The basic recipe for glass is surprisingly simple. Finely ground silica sand (silicon dioxide), soda ash (sodium carbonate) and limestone (calcium carbonate) are mixed together and melted at temperatures above 1,400 degrees Celsius. Under the influence of heat, the carbonates are converted to oxides and carbon dioxide is liberated. As the melt cools it becomes increasingly viscous and ultimately solidifies without crystallizing.
Glass experts call this a “supercooled melt.” The formulation of special types of glass requires other substances such as potash (potassium carbonate, obtained from plant ashes in the past) uranium oxide or lead oxide.
The exceptionally heat-resistant quartz glass, for example, consists of pure silicon dioxide. Lead crystal has a particularly high proportion of lead oxide. Its luminosity is due to strong light refraction, which can be intensified by special polishing (cut glass). Rhinestone is especially rich in lead and sparkles like a diamond if well cut. That’s why this type of glass is used in the jewelry trade. Colored glass is made by adding small amounts of metal oxide to the molten glass.
Cobalt is used for blue, nickel for violet, chromium and uranium for green. Depending on their valency states, copper gives a red or green glass, iron a green or reddish-brown glass. Colloidal (i.e. finely dispersed) gold is used to obtain the vibrant red of gold ruby glass. Colloidal silver gives yellow.
Glass was already in use in Asia Minor 4,000 years ago. The Romans studied the technique and brought it back to Europe. The glass manufacturing process remained more or less unchanged right up to modern times. Glass foundries were located in densely forested regions that provided an unlimited supply of fuel for melting the raw materials, such as Germany’s Upper Palatinate, Lower Bavaria, Black Forest and Spessart regions.