Silica is found almost everywhere in nature, and represents almost 1/3 the mass of the earth’s crust. Vitreous Silica is the generic term used to describe all types of silica glass, and manufacturers refer to the material as either Fused Quartz or Fused Silica. Fused Quartz is manufactured by melting naturally occuring crystalline silica, such as sand or rock crystal. The production method is either electrically fused or flame fused. Afterward, items will appear transparent, translucent, or opaque; making it possible to create a wide range of products. Fused silica, commonly referred to as synthetic fused quartz, is produced using high purity silica sand that is manufactured from SiCL4. The finished product’s appearance will be transparent. Vitreous Silica, in all its forms, offers a variety of properties such as:

• Permeability
• Extreme Hardness
• Very Low Coefficient of Thermal Expansion
• Resistance to High Temperature
• High Chemical Purity
• High Corrosion Resistance
• Extensive Optical Transmission from Ultra-Violet to Infra-Red
• Excellent Electrical Insulation Qualities
• Remarkable Stability Under Atomic Bombardment

Because fused quartz is used in applications involving internal pressures, it is helpful to know the maximum pressure that can be applied to a selected fused quartz tube. The formula below can approximate this information at room temperature.

The cleaning of fused quartz is critical before it is used in any application. The fused quartz should be cleaned by placing it in a 7% maximum solution of Ammonium Bifluoride for no more than ten (10) minutes, or a 10% volume maximum solution of Hydrofloric Acid for no more than five (5) minutes. After cleaning, using the above method, the fused quartz should be rinsed in deionized or distilled water and then dried.
To further reduce the possibility of contamination, care should be used in handling fused quartz. The use of clean cotton gloves at all times is essential.

In order to increase resistance to devitrification and sag of your quartzware, an even layer of cristobalite must be formed on the O.D. of quartz tubes. Expose a new tube to a temperature of up to 1200°C and rotate it 90° every two (2) hours for the first 12 to 24 hours.

The curves in the figure below represent the average transmissions for a 10mm thick test sample for both commercial and optical grades. Fused quartz is very efficient for the transmission of infrared radiation. Infrared transmission extends out approximately 4 micrometers, with little absorption in the “water band” at 2.73µm.