Silica (SiO2) is a common fundamental constituent of glass. In nature, vitrification of quartz occurs, when lightning strikes sand, forming hollow, and branching rootlike structures.
Fused quartz is a glass made from chemically-pure silica. It has excellent resistance to thermal shock, being able to survive immersion in water while red hot. However, its high melting temperature (1723 °C) and viscosity make it difficult to work with.
Normally, other substances are added to simplify processing. One is sodium carbonate (Na2CO3, "soda"), which lowers the glass-transition temperature. The resulting glass contains about 70 to 74% silica by weight and is called a soda-lime glass.
Soda-lime glasses account for about 90% of manufactured glass.