Its coefficient of thermal expansion is exceptionally low, roughly a twentieth of alumina's, so even a steep temperature gradient produces very little differential expansion and therefore very little internal stress. A hot part can often be quenched without cracking, which no oxide ceramic tolerates.

Clear fused quartz is bubble-free and transparent from the ultraviolet through the infrared, used where optical transmission or maximum purity is required. Opaque fused silica contains fine dispersed bubbles that scatter light and radiant heat, costs less, and is used for crucibles and furnace ware where transparency serves no purpose.

The gradual conversion of the glassy structure to crystalline cristobalite, which starts to matter above roughly 1100 degrees Celsius and accelerates with temperature, contamination and time. The affected surface turns white and opaque, and the part becomes far more prone to cracking on cooling. Surface cleanliness strongly affects how quickly it sets in.

Purity combined with thermal stability. High-purity synthetic grades contribute almost no metallic contamination at process temperature, which matters because a few parts per billion of mobile metal ions will ruin a wafer, and the very low expansion tolerates the repeated thermal cycling that furnace ware sees every shift. Diffusion tubes, wafer carriers, boats and photovoltaic crucibles depend on both properties at once, and no cheaper material offers either.

Clear fused quartz does, well into the UV, and that is a defining reason to choose it - UV lamp envelopes, sterilisation equipment, spectroscopy cells and optical windows all rely on it. Transmission at the shortest wavelengths depends on grade and on hydroxyl content, so UV work should be specified by grade rather than by material name.

Carefully, because it is notch-sensitive: a scratch or chip is a stress concentrator and is where a failure will begin. Surfaces should be kept clean, since alkali contamination - including from fingerprints - accelerates devitrification at temperature. Parts are normally cleaned and handled with gloves before high-temperature use.

By diamond grinding, drilling and lapping, and it is also flame-worked like glass to form tube, bends and closed ends, which is how much furnace ware is produced. It cannot be machined with conventional tooling, and cut edges are usually flame-polished or ground to remove the micro-cracks that would otherwise start a fracture.