Synthetic quartz prepared by flame hydrolysis of silicon tetrachloride can have high (> 1000 ppm) or very low hydroxyl content depending on whether a hot chlorination step is employed to remove it. The hydroxyl level can be so high because the silica particles that result from the hydrolysis reaction are extremely fine and therefore have tremendous surface area capable of absorbing moisture present in the flame.
The main attributes of electrically fused materials are the low hydroxyl content and reduced devitrification rates. The low hydroxyl content increases infrared transparency and viscosity. The higher viscosity results in an increased maximum use temperature as well as helping to inhibit devitrification. Devitrification is also restrained by the neutral/ reducing atmosphere used during melting. This causes the material to slightly oxygen deficient, which helps to restrain devitrification.
The high resistance of quartz glass to elements and compounds is another advantage for high-end applications. Fused quartz is outstandingly resistant to water, salt solutions and acids. It is only attacked by hydrofluoric and phosphoric acid. Metals which are free from oxide, with the exception of alkalis and alkaline-earths, do not react with fused quartz or fused silica.
Quartz glass is sensitive to all alkali and alkaline-earth compounds because even slight traces of them hasten devitrification at high temperatures. It is always advisable to remove fingerprints, which contain traces of alkalis, from quartz glass with alcohol before heating.
