A lot of experience is necessary to do hot forming of fused silica. Due to the high melting point and its steep viscosity, the processing temperature range is rather narrow. If the temperature is too low, fused silica is solid, if it is too high, it gets very soft, almost liquid, and it evaporates to form soot dust.
Additionally hot processing fused silica requires an annealing step, or multiple intermediate annealing steps, to relieve thermal stress. If not done correctly, fracturing of the glass can happen any time.
Welding can join two components of fused silica. It is important to heat the two components and keep the temperature high enough to avoid a critical level of thermal stress. A small rod of fused silica, a welding rod, is melted locally in order to fill the gap between the two glass components. Once the melt seam is done, or for large parts in multiple sections, the part needs to be annealed to relieve thermal stress.
- Glass blowing
Typically, heating a tube locally to the softening temperature and a lower pressure applying to the inside reduces the tube diameter. The process is frequently employed in fiber optic production to collapse the cladding onto the core rod.
In its extreme form, the process is able to yield rods, from a tube. This typically is the case in CVD processing.