Pyrex glass is a borosilicate glass first produced by The Corning Glass Works company. It is made by heating raw materials like silica sand and boric oxide to extremely high temperatures for extended periods of time.
To understand how Pyrex is unique it is important to understand the nature of glass itself. Glass is a state of matter that has characteristics similar to both crystalline solids and liquids. On a macroscopic level glass appears to be like solids. It is rigid and remains in one piece when removed from a container. However, on a molecular level, glasses are more like liquids. In crystalline solids, molecules are arranged in an orderly fashion. In liquids they are randomly arranged. This random arrangement is also a characteristic of glass.
Glass is typically made by heating crystalline compounds to temperatures high enough to melt them. Melting breaks the ordered molecular structure, leaving them in a disordered state. When the melted material is cooled, the molecules become locked in place before they can reform in the ordered crystalline structure. The properties of a specific glass such as hardness, brittleness, clarity, and chemical and thermal resistance are dependent on its chemical composition.
When Pyrex was being developed, scientists were trying to create a glass composition that had a high thermal resistance. At some point it was discovered that glass compositions with boron could be heated to high temperatures without breaking. Boron, which is the fifth element on the periodic chart, has the unique ability to create a variety of chemical bonds. When bonded with oxygen it can create a three dimensional structure that is strong. In a glass composition, this extra strength gives it thermal and chemical resistance that makes it useful for cooking applications, thermometers, and laboratory equipment. Pyrex also has a low alkali content that gives it high corrosion resistance.