Fused Silica (FS) and fused quartz are terms used to describe a family of very hard and transparent glass with excellent optical properties. The raw material is nearly 100% pure silicon dioxide (SiO2), it is fused at high temperatures into an amorphous (non-crystalline) material. All grades of transparent fused quartz/fused silica have near-identical physical properties. Its optical and thermal properties are better than those of other types of glass due to its purity.
Fused quartz/fused silica are transparent over a wide range of wavelengths (from deep ultraviolet to infrared regions 0.16 µm-3.5 µm). Its ultraviolet transmission is superior to that of most other glasses.
The water content of fused quartz and fused silica (and therefore infrared transmission of the material) is determined by the manufacturing process. Flame fused material, as fused silica UV grade, always has a higher water content, but it has a very low metallic impurity content making it transparent deeper into the ultraviolet. Fused quartz IR grade, which is fused electrically, has a greater presence of metallic impurities, limiting its UV transmittance wavelength to around 250 nm, but its much lower water content leads to excellent infrared transmission up to 3.6 µm wavelength.
Due to its extremely low coefficient of thermal expansion (5.5 x 10-7 average from 20 °C to 320 °C), fused quartz/fused silica possesses remarkable ability to undergo large, rapid temperature changes without cracking.
Fused quartz/fused silica are chemically stable and have excellent chemical resistance to most acids, except for hydrofluoric (HF) and phosphoric (H2PO4) acids. Because of its superior optical purity and high thermal, chemical and mechanical stability, fused quartz and fused silica are excellent choices for many critical applications in high-quality optical systems.
Fused quartz/fused silica is used for production of windows, prisms, lenses and other optical items in the range from the ultraviolet to the infrared. Due to its low coefficient of thermal expansion, fused quartz/fused silica is also a widely useful material for precision mirror substrates. Fused quartz has nearly ideal properties for fabricating first surface mirrors for telescopes.