Glass is an amorphous solid that has been around in various forms for thousands of years and has been manufactured for human use since 12,000 BCE. Its status as a liquid, versus a solid, has been hotly debated. The short story is that glass is a supercooled liquid, meaning that it is rigid and static but does not change molecularly between melting and solidification into a desired shape. It is one the most versatile substances on Earth, used in many applications and in a wide variety of forms, from plain clear to tempered and tinted varieties, and so forth.
Naturally occurring glass is created when rocks high in silicates melt at high temperatures and cool before they can form a crystalline structure. Obsidian or volcanic glass is a well known example of naturally occurring types, although it can also be formed by a lightning strike on a beach, which contains silicate-rich sand. Early forms were probably rife with impurities and subject to cracking and other instability, but examples of beads, jars, and eating materials first appeared in ancient Egyptian culture.
When manufactured by humans, glass is a mixture of silica, soda, and lime. Other materials are sometimes added to the mixture to “frost” or cloud the glass or to add color. The elements are heated to 1800° Fahrenheit (982° Celsius). The resulting fused liquid can be poured into molds or blown into various shapes, and when cooled, glass is a strong, minimally conducting substance that will not interact with materials stored inside. As a result, it is frequently used in scientific laboratories to minimize inadvertent chemical reactions and to insulate power lines.
Silica is found in a wide variety of natural sources, including, most commonly, sand. Sodium carbonate, or soda, is used to lower the fusion point of silica, making glass light and workable. Soda is called a flux, because it brings the melting point of the mixture down. Lime is ground from limestone and makes the mixture more viscous, as well as making it less susceptible to the erosive qualities of water and acids.
Glass is a strange substance, defying easy scientific categorization. It is not a solid, not a gas, and not quite a liquid either. Generally, it is classified as a rigid liquid, maintaining liquid properties while acting like a solid. Heat can return it to a liquid and workable form, making it easy to reuse and recycle.
There are many reasons that glass is a favored material. It resists chemical interactions, it is easy to recycle, it does not leach chemicals like plastics do, and it can withstand extremes of heat and cold, although not at the same time. Tempered or safety glass is used in a wide variety of applications, and virtually all consumers use many forms daily.