Glass materials have a random, liquid-like (non-crystalline) molecular structure. They are heated to a temperature sufficient to produce a completely fused melt. Raw materials for glass include cullet (crushed, recycled glass), sand, soda ash, limestone, and additives. At ordinary temperatures, glass materials are relatively strong, inert, biologically inactive, and corrosion resistant. They also provide excellent thermal and electrical insulation, and are characterized by high dielectric strength. During the fabrication of glass materials, the properties of glass can be modified as necessary by additives or heat treatment. Glass materials include embedded glass, tempered glass, borosilicate glass, and soda lime glass. Embedded glass refers to glass materials that are embedded with wire for resistance heating or reinforcement. Tempered glass provides increased strength and can shatter into small pieces when broken. Borosilicate glass offers superior durability, chemical resistance, and heat resistance. These properties make borosilicate glass suitable for use in chemical processes, in the pharmaceutical industry, in high-powered lamps, and in cookware and other heat-resistant products. Soda lime glass is used in everyday products such as bottles, jars, drinking glasses, and window glass.
Lead glass, electronic glass, opal glass, and fused silica glass are types of glass materials. Lead glass (also called lead-alkali glass) has a high percentage of lead oxide (at least 20%) to increase its index of refraction. Lead glass is relatively soft, a better electrical insulator than soda-lime or borosilicate glass, and used for optical applications such as prisms and lenses. Electronic glass is used in fabricating electronic components such as X-ray tubes, display devices, and chip components. Opal glass is used to diffuse light uniformly. Fused silica glass has excellent resistance to thermal shock, withstands high operating temperatures (1,200° C for short periods), and is used in optical waveguides and crucibles for growing crystals.
Several organizations maintain standards for glass materials. ASTM International (formerly called the American Society for Testing and Materials (ASTM) Technical Committee C14 on Glass and Glass Products maintains several standards, including ASTM E708-79(1999) Standard Specification for Waste Glass as a Raw Material for the Manufacture of Glass Containers. The International Organization for Standardization (ISO) supports Technical Committees on glass materials, including TC 48 on quality of glassware and TC 160 on glass in building.
