There appears to be some confusion and overlap between the terms “fused silica”, “fused quartz” and “quartz” within precision optics applications. This can become even more confusing when one carries out an internet search on questions such as “Is quartz and silica the same thing?”, as many of the links in the search engine results refer to a geological context for example.

As a Precision Optics Manufacturer we’ve tried to clarify the differences (and the similarities!) below in the hope that it will clear up some of the confusion as pertinent to the context of precision optics.

Definitions:
• Quartz: Crystalline naturally occurring mineral. Made up of Silicon Dioxide (SiO2) (plus other impurities)
• Silica (SiO2): Silicon Dioxide is the correct chemical name. It is therefore ‘pure’ silica
• Fused Silica: Melted Silica (usually made from other purer silicon-rich compounds/precursors)
• Fused Quartz: Melted Quartz.
• Silica Glass/Quartz Glass: The use of these terms is very often where some of the confusion can occur. The terms “Silica Glass” and “Quartz Glass” can be used in a more generic sense and used differently depending on the context and the author. We should consider therefore that they can be used interchangeably and could refer to either fused silica or fused quartz.

Fused Silica vs Fused Quartz

• Fused Silica is a synthetic, non-crystalline silica glass derived from silicon gas or silica sand (non-crystalline). It offers a very high transmission in the UV spectrum and can be either translucent or opaque. The purity of silica glass makes it useful in manufacturing semiconductor and laboratory equipment.

• Fused Quartz is made from naturally occurring crystalline quartz or silica grains. It has a lower UV transmission and a much lower OH content than fused silica. It is usually transparent and the low coefficient of thermal expansion makes it ideal for precision mirror substrates. Fused quartz has excellent electrical, optical and thermal performance and corrosion resistance.

In essence the difference between fused silica and fused quartz lies in the quality of the source ingredients. Fused silica is made from pure silicon dioxide whereas fused quartz is made from quartz, the naturally occurring mineral, which will have naturally occurring impurities depending on the geological source.

Properties

Both share the following properties which make them ideal for the precision optics industry:
They offer low thermal expansion and excellent optical qualities. Unlike traditional glass which has additives to lower the melt temperature, both are very pure and offer high working and melting temperatures. The softening point is approximately 1,730°C with a maximum use temperature of 1450°C and it can be used for a long time at 1,100°C. Both are corrosion resistant, with an acid resistance 30 times that of ceramics and 150 times that of stainless steel. With the exception of hydrofluoric acid, there is barely any reaction to acids.

Quartz glass (fused silica and/or fused quartz) are commonly used in a variety of industries and applications such as: chemistry; building; military; semiconductor; communication; metallurgy; chemistry; environmental protection; space technology; atomic energy; automation.

Greestals Material can advise you on the best optical glass for your industry and equipment, and offer cost-effective solutions.