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What is glass?

Glass has always been important to man. Obsidian, which is usually classified as a "stone" is actually a natural glass and stone age obsidian arrow heads are one of the earliest known applications for glass.  Early glass used to make bottles and windows was usually formed by adding soda and lime or potash to silicates. We know that the Romans made glass using these methods in the 1st century AD. There are stained glass windows still in service that were installed between 1050 to 1150 in Augsburg Cathedral in Germany. 

Glass is usually produced when a molten material cools very rapidly without sufficient time for a crystal lattice to form. Although glass can be formed from other materials, the glass used for windows, containers and decorative objects today is silica-based. In its pure form glass is a transparent and strong but it is brittle. These properties can be changed by alloy additions or heat treatment. Common glasses have the same chemical composition as quarts and sand. They are about 70% silicon dioxide (SiO2). 

Glass such as Pyrex (used for test-tubes and ovenware) is usually based on boro-silicates or alumino-silicates, which withstand heat better than common glass. Optical glass (telescope lenses, eye glasses) is usually not the same as the glass used in windows and bottles. 

What is crystal?

Crystal is lead glass that is usually hand or machine cut with facets. When lead oxide is added to the molten glass, it creates "lead crystal". Crystal has a much higher index of refraction than normal glass, which produces the "sparkle" that is identified with crystal. The lead addition also makes crystal heavier than glass and it also has a higher degree of clarity. 

There are different degrees of lead crystal content and full lead crystal (24% lead oxide) is much more valuable than crystal with lower levels of crystal content.

Some of the famous manufacturers of lead crystal include Royal Leerdam Crystal in the Netherlands, Steuben Glass in the United States, Waterford Crystal in Ireland, Moser in the Czech Republic, Baccarat crystal in France, and Swarovski in Austria.