The term “super-cooled liquid” is sometimes used to describe quartz glass at lower temperatures but this is not strictly correct. Below 1000 °C the material can be regarded as a true solid.
The viscosity has an exponential dependence on inverse temperature and thus decreases rapidly as temperature rises. The viscosity is significantly affected by trace impurities. Alkalis and halogens such as sodium and chloride lower viscosity while small amounts of aluminium and refractory metals like molybdenum increase viscosity.
The hydroxyl (OH) content lowers the viscosity thus making the method of manufacture an important consideration for defining maximum use temperature (see figure).
