commonly available fused quartz thicknesses polished for sight glass applications. Even our 48-hour delivery option is available for these thicknesses if needed.

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 (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 optical or semiconductor applications, please refer to our Synthetic Fused Silica datasheet 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: April 20, 2024