Schott Guide to Glass

‘Natural glass’ is produced whenever glass-forming rocks melt under high temperature and then solidify quickly. This happens when volcanoes erupt, when lightning strikes into quartz iferous sand or when meteorites hit the surface of the earth. During the stone age, humans used cutting tools made of natural glass of volcanic origin, known as absidian and tektities.

When considered as a material, glass is a collective term for an unlimited number of materials of different compositions in a glassy state. Glassy materials can also occur naturally. For example, obsidian, often found in volcanic areas, has a composition comparable to man-made glass. It consists of sand, and sodium and calcium compounds; and it was fashioned into knives, arrowheads, spearheads, and other weapons in ancient times. Natural glass in the form of obsidian was primarily used by peoples of the Eastern Mediterranean. It was in high demand in this region as an object of trade. The Aztecs in Mexico were also familiar with obsidian and created religious and household items from it.

Melting is the central phase in the production of glass. The individual raw materials combine at high temperatures to form molten glass. The quality of the batch material, the type of heating energy, and the type of melting process used are determined by the glass type to be melted and the product to be made. Removal of the glass from the furnace for further processing and the cooling cycle are stages subsequent to the melting process.

The term flat glass pertains to all glasses produced in a flat form, regardless of the method of manufacture. Unlike hollow-ware, it was not until the Middle Ages that glassblowers were finally successful in making flat sheets of glass for windows and further processing. Mechanical production processes were developed much later, effectively after 1920 (apart from some tentative earlier beginnings). Today manual production of flat glasses is the exception rather than the rule.

The most widespread glass products belong to the hollowware family. Hollowware is encountered everywhere. In the broadest sense, these products are consumer goods such as bottles, or consumer durables such as drinking glasses or glass lamps.

Unlike flat glass, glass fibers or hollowware, special glass is not identified by its appearance. The most significant factor for the various types of special glasses is their application and this determines the requirements on certain properties of the glass. These glass properties can be influenced or adjusted by formulating suitable compositions which, in turn, requires intensive scientific research which few glass companies in the world can support. The result is a range of special glasses with high chemical and thermal durability and with a variety of optical, electrochemical or special application-tailored properties. These glasses are used in many different fields, such as chemistry, pharmacy, electrotechnology, electronics, apparatus and instrument construction, optics, illumination engineering, household appliances, certain sectors of the construction industry and in other technical applications.

Melting and processing glass is to a great extent done in processes generating by-products that are released into the air or into water or become scrap or waste material polluting the environment. Very often they are also hazardous; however, there are specifically developed treatments and techniques which substantially reduce or eliminate their environmental impact. Legal regulations allow the government to influence and control the necessary measures and ways to assure their application.

In all industrialized countries, the glass industry is one of the smaller players in the entire production industry. At most, its value is one percent of total production.