What is the difference between fused quartz and fused silica? This is one of the most frequently asked questions by our customers.
Everybody agrees that fused silica and fused quartz are amorphous (non-crystalline) glass made only from silicon dioxide (SiO2), but beyond that there is no universally agreed upon definition. In fact, many people use the terms fused silica and fused quartz interchangeably. Part of the confusion results from changes in the glass fabrication processes, and over the years the definition has changed.
Old Definition
Originally, these terms were used to distinguish between transparent and opaque grades of the material. Fused quartz products were those produced from crushed quartz crystal to make transparent ware. Fused silica products were those manufactured from fine sand into opaque ware. Today, this differentiation is obsolete, as advances in raw material manufacturing permit transparent fusion from sand, as well as from crushed crystal.
Modern Definition
Fused Quartz
Today, Fused Quartz is defined as a material made by melting high purity, naturally occurring quartz crystal. The melting is conducted at around 2000°C, using either an electrically heated furnace (electrically fused) or a gas/oxygen fueled furnace (flame fused). Fused quartz is amorphous and non-crystalline. Residual impurities from the raw material affect Fused Quartz transparency in the ultraviolet.
Fused Silica
Fused Silica (also known as Synthetic Fused Silica) is made from a silicon-rich chemical precursor, usually using a continuous flame hydrolysis process, which involves chemical gasification of silicon, oxidation of this gas to silicon dioxide, and thermal fusion of the resulting dust (although there are alternative processes). This results in a transparent glass with an ultra-high purity and improved optical transmission in the deep ultraviolet. Water vapor, generated as a byproduct during the fabrication process, affects the transparency of fused silica in the infrared.